Viva · Specialty
Nuclear Medicine Viva Questions
Oral-exam style questions and answers on nuclear medicine for MD, DNB and super-specialty practical examinations. Each answer links back to the playbook or Library entry it was written from.
- How does a triple-phase bone scan differentiate osteomyelitis from cellulitis?
- Osteomyelitis shows increased uptake in flow, blood pool, and delayed phases, whereas cellulitis shows increased flow and blood pool but normal delayed uptake.
- What are the diuretic renography (MAG3) T½ thresholds for interpreting urinary tract obstruction?
- T½ less than 10 minutes indicates no obstruction, greater than 20 minutes suggests obstruction, and 10 to 20 minutes is equivocal.
- What is a 'superscan' on bone scintigraphy and what is a classic clinical cause?
- Diffuse, symmetrically increased skeletal uptake with absent renal and soft-tissue activity, classically caused by widespread osteoblastic metastases such as prostate cancer.
- Why is Tc-99m MAG3 preferred over DTPA for renography in patients with renal impairment?
- MAG3 is cleared by tubular secretion, providing adequate image quality and clearance data when GFR is low, unlike DTPA which relies on glomerular filtration.
- Why is PET more sensitive than SPECT?
- PET uses electronic collimation via coincidence detection, which does not reject the vast majority of photons as physical collimators do in SPECT. This gives PET approximately 10-20 times higher sensitivity.
- What limits PET spatial resolution?
- PET resolution is limited by positron range before annihilation, non-collinearity of the two 511 keV photons (approximately 0.5 degrees deviation from 180 degrees), and detector crystal element size. Clinical systems achieve 4-6 mm FWHM.
- What happens if you image I-131 with a low-energy collimator?
- Severe septal penetration occurs because I-131 emits 364 keV photons that penetrate the thin septa of a low-energy collimator, producing a star artefact and degrading image quality. A high-energy collimator with thicker septa is required.
- How does time-of-flight PET improve image quality?
- TOF PET measures the small arrival-time difference between the two annihilation photons, localising the event along the line of response rather than uniformly distributing it. This improves signal-to-noise ratio, particularly beneficial in large patients.
- What is the partial volume effect and why does it matter clinically?
- Lesions smaller than twice the system resolution (approximately 8-12 mm for PET) have their activity spread over a larger volume, causing underestimation of true SUV. This can lead to under-staging of small lesions or underestimation of treatment response.
- What does the Krenning score measure?
- It is a visual five-point grading of tumour somatostatin receptor uptake on somatostatin receptor scintigraphy, comparing lesion uptake with normal liver and with spleen or kidney.
- List the grades of the Krenning score.
- Grade 0 no uptake; grade 1 very low uptake below liver; grade 2 uptake equal to or less than normal liver; grade 3 uptake greater than normal liver; grade 4 uptake greater than normal spleen or kidney.
- Which reference organs are used, and where do they change?
- Normal liver is the reference for grades 1 to 3; spleen or kidney becomes the reference only at grade 4.
- How was the score used in NETTER-1?
- NETTER-1 required tumour uptake at least equal to normal liver on somatostatin receptor scintigraphy, that is Krenning grade 2 or higher, as an enrolment criterion for 177Lu-DOTATATE.
- Why is the score problematic when applied to 68Ga-DOTATATE PET/CT?
- The scale was derived from 111In-pentetreotide planar and SPECT imaging; PET has different tracer biodistribution, resolution and lesion-to-background contrast, so grades read from PET are an extrapolation and are not a validated eligibility criterion.
- What should be reported when lesions differ in uptake?
- Uptake heterogeneity should be stated explicitly, with the lowest-uptake and any FDG-avid or non-expressing lesions described separately, because a single index lesion grade can overestimate suitability for receptor-targeted therapy.
- What is the biochemical basis of FDG trapping in cells?
- FDG is transported by GLUT transporters and phosphorylated by hexokinase to FDG-6-phosphate, which cannot be further metabolised by glycolysis or dephosphorylated efficiently, resulting in intracellular trapping proportional to glucose metabolism.
- How does the Deauville 5-point scale work?
- Score 1 is no uptake, 2 is uptake at or below mediastinal blood pool, 3 is uptake above blood pool but at or below liver, 4 is moderately increased uptake above liver, and 5 is markedly increased uptake or new lesions. Scores 1-3 are generally considered complete metabolic response.
- Why might PET/MRI underestimate SUV near metallic implants?
- PET/MRI uses segmentation-based attenuation correction rather than CT-derived mu-maps. Susceptibility artefacts near metal cause signal loss on MRI, leading to underestimation of tissue attenuation and consequently underestimated SUV values.
- What is the minimum recommended interval between chemotherapy completion and FDG-PET for response assessment?
- At least 6 to 8 weeks after the last chemotherapy cycle, and at least 12 weeks after completion of radiotherapy, to reduce false-positive inflammatory uptake.
- Name three causes of false-negative FDG-PET.
- Low-grade tumours such as typical carcinoid or lepidic adenocarcinoma, mucinous neoplasms with low cellularity, and lesions below the spatial resolution threshold of approximately 7 millimetres.
- What is the theranostic principle?
- The theranostic principle uses the same molecular target with a diagnostic radionuclide for patient selection and dosimetry, and a matched therapeutic radionuclide for targeted treatment, enabling personalised radionuclide therapy.
- Compare alpha and beta emitters used in radionuclide therapy.
- Alpha emitters such as Ra-223 and Ac-225 deliver high linear energy transfer over 50-100 micrometres, causing dense DNA damage with minimal collateral injury. Beta emitters such as Lu-177 and I-131 have longer ranges of 1-11 millimetres, providing a cross-fire effect useful for heterogeneous tumour uptake.
- Why is amino acid infusion given during Lu-177 DOTATATE therapy?
- Positively charged amino acids (lysine and arginine) competitively inhibit renal tubular reabsorption of the radiolabelled peptide, reducing radiation dose to the renal cortex and protecting against nephrotoxicity.
- What trial established Lu-177 PSMA-617 for metastatic castration-resistant prostate cancer?
- The VISION trial demonstrated improved overall survival and radiographic progression-free survival with Lu-177 PSMA-617 plus best standard of care versus best standard of care alone in PSMA-positive mCRPC after prior ARPI and taxane therapy.
- Why is I-123 preferred over I-131 for diagnostic thyroid imaging?
- I-123 emits a 159 keV gamma photon ideal for imaging with no significant beta emission, providing superior image quality and substantially lower radiation dose. I-131 has higher energy gamma emission and beta particles, making it less suitable for pure diagnostic imaging.