MBBS Radiology Flashcards

24 sample cards from the Radiology deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. Define the Hounsfield unit (HU) scale, giving the reference values for water and air.

   The Hounsfield unit is a normalized measure of X-ray attenuation: $$HU = 1000 \times \frac{\mu_{tissue} - \mu_{water}}{\mu_{water}}$$ By definition, water $= 0\ \text{HU}$ and air $= -1000\ \text{HU}$. Dense bone is around $+1000\ \text{HU}$ or higher.

2. Give the approximate Hounsfield unit values for fat, acute blood, and bone on CT.

   Fat: about $-100$ to $-50\ \text{HU}$. Acute (clotted) blood: about $+50$ to $+80\ \text{HU}$ (hyperdense). Cortical bone: about $+1000\ \text{HU}$ or higher. Simple fluid/water is near $0\ \text{HU}$.

3. What are window width and window level (window centre) in CT image display?

   Window level (centre) is the central HU value displayed as mid-grey; window width is the range of HU values mapped across the grey scale. Narrow windows increase contrast over a small HU range (e.g., brain window), wide windows show a broad range (e.g., lung or bone window).

4. In CT of the head for acute stroke, how do acute haemorrhage and acute infarction appear?

   Acute haemorrhage appears hyperdense (bright/white, ~$50$–$80\ \text{HU}$) immediately. Acute ischaemic infarction appears as subtle hypodensity (dark) with loss of grey–white differentiation, often not obvious in the first few hours.

5. Differentiate the CT appearance and shape of an epidural versus a subdural haematoma.

   Epidural (extradural) haematoma: biconvex/lentiform (lens) shape, does not cross suture lines, often from middle meningeal artery. Subdural haematoma: crescent (concavoconvex) shape, crosses suture lines but not dural reflections, from bridging veins.

6. What is the typical iodine-based contrast use in CT of the chest and abdomen, and one major risk?

   Intravenous iodinated contrast enhances vessels and organs (e.g., for CT pulmonary angiography or organ characterization). Major risks include contrast-induced nephropathy (renal impairment) and anaphylactoid (allergic) reactions.

7. On a CT pulmonary angiogram, what finding confirms a pulmonary embolism?

   An intraluminal filling defect within a pulmonary artery — a hypodense thrombus surrounded by contrast-opacified (bright) blood, sometimes producing a "polo-mint" or "railway track" sign.

8. What is the fundamental physical principle underlying MRI signal generation?

   Nuclear magnetic resonance: hydrogen protons (in water/fat) align with a strong static magnetic field $B_0$; a radiofrequency pulse tips them out of alignment; as they relax back they emit a radiofrequency signal that is spatially encoded by gradient coils to form an image.

9. Define T1 and T2 relaxation times in MRI.

   T1 (longitudinal/spin-lattice relaxation): time for ~63% recovery of longitudinal magnetization along $B_0$. T2 (transverse/spin-spin relaxation): time for transverse magnetization to decay to ~37% of its initial value due to dephasing.

10. On MRI, how does fluid (e.g., CSF) appear on T1-weighted versus T2-weighted images?

    On T1-weighted images CSF/fluid is dark (low signal/hypointense). On T2-weighted images CSF/fluid is bright (high signal/hyperintense). Mnemonic: "WW2" — Water is White on T2.

11. How does fat appear on standard T1- and T2-weighted MRI sequences?

    Fat is bright (hyperintense) on T1-weighted images and remains relatively bright on conventional (fast/turbo spin-echo) T2-weighted images. Fat-suppression sequences (e.g., STIR) are used to null fat signal.

12. What MRI contrast agent is most commonly used, and what is its primary safety concern?

    Gadolinium-based contrast agents (paramagnetic, shorten T1). The main concern is nephrogenic systemic fibrosis (NSF) in patients with severe renal impairment; gadolinium retention is also a consideration.

13. Why is MRI superior to CT for evaluating the brain and spinal cord and musculoskeletal soft tissues?

    MRI provides far superior soft-tissue contrast resolution, multiplanar imaging without ionizing radiation, and excellent differentiation of grey/white matter, ligaments, cartilage, marrow, and cord pathology that CT cannot resolve well.

14. In MRI of the brain, what does a DWI (diffusion-weighted imaging) sequence detect in acute stroke?

    DWI detects restricted diffusion of water in acutely ischaemic (cytotoxic oedema) tissue, appearing bright on DWI and dark on the corresponding ADC map. This is the earliest and most sensitive sign of acute infarction, positive within minutes.

15. List common absolute MRI contraindications.

    Non-MRI-conditional cardiac pacemakers/ICDs, certain cochlear implants, intraocular/intracranial ferromagnetic foreign bodies (e.g., metal shards near the eye), some aneurysm clips, and other ferromagnetic implanted devices. Each device must be checked for MR conditionality.

16. What is the fundamental physical principle of diagnostic ultrasound imaging?

    A piezoelectric transducer emits high-frequency sound waves (typically $2$–$15\ \text{MHz}$) into tissue; reflected echoes at tissue interfaces of differing acoustic impedance return to the transducer and are converted to an image based on echo amplitude and time-of-flight (depth).

17. Explain the trade-off between ultrasound frequency, resolution, and depth of penetration.

    Higher-frequency transducers give better spatial resolution but less tissue penetration (greater attenuation), so they are used for superficial structures. Lower-frequency transducers penetrate deeper (e.g., abdomen) but give lower resolution.

18. Define the ultrasound terms echogenic (hyperechoic), hypoechoic, and anechoic.

    Echogenic/hyperechoic: produces strong echoes, appears bright (e.g., bone, gas, calcification). Hypoechoic: fewer echoes, appears darker than surrounding tissue. Anechoic: no internal echoes, appears black (e.g., simple fluid/cyst, urine, bile).

19. What is posterior acoustic shadowing versus posterior acoustic enhancement on ultrasound?

    Acoustic shadowing: a dark band behind a strongly attenuating/reflecting structure (e.g., gallstone, calcification) that blocks sound. Acoustic enhancement: increased brightness behind a fluid-filled (low-attenuation) structure (e.g., cyst, bladder) because sound is less attenuated.

20. What is the Doppler effect as applied to ultrasound, and what is it used to assess?

    The Doppler effect is the frequency shift of reflected sound caused by moving reflectors (e.g., red blood cells). It is used to detect and quantify blood flow direction and velocity, e.g., in vessels and the heart, with colour and spectral Doppler.

21. On abdominal ultrasound, what features characterize a simple (benign) renal or hepatic cyst?

    Anechoic (black) content, thin/imperceptible smooth wall, well-defined back wall, and posterior acoustic enhancement. Absence of internal septations, solid components, or vascularity supports a benign simple cyst.

22. What is the typical ultrasound finding of acute cholecystitis?

    Gallstones with gallbladder wall thickening (greater than ~3 mm), pericholecystic fluid, and a positive sonographic Murphy sign (maximal tenderness when the transducer presses over the gallbladder). An echogenic stone casts a posterior acoustic shadow.

23. Why is ultrasound the preferred initial imaging modality in pregnancy and paediatrics?

    It uses no ionizing radiation, is real-time, portable, relatively inexpensive, and provides good soft-tissue and fluid characterization, making it safe for the fetus and children.

24. Rank CT, MRI, ultrasound, and plain radiography by ionizing radiation exposure.

    Highest to none: CT (highest radiation dose) > plain radiography (low dose) > ultrasound and MRI (no ionizing radiation). MRI uses magnetic fields/radiofrequency, and ultrasound uses sound waves, so neither involves ionizing radiation.

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