ECG (Obstetrics & Gynecology Physics Chapter)

1. What ECG Actually Records – the ONE Big Idea

ECG = Electrical activity of the heart → displayed as waves on paper.

Not contraction. Not pressure. Only depolarisation + repolarisation.

👉 If you know what each wave means, you can answer 80% of exam questions.

2. The Waves You MUST Know (Simple Meaning)

This table alone → repeatedly tested.

3. Relationship to Cardiac Cycle (Essential Pathway)

Electricity flows SA node → atria → AV node → His → RBB/LBB → Purkinje → ventricles.

Mnemonic: “SA → AV → His → Branches → Purkinje → Boom.”

Exams test:

• Which node is the pacemaker? → SA node
• What delays conduction? → AV node
• What spreads activation fast? → Purkinje system

4. ECG Paper Speed and Measurements — MOST EXAM QUESTIONS COME FROM THIS

Paper speed

• 25 mm/s

Squares

• 1 small square = 0.04 s
• 1 large square = 0.20 s

These two numbers — 0.04 and 0.20 — are exam gold.

5. Duration of Key Intervals (the only ones examiners adore)

If you know these 3 → you can answer >80% of interval-related MCQs.

6. Lead Groupings — the Most Important, Exam-Targeted Table

👉 You must memorise this mapping. It is extremely common in exams.

7. Bipolar vs Unipolar Leads (You only need this)

• Bipolar leads: I, II, III

→ Compare electrical activity between two electrodes

• Unipolar leads: aVR, aVL, aVF + V1–V6

→ Compare one exploring electrode with an average reference

This distinction repeatedly appears.

8. Electrode Placement (Only the Exam-High-Yield Facts)

• Limb leads + augmented leads → frontal plane
• Chest (V1–V6) leads → horizontal plane

Knowing the plane = marks.

Also:

V1 = right sternal edge, 4th intercostal space

V6 = mid-axillary line, 5th intercostal space

(Only V1 & V6 are usually tested.)

9. Features of Normal Sinus Rhythm (Questions Always Come)

You only need these 5 points:

1. Rate: 60–99 bpm
2. Rhythm: Regular
3. P wave upright in I and II
4. Each P followed by QRS
5. Normal PR, QRS, QT intervals

If all 5 are present → answer = “Normal Sinus Rhythm.”

10. Normal Variations in Healthy People — Often Asked to Avoid Over-diagnosis

These are normal, especially in athletes:

• Sinus arrhythmia (RR varies with breathing)
• Sinus bradycardia
• Tall R waves
• Prominent U waves
• ST elevation (benign early repolarisation)
• First-degree heart block (prolonged PR interval)

Exams love asking which of these is normal.

USS

1. What Ultrasound REALLY Is (Core Concept)

If you understand this ONE sentence, the rest becomes easy:

👉 Ultrasound = mechanical sound waves (>20 kHz) produced by a piezoelectric crystal → travel through tissues → reflect/scatter → return → converted into image.

This ONE idea appears in almost every exam question.

2. Why Ultrasound Is Ideal for Obstetrics & Gynecology (Always Tested)

• Noninvasive
• No ionising radiation → safe for pregnancy
• Repeatable
• Real-time imaging (movement, fetal heart)

Exam answers: “safe + repeatable + real-time.”

3. Functions of Ultrasound You Must Know (High-Yield)

• Distinguish cystic vs solid
• Assess movement (fetal heart, bowel)
• Measure blood flow (Doppler)
• Measure structures (follicle diameter, femur length)

4. Piezoelectric Crystal – the Heart of Ultrasound (Frequently Tested)

A piezoelectric crystal converts electrical energy ↔ mechanical (sound) energy.

Key points:

• Vibrates when voltage applied
• Frequency controlled by thickness of the crystal
• Located inside the transducer (probe)

If a question mentions “generation of ultrasound,” answer = piezoelectric crystal in the transducer.

5. Types of Ultrasound (Only One is for Imaging)

👉 Exams love: Imaging uses pulsed-wave ultrasound, not continuous.

Pulsed → packets → gives spatial information → creates images.

Active Recall – Fill in the Blanks

Types of Ultrasound (Only ONE is for Imaging)

👉 Exam favourite:

Imaging uses __________-wave ultrasound, not __________-wave.

Why?

__________ → sent in __________ → allows __________ information → forms __________.

6. How Ultrasound Interacts With Tissue (Ultra-High Yield)

These THREE interactions = 80% of the physics questions.

1. Reflection

Happens at boundaries between tissues.

Depends on acoustic impedance (Z = ρ × c).

Big difference in Z = strong reflection.

the sound waves cannot travel deeper due to impedence(shadowing happens)

👉 Why gel is used?

To remove air → huge impedance mismatch → prevents reflection loss.

2. Scatter

Occurs with very small structures (cells, parenchyma).

Rayleigh scattering = 360° scatter.

↑ Frequency → ↑ scatter → limits usable frequency.

3. Absorption

Ultrasound → converted to heat.

↑ Frequency → ↑ absorption → ↓ penetration.

Upper limit for clinical ultrasound ≈ 20 MHz.

Exam tip: High frequency = good resolution but poor penetration.

Ultrasound–Tissue Interaction: Active Recall (Fill in the blanks)

Core concept

1. The THREE key tissue interactions are: ________, ________, and ________.

1) Reflection

1. Reflection happens at tissue ________.
2. Reflection depends on acoustic impedance (Z).
3. Acoustic impedance formula: Z = ρ × ________.
4. If the difference in Z between two tissues is ________, reflection is strong.
5. When there is strong impedance mismatch, sound waves cannot travel ________ → shadowing occurs.
6. Gel is used to remove ________.
7. Air causes a huge impedance ________ leading to reflection loss.

2) Scatter

1. Scatter occurs with very ________ structures (cells / parenchyma).
2. Rayleigh scattering causes ________° scatter.
3. ↑ Frequency → ↑ ________ → limits usable frequency.

3) Absorption

1. Absorption converts ultrasound energy into ________.
2. ↑ Frequency → ↑ ________ → ↓ penetration.
3. Upper frequency limit for clinical ultrasound ≈ ________ MHz.

Golden Exam Rule

1. High frequency = good ________ but poor ________.

7. B-Mode Real-Time Scanning – How the Image is Formed (Critical)

• About 200 ultrasound beams sweep across field → produce image.
• Transducer sends → receives echoes → converts back to electrical signals.
• Signals stored in pixels in the scan converter.
• Grey scale determined by echo amplitude.

B-Mode Real-Time Scanning — Fill in the Blanks

1. In B-mode scanning, about ________ ultrasound beams sweep across the field to produce the image.
2. The ________ sends ultrasound waves into the body.
3. The transducer then ________ echoes returning from tissues.
4. The transducer converts echoes back into ________ signals.
5. These signals are stored as ________ in the scan converter.
6. Each pixel corresponds to a stored signal ________.
7. The grey scale in the final image is determined mainly by echo ________.
8. High amplitude echoes appear more ________ on the image.(bright)
9. Low amplitude echoes appear more ________ on the image. (dark)

8. Gain vs Time-Gain Compensation (TGC) — Common MCQ

👉 Most exam candidates confuse these two — now you won’t.

9. What Limits Ultrasound Image Quality (Always Tested)

1. Spatial Resolution

Axial resolution (along beam)

• Depends on pulse length
• Higher frequency → shorter pulses → better axial resolution

Lateral resolution (side-by-side)

• Depends on beam width and focus

2. Penetration

↓ Frequency → better penetration

↑ Frequency → better resolution but poor penetration

Exam loves asking:

“Which frequency for deep pelvis?” → Lower frequency (e.g., 3.5 MHz)

“Which frequency for follicle/ovary?” → Higher frequency (7–10 MHz)

3. Frame Rate, Line Density, Field of View

These three are interdependent:

• ↑ Line density → ↑ resolution but ↓ frame rate
• ↑ Field of view → ↓ frame rate
• ↑ Frame rate → ↓ line density or ↓ field of view, ↓ resolution

Clinically:

• Fast-moving structures (fetal heart) → need ↑ frame rate
• Detailed organ imaging → need ↑ line density

What Limits Ultrasound Image Quality — Fill in the blanks

1) Spatial Resolution

1. Spatial resolution has two main types: ________ resolution and ________ resolution.

Axial resolution

1. Axial resolution is resolution along the ________.
2. Axial resolution mainly depends on pulse ________.
3. Higher frequency → shorter pulses → better ________ resolution.

Lateral resolution

1. Lateral resolution is resolution side-by-side (perpendicular to the ________).
2. Lateral resolution depends mainly on beam ________ and ________.

2) Penetration

1. Decreasing frequency → better ________.
2. Increasing frequency → better ________ but poor ________.
3. For deep pelvis, best choice is ________ frequency (example ________ MHz).
4. For follicle/ovary imaging, best is ________ frequency (– MHz).

3) Frame Rate / Line Density / Field of View

1. The three interdependent parameters are: ________ rate, ________ density, and field of ________.
2. ↑ Line density → ↑ ________ but ↓ ________ rate.
3. ↑ Field of view → ↓ ________ rate.
4. ↑ Frame rate → ↓ ________ density OR ↓ field of ________.

Clinical application

1. Fast-moving structures (e.g., fetal ________) require ↑ ________ rate.
2. Detailed organ imaging requires ↑ ________ density.

10. Ultimate Golden Formula to Remember

High Frequency = High Resolution, Low Penetration

Low Frequency = Low Resolution, High Penetration

This appears in every exam paper.

Doppler

1️⃣ Core Doppler Idea – What It Really Does

Doppler ultrasound = using frequency change of echoes to tell:

• ✅ Speed of blood flow
• ✅ Direction of blood flow

Because:

👉 If blood cells move towards the probe → frequency of returning echo increases

👉 If they move away → frequency decreases

That difference is the Doppler frequency shift (fₑ or f_d) → used to calculate velocity.

Active Recall – Fill in the Blanks

Doppler ultrasound = using __________ change of __________ to determine:

• ✅ __________ of blood flow
• ✅ __________ of blood flow

Because:

👉 If blood cells move __________ the probe → frequency of returning echo __________

👉 If they move __________ → frequency __________

That difference is called the __________ frequency shift (__________ or __________) → used to calculate __________.

2️⃣ The Doppler Equation – What Matters & How to Think About It

Doppler frequency shift (f d) = (2 × transmitted frequency × blood velocity × cosine of the angle) ÷ speed of sound in tissue

Or even simpler:

f d = 2 × f t × v × cos θ divided by c

Where:

• f_d = Doppler frequency shift
• f_t = transmitted ultrasound frequency
• v = velocity of red blood cells (what we want)
• θ = angle between beam and blood flow (angle of insonation)
• c = speed of sound in soft tissue (≈ constant)

What you must understand (for exams + real life):

• Only the component of velocity along the beam (v cosθ) is measured.
• If θ = 0° (beam parallel to flow) → cosθ = 1 → maximum Doppler signal (best).
• If θ = 90° → cosθ = 0 → no Doppler shift → you see nothing even though blood is moving.
• So: correct angle is critical. Bigger angle → more error.

That’s one of the most important practical points.

🧠 Doppler Equation – Active Recall (Fill in the blanks)

1) Core equation

1. Doppler frequency shift (____) = (2 × ____ frequency × blood ____ × cosine of the angle) ÷ speed of sound in tissue
2. Simplified: ____ = 2 × ____ × ____ × cos ____ ÷ ____
3. The “2” in the Doppler equation is because Doppler shift occurs during ____ and again during ____ (double shift).

2) Meaning of symbols

Fill in:

• f_d = Doppler frequency ____
• f_t = ____ ultrasound frequency
• v = ____ of RBCs (what we want)
• θ = ____ between beam and blood flow (angle of insonation)
• c = ____ of sound in soft tissue (≈ ____)

3) What is actually measured

1. Doppler measures only the component of velocity ____ the beam.
2. That measured component is written as: v ____ θ
3. Therefore the Doppler signal depends on cos ____.

4) Best insonation angle facts

1. If θ = ____°, beam is parallel to flow → cosθ = ____ → Doppler signal is ____.
2. If θ = ____°, beam is perpendicular to flow → cosθ = ____ → Doppler shift is ____.
3. You can see ____ Doppler shift even though blood is ____ if θ is 90°.

5) Practical exam punchlines

1. Correct ____ is critical.
2. Bigger angle → more ____.
3. Doppler frequency shift is directly proportional to:
• transmitted frequency (____)
• blood velocity (____)
• cos of the insonation angle (cos____)

3️⃣ Types of Doppler & Colour Flow – What They Actually Show

🔹 Continuous-wave Doppler

• Simple, no imaging, just detects flow.
• Used for:
• Fetal heart rate
• Basic pulse/blood flow detection
• Cannot localise exact depth (no range resolution).

🔹 Pulsed-wave Doppler + B-mode + Colour flow = Modern machines

Colour flow scanner combines:

• B-mode → anatomy/structure (grey scale)
• Pulsed Doppler → spectral waveforms (velocity, pattern)
• Colour Doppler → direction + velocity overlaid on image

Colour coding (classic exam favourite):

• Flow towards transducer → Red
• Flow away from transducer → Blue
• Darker shades → slower velocities
• Brighter shades → higher velocities

(Exact shades depend on the colour scale, but this conceptual pattern is what exams want.)

🔹Continous-wave Doppler

• __________, no __________, just detects __________.
• Used for:
• __________ heart rate
• Basic __________ / blood flow detection
• Cannot localise exact __________ (no __________ resolution).

🔹 Pulsed wave Doppler + B mode + color flow = Modern machines

A __________ flow scanner combines:

• __________ → anatomy / structure (__________ scale)
• __________ Doppler → __________ waveforms (__________, pattern)
• __________ Doppler → __________ + __________ overlaid on image

Colour coding (exam favourite):

• Flow __________ transducer → __________
• Flow __________ transducer → __________
• __________ shades → slower __________
• __________ shades → higher __________

4️⃣ Safety: Why Doppler Is More Concerning Than B-mode

Key idea:

Doppler imaging uses higher intensities than B-mode or M-mode,

so safety becomes more important, especially in pregnancy.

Potential issues come from two mechanisms:

1. Heating (thermal effects)
2. Mechanical effects (mainly cavitation)

🧠 Doppler Safety – Active Recall (Fill in the blanks)

1) Core safety concept

1. Doppler imaging uses higher ____ than B-mode or M-mode.
2. Therefore Doppler has greater safety concerns, especially in ____.

2) Why Doppler is more concerning

1. Compared to B-mode, Doppler uses higher ____ (power/intensity).
2. Higher intensity increases risk of biological effects, mainly related to ____ and ____.

3) Two mechanisms of potential harm

1. Doppler safety issues come mainly from two mechanisms:
• ____ (thermal effects)
• ____ effects (mainly cavitation)

4) Thermal (heating) effects

1. Heating effects are called ____ effects.
2. These occur because ultrasound energy is absorbed by tissue and converted into ____.

5) Mechanical effects

1. Mechanical effects are mainly due to ____.
2. Cavitation refers to formation and activity of ____ in tissue/fluid.

5️⃣ Heating & Thermal Index (TI) – What You Need to Know

Absorption of ultrasound → heat in tissues.

⚠️ Particularly important where:

• Absorption is high → e.g. bone
• Surrounding soft tissues may be heated by conduction from bone
• In obstetrics → as fetal bone develops, risk increases.

So they created the Thermal Index (TI):

TI = (emitted acoustic power) ÷ (power needed to raise target tissue by 1°C)

There are three types:

• TIS – soft tissue
• TIB – bone
• TIC – cranium

In obstetrics:

• First 8 weeks post-conception → watch TIS
• After 8 weeks (fetal bones forming) → watch TIB

👉 Operator’s responsibility:

Always keep TI as low as is consistent with getting a diagnostic image (shorter time, lower power if possible).

Heating & Thermal Index (TI) – Active Recall (Fill-in-the-Blanks)

Absorption of ultrasound → ______ in tissues.

⚠️ Heating risk is particularly important where:

• Absorption is high → e.g. ______
• Surrounding soft tissues may be heated by ________ from bone
• In obstetrics → as fetal ______ develops, risk increases

Thermal Index (TI)

TI = (________ acoustic power) ÷ (power needed to raise target tissue by ___ °C)

Three Types of TI

• TIS → ______ tissue
• TIB → ______
• TIC → ______

Obstetric Application

• First ___ weeks post-conception → monitor ______
• After ___ weeks (fetal bones forming) → monitor ______

Operator Responsibility

• Always keep TI as ______ as is consistent with getting a ______ image
• Reduce:
• Exposure ______
• Acoustic ______ (if possible)

6️⃣ Cavitation & Mechanical Index (MI) – The Other Big Safety Issue

Cavitation = behaviour of gas bubbles in tissue exposed to ultrasound.

• Tiny gas bubbles can:
• Oscillate → shear forces
• Collapse → very high local pressure & temperature
• Effects:
• Can damage cell membranes
• Can generate free radicals

More likely:

• In tissues with gas (lung, bowel)
• In presence of microbubble contrast agents

So we use the Mechanical Index (MI):

• Related to maximum negative pressure and ultrasound frequency
• Higher MI → higher risk of mechanical (non-thermal) effects

Key exam line (from EFSUMB-type statements):

Non-thermal bioeffects have been shown in animals,

but no harmful effects in humans have been proven without contrast agents.

🧠 Cavitation & Mechanical Index (MI) – Active Recall (Fill in the blanks)

1) Definition

1. Cavitation = behaviour of ____ bubbles in tissue exposed to ____.
2. Cavitation is part of the ____ (thermal / mechanical) safety issue.

2) What gas bubbles can do

1. Tiny gas bubbles can ____ → causes ____ forces.
2. Tiny gas bubbles can ____ → produces very high local ____ and ____.

3) Effects of cavitation

1. Cavitation can damage ____ ____.
2. Cavitation can generate ____ ____.

4) When cavitation is more likely

1. Cavitation is more likely in tissues containing ____ (e.g., ____ and ____).
2. Cavitation risk increases in presence of ____ contrast agents.

5) Mechanical Index (MI)

1. MI is used to estimate risk of ____ (non-thermal) effects.
2. MI is related to maximum negative ____ and ultrasound ____.
3. Higher MI → higher risk of ____ (thermal / mechanical) effects.

X-RAY

1️⃣ X-RAYS – The Most Tested Core Facts

What X-rays actually are

• Electromagnetic radiation (not sound, not particles).
• Wavelength 0.01–10 nm (shorter than UV, longer than gamma).
• Energies: 120 eV – 120 keV.
• Ionising radiation → carcinogenic.
• Unit of dose:
• Sievert (Sv)
• 1 Sv = 100 rem
• Diagnostic exposures are in microsieverts (µSv).
• Average background radiation per person ≈ 3 mSv/year.

👉 Main exam point: X-rays = ionising electromagnetic radiation.

1️⃣ X-RAYS – Active Recall (Fill in the Blanks)

What X-rays actually are

1. X-rays are a form of ________ ________ radiation (not sound, not particles).
2. Their wavelength range is ________ – ________ nm.
3. X-rays have wavelengths shorter than ________ but longer than ________.
4. Their energy range is approximately ________ eV – ________ keV.
5. X-rays are ________ radiation, therefore they are ________.
6. The SI unit used to measure radiation dose is ________ (____).
7. 1 Sv = ________ rem.
8. Diagnostic imaging exposures are usually measured in ________ (µSv / mSv / Sv).
9. The average background radiation exposure per person per year is approximately ________ mSv/year.

One-line exam reflex

👉 X-rays = __________ __________ radiation that is __________ and __________ risk.

2️⃣ How X-rays are produced (super high yield)

In an X-ray tube:

1. Electrons are accelerated toward a metal target.
2. Electrons suddenly decelerate → produce X-ray photons (bremsstrahlung).
3. Inner-shell electrons may be ejected → outer electrons fall in → X-rays released.

⚠️ Process is highly inefficient → lots of heat generated.

X-ray production – Fill in the blanks

1. In an __________ tube, __________ are accelerated toward a __________ target.
2. When electrons suddenly __________, they produce __________ photons → called __________ radiation.
3. This mechanism is also called __________ (“braking radiation”).
4. Some electrons can eject __________-shell electrons from the target atom.
5. Then __________-shell electrons fall inward to fill the vacancy → __________ X-rays are released.
6. The process is highly __________ → therefore lots of __________ is generated.

3️⃣ How X-ray images work (shadow principle)

• Dense tissues (bone) absorb more X-rays → appear white.
• Soft tissues absorb less → grey.
• Air absorbs least → black.

👉 Black = high exposure

👉 White = blockage/attenuation.

How X-ray Images Work (Shadow Principle) – Active Recall

1. Dense tissues such as ________ absorb ________ X-rays and therefore appear ________ on X-ray images.
2. ________ tissues absorb fewer X-rays and appear ________.
3. ________ absorbs the least amount of X-rays and therefore appears ________.
4. On an X-ray image, areas that appear black represent ________ exposure to X-rays.
5. Areas that appear white represent ________ or attenuation of X-rays.

One-line exam lock

👉 Black = ________ exposure | White = ________ / attenuation

4️⃣ Key Clinical Uses (must know)

Useful for:

• Skeletal disease
• Chest pathology
• Abdominal obstruction / stones (some)

Not useful for:

• Brain
• Muscles
• Small soft tissue lesions

These require CT/MRI/ultrasound instead.

X-ray Clinical Uses — Fill in the blanks

✅ Useful for:

1. X-rays are most useful for __________ disease.
2. X-rays are commonly used for __________ pathology.
3. X-rays can help detect abdominal __________.
4. X-rays can detect some abdominal __________.

❌ Not useful for:

1. X-rays are not useful for imaging the __________.
2. X-rays are not useful for imaging __________.
3. X-rays are not useful for detecting small __________ tissue lesions.

Better alternatives:

1. These limitations require __________ / __________ / __________ instead.

5️⃣ Contrast Use in X-ray Imaging (OG-relevant)

Radiopaque agents (iodine, barium) highlight cavities:

• Barium meal/enema
• Hysterosalpingography (HSG)
• Uterine artery embolisation (fluoroscopy)

6️⃣ HYSTEROSALPINGOGRAPHY – Very High Yield for O&G Exams

Benefits

• Minimally invasive
• Quick
• No post-procedure radiation
• Shows uterine cavity + tubal patency
• Very low immediate adverse effects

Timing

• Done within first 10 days of cycle → avoid early pregnancy irradiation.

Radiation dose

• Approximately 1 mSv (same as background radiation for ~4 months)

Risks

• Minimal cancer risk (very small)
• Flare-up of PID
• Undiagnosed early pregnancy exposure
• Small teratogenic risk to ovaries

This is always tested.

HYSTEROSALPINGOGRAPHY (HSG) — Fill in the blanks

Benefits

1. HSG is a __________ invasive test.
2. It is a __________ procedure.
3. There is no __________-procedure radiation.
4. HSG demonstrates __________ cavity and tubal __________.
5. It has very low immediate __________ effects.

Timing

1. HSG is performed within the first __________ days of the menstrual cycle.
2. The reason is to avoid irradiating an early __________.

Radiation dose

1. Approximate radiation dose of HSG is __________ mSv.
2. This equals background radiation exposure for about __________ months.

Risks

1. There is a minimal risk of __________ (very small).
2. HSG can cause a flare-up of __________.
3. One risk is exposure in undiagnosed early __________.
4. There is a small __________ risk to the ovaries.

DEXA (Dual-Energy X-ray Absorptiometry) – Bone Density Scan

How it works

• Uses two low-dose X-ray beams of different energies.
• One beam absorbed by soft tissue → subtracted out.
• Remaining absorption = bone mineral density (BMD).

Scores (Very High Yield)

T score → compare to young adult

• –1 = normal
• –1 to –2.5 = osteopenia
• < –2.5 = osteoporosis

Z score → compare to same age, size, gender

• Used for children + premenopausal women
• Avoids false diagnosis of osteopenia.

Important limitation

• DEXA uses area (2D) not volume (3D).
• Overestimates BMD in tall people
• Underestimates in small people.

Types of DEXA Machines

• Central DEXA (hip + spine) = most accurate.
• Peripheral DEXA (heel, wrist, fingers) = screening only.

DEXA (Dual-Energy X-ray Absorptiometry) – Active Recall

How it works

• DEXA uses _____ low-dose X-ray beams of _____ energies.
• One beam is absorbed by _____ tissue and is _____ out.
• The remaining absorption represents _____ mineral density (_____).

Scores (Very High Yield)

T score → comparison with _____ adult

• T = _____ → normal
• T between _____ and _____ → osteopenia
• T < _____ → osteoporosis

Z score → comparison with same _____, _____, and _____

• Mainly used for _____ and _____ women
• Prevents _____ diagnosis of osteopenia

Important Limitation

• DEXA measures _____ (2D) rather than _____ (3D).
• This causes _____ estimation of BMD in _____ people
• And _____ estimation of BMD in _____ people

Types of DEXA Machines

• _____ DEXA measures _____ and _____ → most _____
• _____ DEXA measures _____, _____, or _____ → used for _____ only

CT &MRI

1️⃣ CT SCANNING – THE ESSENTIALS

What CT actually does

• Uses X-rays (ionising radiation).
• Produces cross-sectional (slice) images by reconstructing attenuation of thin X-ray beams.
• Gives much higher contrast resolution than plain X-rays.

👉 Key line for exams:

Ultrasound = first line for pelvic disease, but CT helps in pelvic tumours + abdominal disease.

How CT images are built

• Many thin slices stacked → form 3D volume.
• Software can use:
• Surface rendering
• Volume rendering
• Segmentation (remove structures digitally)

👉 Exam highlight:

CT = fast, high-resolution, ionising radiation.

✅ CT Basics (Fill in the blanks)

1. CT uses _______ (type of radiation).
2. CT radiation is _______ (ionising / non-ionising).
3. CT produces _______-sectional (“_______”) images.
4. CT works by reconstructing _______ of thin X-ray beams.
5. CT has much higher _______ resolution than plain X-rays.
6. Plain X-ray gives mainly a _______-dimensional image.
7. CT gives _______ (fast / slow) imaging.
8. CT provides high _______ images.

✅ Exam Key Line (Fill in the blanks)

1. _______ = first line for pelvic disease.
2. _______ helps in pelvic tumours + abdominal disease.
3. CT is most useful when disease extends to _______ (organ system).
4. CT is commonly used for staging _______.

✅ How CT Images are Built (Fill in the blanks)

1. CT is made of many thin _______ stacked together.
2. These stacked slices form a _______-D volume.
3. Software can create 3D images using _______ rendering.
4. Another 3D method is _______ rendering.
5. _______ = digitally removing structures.

✅ Super high yield one-liner (Fill in the blanks)

CT = _______, high-resolution, _______ radiation.

2️⃣ MRI – THE HIGH-YIELD CONCEPTS

How MRI works (the MOST important thing to understand)

MRI does NOT use X-rays.

It uses:

1. Strong magnetic field → aligns protons (H⁺) in water molecules.
2. Radiofrequency pulse → disturbs alignment.
3. Protons return to alignment → release radiofrequency signal.
4. Signal is detected → image produced.

👉 No ionising radiation.

👉 Excellent soft-tissue contrast.

Why MRI is powerful

• Contrast resolution is better than CT → especially for soft tissues.
• Can distinguish subtle differences between tissues.
• Uses different pulse sequences to characterise tissues.

Used for:

• Brain tumours
• Ovarian tumours
• Uterus
• Pelvic abnormalities
• Congenital fetal issues

3️⃣ MRI IN PREGNANCY – EXAM GOLD

Is MRI safe in pregnancy?

• No proven harmful fetal effects.
• Avoid in first trimester unless necessary (precautionary).
• Uses no ionising radiation.

Contrast agents

• Gadolinium crosses the placenta.
• European Society of Radiology:
• Appears safe
• Rapidly excreted by fetus,
• Use only if essential for maternal diagnosis.

This is frequently tested in O&G examinations.

4️⃣ Interventional MRI – What You MUST Know

Because MRI is safe (no radiation), it is used to guide:

• Minimally invasive procedures
• Real-time or intraoperative imaging (some systems allow scanning during surgery)

Important rule:

👉 No ferromagnetic instruments allowed.

5️⃣ MRI in RADIATION THERAPY PLANNING

MRI is used to:

• Locate tumour precisely
• Map its shape/size
• Correct spatial distortion
• Mark triangulation points for targeted radiotherapy

This is always tested in imaging physics.

6️⃣ MRI-GUIDED FOCUSED ULTRASOUND (Very modern + high-yield)

MRI helps guide high-energy ultrasound beams to:

• Heat tissue to >65°C
• Destroy target (e.g., uterine fibroids)
• Monitor treatment accuracy in real time
• Ensure precise ablation

7️⃣ Drawbacks of MRI – YOU MUST KNOW THIS TABLE

Main limitations:

• Claustrophobia (major reason patients refuse)
• Longer scan times (though now improved)
• High noise levels
• Difficult for:
• Children
• Obese patients
• Pregnant women (comfort issues)
• Contraindications:
• Ferromagnetic clips
• Pacing wires
• Metal fragments (e.g., welders)
• Some prostheses

Absolute exam point:

👉 MRI is contraindicated if metallic objects are in the eye or neurosurgical clips are present.

📊 MASTER IMAGING TABLE — X-RAY · DEXA · CT · MRI

🩻 CLINICAL USES & LIMITATIONS

🧪 SPECIAL PROCEDURES (EXAM FAVOURITES)

Hysterosalpingography (HSG) — X-ray based

DEXA — Scores (VERY HIGH YIELD)

🧠 CT vs MRI — EXAM LOCK COMPARISON

🤰 MRI IN PREGNANCY — GOLD POINTS

⚠️ MRI DRAWBACKS (NEVER MISS)

🧠 ONE-LINE EXAM REFLEX BLOCK

• X-ray / CT → ionising
• DEXA → bone density, T-score
• CT → fast, high-resolution, radiation
• MRI → best soft tissue, no radiation, metal contraindicated

Electrosurgery

1️⃣ What Electrosurgery Actually Is (Core Concept)

A diathermy machine converts normal main electricity

(230 V, 50 Hz) into high-frequency radiofrequency current

(200 kHz – 3.3 MHz).

👉 High frequency = heat without nerve or muscle stimulation

(because ions cannot move fast enough to depolarise nerves).

This single concept appears in almost every exam.

Electrosurgery / Diathermy – Active Recall (Fill in the blanks)

1. A diathermy machine converts normal main electricity (_____ V, _____ Hz) into high-frequency radiofrequency current.
2. The normal mains electricity is (_____ V, _____ Hz).
3. The output current produced by a diathermy machine is radiofrequency current of _____ kHz – _____ MHz.
4. Electrosurgery uses _____ frequency current.
5. High frequency causes _____ production without _____ or _____ stimulation.
6. High frequency means heat is produced without stimulation of _____ or _____.
7. The reason high frequency does not stimulate nerves is because _____ cannot move fast enough to depolarise _____.
8. In electrosurgery, ions cannot move fast enough to cause _____.
9. The frequency range used in electrosurgery is _____ kHz to _____ MHz.
10. Core concept: High frequency = _____ without nerve/muscle stimulation.

2️⃣ Why High Frequency Produces Heat (Must Understand)

• Low frequency → ions move → depolarisation → muscle contraction, tetany.
• High frequency → ions cannot move → no stimulation, but

collisions = heat → cutting, coagulation, vaporisation.

👉 Electrosurgery works by heating tissues, not by electrical shock.

Why High Frequency Produces Heat – Fill in the Blanks (Active Recall)

1. Low frequency current causes ions to _____ → leading to _____ → causing muscle _____ and _____.
2. Low frequency → ions move → depolarisation → _____ contraction and _____.
3. In high frequency current, ions _____ move → therefore there is _____ stimulation.
4. High frequency → ions cannot move → _____ stimulation.
5. Even though there is no stimulation in high frequency, tissue heating occurs due to _____.
6. Collisions produce _____.
7. Heat produced by electrosurgery can cause _____, _____, and _____.
8. Electrosurgery achieves its effect mainly by heating _____.
9. Electrosurgery does not work by giving an electrical _____.
10. Core concept: Electrosurgery works by _____ tissues, not by electrical _____.

3️⃣ Monopolar vs Bipolar (Extremely High Yield)

Monopolar

• Current flows from active electrode → patient → return plate → machine.
• Small tip = high current density = more heat.
• Can cut and coagulate.

⚠️ Risks: burns if plate poorly applied, capacitive coupling, insulation failure.

Bipolar

• Current passes only between forceps tips.
• Tissue between blades conducts current.
• Only coagulation, no cutting.
• Much safer (no return plate).

👉 Exam rule: Bipolar safer than monopolar.

Monopolar vs Bipolar – Fill in the Blanks (Active Recall)

Monopolar

1. In monopolar diathermy, current flows from the _____ electrode → _____ → _____ plate → _____.
2. The current pathway in monopolar is: active electrode → _____ → return plate → _____.
3. In monopolar, a small tip produces high current _____ leading to more _____.
4. Small tip = high current density = more _____.
5. Monopolar can be used for both _____ and _____.
6. A major risk in monopolar is _____ if the return plate is poorly applied.
7. Two other monopolar hazards are capacitive _____ and _____ failure.
8. Monopolar requires a return _____.

Bipolar

1. In bipolar diathermy, current passes only between the _____ tips.
2. In bipolar, the tissue between the _____ conducts current.
3. Bipolar is mainly used for _____ only (not cutting).
4. Bipolar does not require a return _____.
5. Bipolar is much _____ than monopolar.

Exam Rule

1. Exam rule: Bipolar is _____ than monopolar.

4️⃣ Waveforms = Tissue Effects (Exam Favourite)

Cutting current

• Continuous, unmodulated sine wave
• Low peak voltage
• 100% duty cycle
• Produces electric arcs
• Causes rapid intracellular heating → steam formation → cell vaporisation
• Results in a clean, precise cut
• Minimal lateral heat spread
• ❌ Poor haemostasis
• ❌ No carbonisation
• Depth of injury: very shallow
• Typical use: skin incision, sharp dissection

Cutting Current – Fill in the Blanks (Active Recall)

1. Cutting current is _____ and unmodulated _____ wave.
2. Cutting current has _____ peak voltage.
3. Cutting current has _____% duty cycle.
4. Cutting current produces electric _____.
5. Mechanism: rapid intracellular _____ → _____ formation → cell _____.
6. Cutting results in a clean, precise _____.
7. Cutting current produces minimal lateral heat _____.
8. Cutting current has poor _____.
9. Cutting current causes _____ carbonisation.
10. Depth of injury with cutting current is very _____.
11. Typical uses of cutting current include skin _____ and sharp _____.

Coagulation current

• High peak voltage
• Intermittent waveform (≈ 6% duty cycle)
• Causes slower tissue heating
• Leads to protein denaturation + tissue drying
• Results in coagulation → haemostasis
• ❌ No vaporisation
• ± Minimal carbonisation
• Depth of injury: moderate, controlled
• Typical use: control of bleeding vessels

Coagulation Current – Fill in the Blanks (Active Recall)

1. Coagulation current has _____ peak voltage.
2. Coagulation uses an _____ waveform.
3. Duty cycle in coagulation current is approximately _____%.
4. Coagulation current causes _____ tissue heating.
5. Mechanism: leads to protein _____ + tissue _____.
6. The final result is _____ → _____.
7. Coagulation current causes _____ vaporisation.
8. Carbonisation in coagulation is _____ / minimal.
9. Depth of injury with coagulation current is _____ and controlled.
10. Typical use: control of bleeding _____.

Desiccation / Soft coagulation

• Direct contact between electrode and tissue
• Tissue heated to 70–100°C
• Heat generated by tissue resistance (resistive heating)
• Causes cellular dehydration
• Produces coagulation without sparks
• ❌ Minimal carbonisation
• Depth of injury: localized, contact-dependent
• Typical use: small vessels, tissue drying near vital structures

Desiccation / Soft Coagulation – Fill in the Blanks (Active Recall)

1. Desiccation involves _____ contact between electrode and tissue.
2. In desiccation, tissue is heated to – °C.
3. Heat is generated mainly by tissue _____ (_____ heating).
4. Desiccation causes cellular _____.
5. It produces coagulation without _____.
6. Carbonisation is _____.
7. Depth of injury is _____ and depends on _____.
8. Typical use: coagulation of _____ vessels.
9. Desiccation can be used for tissue drying near _____ structures.

Blended waveform

• Combination of cut + coag
• Alternates continuous and intermittent currents
• Produces tissue vaporisation with simultaneous haemostasis
• Depth of injury: intermediate
• Typical use: incision where bleeding control is required

Blended Waveform – Fill in the Blanks (Active Recall)

1. Blended waveform is a combination of _____ + _____.
2. It alternates between _____ and _____ currents.
3. Blended waveform produces tissue _____ with simultaneous _____.
4. Depth of injury in blended waveform is _____.
5. Typical use: _____ where _____ control is required.

Fulguration

• High power, high voltage
• Electrode does NOT touch tissue
• Electrical sparks jump through air
• Causes superficial tissue charring
• Produces wide-area haemostasis
• ✅ Marked carbonisation
• Depth of injury: superficial but broad
• Typical use: diffuse oozing surfaces, tumour beds

Fulguration – Fill in the Blanks (Active Recall)

1. Fulguration uses _____ power and _____ voltage.
2. In fulguration, the electrode does _____ touch the tissue.
3. Electrical _____ jump through _____.
4. Fulguration causes superficial tissue _____.
5. It produces _____-area haemostasis.
6. Carbonisation is _____.
7. Depth of injury is _____ but _____.
8. Typical use: diffuse _____ surfaces.
9. Another typical use is _____ beds.

🔥 Comparative High-Yield Add-Ons (Exam Traps)

Haemostasis (least → most)

Cut < Blend < Desiccation < Coagulation < Fulguration

Carbonisation

• Cut → ❌ None
• Coagulation → ± Minimal
• Desiccation → ❌ Minimal
• Fulguration → ✅ Marked

Thermal Precision

• Most precise: Cutting
• Most controlled coagulation: Soft coagulation
• Widest tissue effect: Fulguration

🧠 Master Memory Lock

“Cut vaporises, Coag cooks, Desiccation dries, Fulguration burns.”

5️⃣ Grounded vs Isolated Systems (High-Yield Safety)

Grounded (old) systems

• Return path = earth.
• If patient plate fails → current may return through skin → severe burns.

Isolated systems (modern)

• Circuit isolated from earth.
• Much safer – current cannot travel into earth.

Grounded vs Isolated Systems – Fill in the Blanks (Active Recall)

Grounded (old) systems

1. In grounded systems, the return path is the _____.
2. Grounded systems are considered _____ (old/modern).
3. If the patient plate fails in a grounded system, current may return through the _____.
4. This can cause severe _____.

Isolated systems (modern)

1. In isolated systems, the circuit is isolated from _____.
2. Isolated systems are considered _____ (old/modern).
3. Isolated systems are much _____.
4. In isolated systems, current cannot travel into the _____.

6️⃣ Safety Hazards You Must Know

1️⃣ Direct coupling

• Active electrode contacts another metal instrument
• Electrical current travels along the second instrument
• Heat delivered away from the intended surgical field
• Causes unexpected burns to tissue or organs

When it happens

• Laparoscopic instruments touching
• Activation while another metal tool is in contact

Prevention

• ❌ Never activate electrode while touching another instrument
• Maintain visual control of active electrode
• Separate instruments before activation

Direct Coupling – Fill in the Blanks (Active Recall)

1. Direct coupling occurs when the active electrode contacts another _____ instrument.
2. In direct coupling, electrical current travels along the _____ instrument.
3. Heat is delivered away from the intended surgical _____.
4. This can cause unexpected _____ to tissues or organs.

When it happens

1. Direct coupling commonly occurs when laparoscopic instruments are _____.
2. It can occur if activation happens while another _____ tool is in contact.

Prevention

1. Prevention rule: Never _____ the electrode while touching another instrument.
2. Always maintain _____ control of the active electrode.
3. Instruments should be _____ before activation.

2️⃣ Insulation failure

• Cracks, breaks, or wear in electrode insulation
• Electrical current leaks through damaged insulation
• Causes stray thermal injury
• Burn may be deep and unrecognised intraoperatively

High-risk situations

• Repeated instrument use
• Reprocessing damage
• Laparoscopic surgery (hidden injuries)

Prevention

• Inspect insulation before use
• Replace damaged electrodes
• Use active electrode monitoring (AEM) systems

Insulation Failure – Fill in the Blanks (Active Recall)

1. Insulation failure occurs due to _____, _____, or _____ in electrode insulation.
2. In insulation failure, electrical current _____ through damaged insulation.
3. This results in _____ thermal injury.
4. The burn may be _____ and _____ intraoperatively.

High-risk situations

1. Risk increases with repeated instrument _____.
2. Risk increases with _____ damage.
3. Insulation failure is especially dangerous in _____ surgery because injuries can be _____.

Prevention

1. Prevention: Inspect _____ before use.
2. Replace _____ electrodes.
3. Use active electrode monitoring systems (_____).

3️⃣ Capacitive coupling ⭐ (classic exam favourite)

• Active electrode is insulated
• Passed inside a metal cannula
• Electrical energy transfers through insulation
• Current accumulates in the cannula
• Causes unexpected tissue burns when discharged

Key feature

• ❌ No direct contact required
• Occurs even with intact insulation

Capacitive Coupling – Fill in the Blanks (Active Recall)

1. In capacitive coupling, the active electrode is _____.
2. The insulated electrode is passed inside a metal _____.
3. Electrical energy transfers through the _____.
4. The current then accumulates in the _____.
5. This can cause unexpected tissue _____ when discharged.

Key feature

1. Capacitive coupling requires _____ direct contact.
2. Capacitive coupling can occur even with _____ insulation.

🛑 How to Prevent Capacitive Coupling

• Use all-metal cannula systems
• OR use all-plastic cannula systems
• Avoid hybrid metal–plastic combinations
• Use active electrode monitoring systems
• Never activate the electrode unless the tip is clearly visible
• Avoid activating near other instruments

How to Prevent Capacitive Coupling – Fill in the Blanks (Active Recall)

1. To prevent capacitive coupling, use _____-metal cannula systems.
2. Alternatively, use _____-plastic cannula systems.
3. Avoid _____ metal–plastic combinations.
4. Use active electrode _____ systems.
5. Never _____ the electrode unless the tip is clearly _____.
6. Avoid activating near other _____.

🧠 One-Line Examiner Memory Lock

• Direct coupling → metal touching metal
• Insulation failure → cracked coating
• Capacitive coupling → hidden energy transfer

🎯 Ultra-High-Yield Exam Sentence

Most laparoscopic electrosurgical injuries occur due to insulation failure or capacitive coupling and are often unrecognised at the time of surgery.

7️⃣ Practical Safety Rules (Always Tested)

• Use lowest effective power (e.g., 30 W).
• Activate electrode only when needed.
• Do NOT activate when touching metal.
• Use well-applied, large return plate (>69 cm²).
• Check isolation before surgery.
• Bipolar safer than monopolar.
• Monitor for low-frequency leaks → modern machines do this.

🔥 ELECTROSURGERY — MASTER HIGH-YIELD TABLES (ZERO OMISSION)

1️⃣ Core Concept & Physics

2️⃣ Frequency vs Tissue Effect (Mechanism Logic)

3️⃣ Monopolar vs Bipolar Diathermy

Exam Rule

4️⃣ Waveforms → Tissue Effects (Exam Favourite)

5️⃣ Comparative Exam Traps

Haemostasis (Least → Most)

Order

Cut < Blend < Desiccation < Coagulation < Fulguration

Carbonisation

Thermal Precision

6️⃣ Grounded vs Isolated Systems

Grounded (Old)

Isolated (Modern)

7️⃣ Electrosurgical Hazards (Must Know)

A. Direct Coupling

B. Insulation Failure

C. Capacitive Coupling (Exam Favourite)

unintended transfer of electrical energy from an active electrode to nearby conductive objects through intact insulation, due to a capacitor effect.

Prevention of Capacitive Coupling

Measure

All-metal cannula OR all-plastic cannula

Avoid metal–plastic hybrids

Active electrode monitoring

Activate only when tip visible

Avoid activating near other instruments

8️⃣ Practical Safety Rules (Always Tested)

Rule

Use

lowest effective power

Activate only when needed

Do not activate when touching metal

Large, well-applied return plate (>69 cm²)

Check isolation

Bipolar safer than monopolar

Modern machines monitor low-frequency leaks

🧠 Final Examiner Memory Lock

“Cut vaporises, Coag cooks, Desiccation dries, Fulguration burns.”

LASERS IN ENDOSCOPIC SURGERY

1. What a laser actually is (core idea)

LASER = Light Amplification by Stimulated Emission of Radiation

In OG surgery, lasers are used to:

• Vaporise tissue
• Cut/dissect tissue
• Coagulate (hemostasis)

Bonus effects:

• Sterilises (kills bacteria/viruses/fungi)
• Seals nerve endings → ↓ postoperative pain

Most surgical lasers used are Class 4 → can cause fire, burns, eye damage.

2. CO₂ LASER – Key Properties & Uses

• Type: Gas laser
• Wavelength: 10,600 nm (far infrared)
• Strongly absorbed by water → therefore strongly absorbed in soft tissue
• Main effects:
• Very precise cutting & dissection
• Minimal lateral thermal damage
• Good for superficial vaporisation – e.g. endometriosis near ureter

Limitations:

• Beam delivery via articulated arms + mirrors + semi-flexible fibre

→ Expensive, hard to clean

• Invisible beam → need visible aiming beam
• Shallow penetration → not suitable for laparoscopy (depth too limited)

👉 Think: CO₂ = shallow, precise, water-loving, “surface sculptor”.

3. Nd:YAG LASER – Deep & Powerful

• Type: Solid-state
• Medium: Neodymium-doped Yttrium Aluminium Garnet
• Wavelength: 1064 nm (infrared)
• Can travel down fibreoptic cable → convenient for endoscopy

Effects:

• Cutting + coagulation + vaporisation
• Deeper penetration, especially with bare quartz fibre
• Good for hysteroscopic surgery
• Dangerous in laparoscopy if not focused properly → deep unintended burns

In laparoscopy, we focus the beam at the tip (small focal spot size) to limit penetration to approx 0.2–1 mm.

4. KTP LASER – “Half the Wavelength, Middle Behaviour”

• Nd:YAG beam passed through KTP crystal → frequency doubled, wavelength halved
• Wavelength: 532 nm (visible green light)
• Tissue penetration: about 1–2 mm
• Delivered through flexible fibreoptic cable

Advantages:

• Visible beam = easier aiming
• Can cut, coagulate, vaporise
• Uses single, bare, reusable quartz fibre

Overall position:

KTP = between CO₂ and Nd:YAG in behaviour (depth & effect).

5. Key Laser Safety Points (high-yield)

• Class 4 → risk of fire, burns, retinal injury
• Staff and patient need protective goggles (Nd:YAG/KTP especially – retinal hazard)
• Beam must be confined and controlled
• Good smoke evacuation needed (not in text, but clinically relevant)

LASERS IN ENDOSCOPIC SURGERY — COMPARATIVE MASTER TABLE

Universal Laser Safety (Applies to ALL Above)

One-line exam locks

• CO₂ → water-absorbed, shallow, ultra-precise
• Nd:YAG → deep penetration, powerful, risky if unfocused
• KTP → visible green, intermediate depth, versatile

🟣 PART 2 – PRINCIPLES & USE OF RADIOTHERAPY

1. What radiotherapy is and what it does

• Radiotherapy = use of ionising radiation to treat disease, mainly cancer
• Dose measured in Gray (Gy)
• Limited use in benign disease due to cancer risk

Main mechanism: DNA damage

• Direct: radiation hits DNA
• Indirect (most important):
• Radiation ionises water (H₂O) → produces H⁺, OH⁻, free radicals (H·, OH·)
• These free radicals damage DNA
• Leads to loss of reproductive integrity → cell death

Radiation also affects:

• Proliferation
• Senescence
• Apoptosis
• DNA repair capacity

2. Factors that influence cell killing

Cell survival after radiation depends on:

• Dose
• Position in cell cycle (some phases more sensitive)
• Oxygen tension
• Intrinsic radiosensitivity
• Cellular environment

Hypoxia = radioresistant

• Oxygen fixes DNA damage → makes it permanent
• Without oxygen, damage is more reversible

Fractionation (spreading dose out over days) helps:

• Exploit differences in repair and repopulation
• Improve tumour control with less normal tissue damage
• Also improves tissue oxygenation over time

3. Proton vs Photon (classic exam comparison)

• Photons (X-rays): deposit energy along path → more dose to normal tissue beyond tumour.
• Protons:
• Release most of their energy at a precise depth (Bragg peak)
• Rapid fall-off beyond tumour → less damage to normal tissues
• Allows higher dose to tumour with fewer side-effects.

4. Roles of Radiotherapy in Cancer Treatment

You must know these words:

1. Primary therapy
• Radiotherapy alone aimed to cure or palliate.
2. Adjuvant therapy
• Given after surgery
• Aim: reduce recurrence risk, improve survival
• Many patients might already be cured by surgery; RT is “insurance”.
3. Neoadjuvant therapy
• Given before main treatment (usually surgery)
• Aim: shrink tumour → easier/more effective surgery.
4. Concomitant / Concurrent therapy
• Radiotherapy given at the same time as chemo / hormonal therapy.

These labels come up all the time in exam stems.

5. How Radiotherapy is Delivered

A. External-beam radiotherapy

• Delivered via linear accelerator (linac)
• Beams directed from 2–4 (or more) directions

Conventional RT:

• Use simulator (X-ray unit with same geometry)
• Simulate treatment fields

3D Conformal RT (3D CRT):

• Uses CT/MRI volumes
• Beam shape matched to tumour (conformal)
• Reduces normal tissue toxicity → allows higher tumour dose

IMRT (Intensity-Modulated RT):

• Next generation of 3D CRT
• Beam intensity varies within each field
• Allows extremely precise dose sculpting around organs at risk.

4D / Adaptive RT (future-oriented but examinable):

• Accounts for:
• Tumour movement (e.g. breathing)
• Shrinkage over time
• Uses real-time imaging + beam adjustment
• Aim: match actual daily delivered dose to tumour shape/position.

B. Internal-beam radiotherapy

Two types:

1. Sealed-source → Brachytherapy
2. Unsealed-source → Radioisotope therapy

1. Brachytherapy (very important in Gynae)

• Radioactive source placed in or near tumour.
• Advantages:
• Very short distance
• Less normal tissue irradiated
• Lower photon energy → better local control
• Used for many gynaecological cancers:
• Cervix
• Vagina
• Uterus
• Ovary (selected cases)

Common isotopes:

• Caesium-137 (¹³⁷Cs)
• Iridium-192 (¹⁹²Ir)
• Iodine-125 (¹²⁵I) – permanent seeds (activity decays over weeks–months, then biologically inert).

2. Radioisotope therapy (unsealed)

• Radioactive substance given by:
• IV (e.g. ¹³¹I, ¹⁷⁷Lu, ⁹⁰Y)
• Oral (e.g. iodine-131 for thyroid)

Examples:

• I-131 → thyroid cancer, thyrotoxicosis
• ¹⁷⁷Lu / ⁹⁰Y-labeled hormones → neuroendocrine tumours (peptide receptor radionuclide therapy)
• Radioactive spheres via hepatic artery → radioembolisation of liver tumours
• Radioimmunotherapy: monoclonal antibody against CD20 + ¹³¹I / ⁹⁰Y for refractory Non-Hodgkin lymphoma

6. Side-Effects of Radiotherapy (Must-Know List)

Acute effects (during/soon after RT)

• Damage to epithelial surfaces (skin, mucosa, bowel)
• Oedema & inflammatory swelling (may improve with steroids)
• Infertility (gonadal damage)
• Fatigue

Medium & long-term effects

• Fibrosis & scarring of irradiated tissue
• Hair loss in treated area
• Dryness (salivary glands, lacrimal glands, vaginal dryness)
• Persistent fatigue
• Secondary malignancies
• In extreme cases, death

Re-irradiation:

• Higher risk of worsening existing damage
• Requires very careful monitoring.

🧠 30-Second Super Summary

• Laser = light-based; used to cut, coagulate, vaporise with some sterilising & analgesic benefits.
• CO₂ laser: 10,600 nm, water-absorbed, very superficial, precise; not ideal for laparoscopy.
• Nd:YAG: 1064 nm, deeper, fibreoptic, powerful; good for hysteroscopy but risky in laparoscopy unless tightly focused.
• KTP: 532 nm, visible green, moderate penetration (1–2 mm), flexible fibres; cuts + coagulates.
• Radiotherapy = ionising radiation → DNA damage (mostly via water + free radicals).
• Oxygen makes DNA damage permanent → hypoxic tissue = radioresistant.
• Protons deposit dose at tumour (Bragg peak) → spare normal tissue.
• Roles: primary, adjuvant, neoadjuvant, concurrent.
• External-beam: Conventional → 3D CRT → IMRT → 4D adaptive RT.
• Internal: brachytherapy (sealed) & radioisotope therapy (unsealed).
• Side-effects: acute (mucosa, oedema, infertility, fatigue) + late (fibrosis, dryness, secondary cancers).

Radiotherapy — Principles, Use, Techniques & Effects (Master Table)

PULSE oximeter

🔴 What is a Pulse Oximeter?

A pulse oximeter is a non-invasive device that measures:

1. Oxygen saturation of arterial blood (SpO₂)
2. Pulse rate (heart rate)

👉 It does NOT measure PaO₂ directly.

🔴 What does SpO₂ mean?

• SpO₂ = % of haemoglobin saturated with oxygen
• Normal adult range:
• 95–100% (room air, sea level)
• <90% = hypoxaemia (clinically significant)

🔴 Principle (VERY EXAM-FAVOURITE)

🔑 Based on Beer–Lambert Law

Amount of light absorbed ∝ concentration of absorbing substance

Uses two wavelengths of light

🔴 How does it work? (Step-by-step logic)

1. Probe emits red + infrared light
2. Light passes through pulsatile arterial blood
3. Sensor detects transmitted light
4. Device isolates arterial pulsation

(subtracts venous + tissue absorption)

5. Calculates ratio of absorption
6. Converts ratio → SpO₂ value using internal algorithm

📌 Key point:

It only works because arterial blood is pulsatile

🔴 What exactly does it measure?

✔ Arterial oxygen saturation

✔ Pulse rate

❌ NOT PaO₂

❌ NOT ventilation

❌ NOT CO₂ levels

❌ NOT haemoglobin concentration

🔴 Components

• Light-emitting diodes (LEDs)
• Photodetector
• Microprocessor
• Display screen

🔴 Types of probes

🔴 Plethysmographic waveform (clinically important)

• Represents arterial pulse
• Helps assess:
• Signal quality
• Peripheral perfusion
• Arrhythmias
• Volume status (rough idea)

📌 Flat or poor waveform → unreliable reading

🔴 Indications (exam + ward)

• Monitoring hypoxia
• Anaesthesia & recovery
• ICU / HDU monitoring
• COPD / pneumonia / asthma
• COVID-19 monitoring
• Post-operative care
• Labour & obstetric emergencies
• Neonatal monitoring

🔴 Limitations & causes of false readings ⚠️

🔻 Falsely LOW SpO₂

• Poor perfusion (shock, cold extremities)
• Motion artefact
• Nail polish (especially dark colours)
• Severe anaemia (signal weak)
• Venous congestion

🔺 Falsely NORMAL / HIGH SpO₂ (DANGEROUS)

📌 CO poisoning → SpO₂ may look normal

🔴 Important haemoglobin variants

🔴 Oxygen–Haemoglobin dissociation curve relevance

• SpO₂ reflects plateau region
• Large ↓ PaO₂ may cause small change in SpO₂
• Once SpO₂ <90%, PaO₂ falls rapidly

📌 Hence:

Normal SpO₂ ≠ normal oxygen delivery

🔴 Factors NOT detected by pulse oximeter

❌ Hypercapnia (↑CO₂)

❌ Hypoventilation

❌ Acidosis

❌ Anaemia severity

❌ Tissue hypoxia

👉 A patient can be pink, SpO₂ normal, but hypoxic at tissue level

🔴 Clinical pearls (EXAM GOLD 💎)

• SpO₂ is an estimate, not a direct measurement
• Always correlate with:
• Clinical condition
• ABG if sick
• In CO poisoning → rely on co-oximetry
• Poor waveform = unreliable number
• Pulse oximeter measures saturation, not oxygen content

🔴 One-line exam answers

• Principle: Beer–Lambert law
• Wavelengths: Red 660 nm, Infrared 940 nm
• Measures: SpO₂ + pulse rate
• Cannot detect: CO₂, PaO₂, anaemia
• Fails in: CO poisoning, poor perfusion