NEET PG 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. Why is a non-contrast CT head the first-line investigation in acute stroke?

   It rapidly excludes haemorrhage (which appears hyperdense/bright) before thrombolysis, and may show early ischaemic signs (loss of grey-white differentiation, hyperdense vessel sign, sulcal effacement). Acute infarct itself becomes hypodense over hours.

2. On a non-contrast CT head, how do acute blood, acute infarct, and CSF appear?

   Acute haemorrhage is hyperdense (bright, ~50-90 HU), acute infarction is hypodense (dark) with loss of grey-white differentiation, and CSF is hypodense (near water, ~0-15 HU).

3. What are the standard intravenous contrast phases in CT of the abdomen and what is each best for?

   Arterial phase (~25-35 s) for vascular/hypervascular lesions (e.g., HCC); portal venous phase (~60-70 s) for liver parenchyma/most metastases and solid organs; delayed/excretory phase (several minutes) for the urinary collecting system and washout assessment.

4. On what physical phenomenon is MRI based?

   Nuclear magnetic resonance of hydrogen protons: in a strong magnetic field protons align, a radiofrequency pulse tips them, and the signal emitted as they relax is spatially encoded by gradient coils to form the image.

5. Define T1 and T2 relaxation in MRI.

   T1 (longitudinal/spin-lattice) relaxation is the recovery of longitudinal magnetisation along the main field. T2 (transverse/spin-spin) relaxation is the decay of transverse magnetisation due to spin dephasing. Different tissues differ in T1 and T2, giving contrast.

6. How do fat and water/CSF appear on T1-weighted versus T2-weighted MRI?

   On T1: fat is bright, water/CSF is dark. On T2: water/CSF is bright, fat is intermediate-to-bright. Mnemonic: 'WW2' - Water is White on T2.

7. What is the main MRI contrast agent, how does it work, and a key safety concern?

   Gadolinium-based chelates shorten T1, causing enhancement (bright on T1) of vascular/leaky tissues. A key risk is nephrogenic systemic fibrosis in patients with severe renal impairment; gadolinium also crosses the placenta and is generally avoided in pregnancy.

8. What is diffusion-weighted imaging (DWI) and its key role in brain MRI?

   DWI measures the random (Brownian) motion of water. Acute ischaemic stroke shows restricted diffusion: bright on DWI and dark on the ADC map, making it the most sensitive sequence for early infarction (positive within minutes).

9. In MRI of the spine, what sequence best shows the cord and CSF, and how does a disc herniation appear?

   T2-weighted sagittal images show CSF bright, outlining the cord and any compression. A herniated/extruded disc appears as a focal protrusion of disc material indenting the thecal sac and compressing nerve roots or the cord.

10. Why is MRI the modality of choice for musculoskeletal soft-tissue injury (e.g., menisci, ligaments)?

    MRI provides superior soft-tissue contrast without ionising radiation, directly visualising cartilage, menisci, ligaments, tendons, marrow oedema (bright on fluid-sensitive/STIR sequences), and occult fractures not seen on plain film.

11. What is the STIR sequence used for in MSK MRI?

    Short Tau Inversion Recovery is a fat-suppression technique that makes fluid/oedema conspicuously bright while nulling fat signal. It is highly sensitive for bone marrow oedema, occult fractures, infection, and tumours.

12. On what physical principle does diagnostic ultrasound imaging work?

    A transducer emits high-frequency sound (~2-18 MHz) using the piezoelectric effect; echoes reflected at tissue interfaces (acoustic impedance differences) return to the transducer and are timed/processed into a real-time image. No ionising radiation.

13. What is the trade-off between high- and low-frequency ultrasound transducers?

    Higher frequencies give better spatial resolution but less penetration (good for superficial structures, e.g., thyroid). Lower frequencies penetrate deeper but with lower resolution (good for abdominal/obstetric imaging).

14. Define the ultrasound terms anechoic, hypoechoic, hyperechoic, and acoustic shadowing.

    Anechoic = no internal echoes/black (e.g., simple cyst, fluid). Hypoechoic = darker than surrounding tissue. Hyperechoic = brighter (e.g., gallstone, fat). Acoustic shadowing = dark band behind a strongly reflective/attenuating structure such as a stone or gas.

15. What is the classic ultrasound appearance and key sign of acute cholecystitis?

    Gallbladder wall thickening (>3 mm), pericholecystic fluid, gallstones, and a positive sonographic Murphy's sign (maximal tenderness when the probe is over the gallbladder).

16. What is the Doppler effect in ultrasound and its clinical use?

    A frequency shift of reflected sound caused by moving blood; the shift is proportional to flow velocity. It is used to assess blood flow direction and velocity (colour and spectral Doppler), e.g., carotid stenosis, DVT, fetal vessels.

17. Why is ultrasound the modality of choice in obstetrics, and what is measured to date a pregnancy in the first trimester?

    It is real-time, widely available, and uses no ionising radiation (safe for the fetus). In the first trimester, the crown-rump length (CRL) is the most accurate measurement for dating gestational age.

18. What is a radiopharmaceutical, and what is the most widely used radionuclide in nuclear medicine?

    A radiopharmaceutical is a radioactive isotope bound to a pharmaceutical carrier that targets a specific organ/process. Technetium-99m (Tc-99m) is the most widely used, due to its ideal 140 keV gamma energy and ~6-hour half-life; it is eluted from a molybdenum-99 generator.

19. How does a gamma camera (Anger camera) form an image?

    A collimator selects gamma photons travelling in a known direction; they strike a sodium iodide (NaI(Tl)) scintillation crystal producing light, which photomultiplier tubes convert and localise into a 2D projection map of radiotracer distribution.

20. How does SPECT differ from planar gamma-camera imaging?

    Single-Photon Emission CT rotates one or more gamma-camera heads around the patient to acquire multiple projections, then reconstructs tomographic (3D cross-sectional) images of tracer distribution, improving localisation and contrast over flat planar images.

21. What is the Seldinger technique used in image-guided vascular procedures?

    A method of gaining vascular/cavity access: puncture with a needle, pass a guidewire through it, remove the needle, then advance a catheter/sheath over the wire into the vessel. It underlies most angiographic and interventional procedures.

22. Name two minimally invasive image-guided therapies for tumours and how they work.

    Radiofrequency/microwave ablation uses image-guided needle electrodes to heat and coagulate tumour tissue. Transarterial chemoembolisation (TACE) delivers chemotherapy and embolic particles via a catheter into the tumour's feeding artery to cause ischaemia and local drug effect.

23. What is the radiobiological rationale for fractionation in radiation therapy (the four R's)?

    Splitting the total dose into fractions exploits the four R's: Repair of sublethal damage (favours normal tissue), Reassortment/redistribution into radiosensitive cell-cycle phases, Repopulation, and Reoxygenation of hypoxic tumour cells (oxygen enhances radiosensitivity).

24. What is the difference between external beam radiotherapy and brachytherapy, and what imaging is used for treatment planning?

    External beam (teletherapy) delivers radiation from a distant source such as a linear accelerator; brachytherapy places sealed radioactive sources within or next to the tumour. CT (often fused with MRI/PET) is used to define target volumes and organs at risk for dose planning.

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