Nuclear medicine is concept-dense and rewards focused revision — PET-CT, radiotracers, and theranostics recur in NEET SS and INI-SS. These free samples from the RadioQBank published set come with full explanations, exam pearls, and references.
Question 1 · Thyroid Scan · easy
A Tc-99m pertechnetate thyroid scan in a 35-year-old woman with hyperthyroidism shows diffusely enlarged thyroid with homogeneously increased uptake in both lobes. The 24-hour radioiodine uptake (RAIU) is 72%. Which diagnosis does this scan pattern most support?
- a. Toxic multinodular goitre
- b. Graves disease
- c. Subacute (De Quervain) thyroiditis
- d. Solitary toxic adenoma
Answer: B. Graves disease is an autoimmune condition in which TSH receptor antibodies (TRAb) mimic TSH, causing diffuse thyroid hyperplasia and autonomous stimulation. On thyroid scintigraphy, this produces a diffusely enlarged gland with homogeneously increased uptake throughout both lobes — the classic Graves pattern. RAIU is markedly elevated (typically >50%) because the entire gland is uniformly overstimulated. This distinguishes it from toxic multinodular goitre (patchy uptake with suppressed background) and from solitary adenoma (single hot nodule with suppressed remainder).
Exam pearl: Graves disease = diffuse homogeneous uptake + elevated RAIU; subacute thyroiditis = suppressed uptake + low RAIU.
Grainger & Allison's Diagnostic Radiology 7th ed, Chapter 73; AJR 2012;199:1170–1179
Question 2 · Theranostics — PSMA · easy
A 72-year-old man with castration-resistant prostate cancer (CRPC) and PSA 45 ng/mL undergoes 177Lu-PSMA-617 therapy. Which pre-treatment scan is MANDATORY to confirm target expression before therapy?
- a. 18F-FDG PET/CT
- b. 99mTc-MDP bone scan
- c. 68Ga-PSMA-11 or 18F-DCFPyL PET/CT
- d. 68Ga-DOTATATE PET/CT
Answer: C. 177Lu-PSMA-617 is a theranostic agent — the therapeutic lutetium-177 counterpart to the diagnostic 68Ga-PSMA tracer. The VISION trial (NEJM 2021) established that patients must demonstrate PSMA-positive disease (all metastatic lesions PSMA-avid exceeding liver background, with no PSMA-negative metastases on FDG) on pre-treatment 68Ga-PSMA-11 or 18F-DCFPyL PET/CT. This theranostic pair principle ensures the therapeutic agent reaches the target.
Exam pearl: Theranostic pair: 68Ga-PSMA (diagnose) + 177Lu-PSMA-617 (treat) — VISION trial mandates pre-therapy PSMA PET to confirm target expression before 177Lu therapy.
Sartor O et al. NEJM 2021 (VISION trial); J Nucl Med 2022
Question 3 · PET-CT Neurology · hard
A 70-year-old man with worsening Parkinson's disease and no dementia undergoes 18F-flortaucipir (AV-1451) tau-PET. Imaging shows high tracer retention in the entorhinal cortex and hippocampus with minimal involvement of association cortices. The Braak stage implied by this distribution is most likely:
- a. Braak stage I–II (entorhinal/transentorhinal) — early limbic involvement, compatible with normal cognition or MCI
- b. Braak stage V–VI (neocortical) — extensive cortical involvement, compatible with severe dementia
- c. Braak stage III–IV (limbic) — hippocampal involvement indicates moderate tau burden with established dementia
- d. Tau PET cannot be Braak staged — flortaucipir maps to NFT density, not Braak regions
Answer: A. Braak staging describes the hierarchical spread of neurofibrillary tangles (NFT) through the brain. Stages I–II involve the transentorhinal and entorhinal cortex. Stages III–IV extend to hippocampus and limbic structures. Stages V–VI involve neocortex and association cortices. 18F-flortaucipir (AV-1451) tau-PET signal correlates with Braak staging — entorhinal and hippocampal retention without association cortex involvement maps to Braak I–II, compatible with normal cognition or early MCI. In a 70-year-old Parkinson's patient without dementia, isolated entorhinal-hippocampal tau is expected and does not diagnose AD dementia.
Exam pearl: Tau-PET Braak I–II = entorhinal/hippocampal. Braak V–VI = neocortical. AD dementia requires Braak III–VI. Tau spread is hierarchical and predictable.
Braak & Braak Acta Neuropathol 1991; Radiographics 2020 PMID:32822285
Question 4 · PET-CT Cardiac · hard
A 66-year-old man with hypertrophic cardiomyopathy (HCM) undergoes cardiac PET with 18F-sodium fluoride (18F-NaF). Focal uptake is noted in the interventricular septum at the site of maximal hypertrophy. What does 18F-NaF uptake in this context most likely represent?
- a. Active calcification and microcalcification in myocardial fibrosis — marker of active disease remodelling
- b. Myocardial ischaemia — 18F-NaF is a perfusion tracer equivalent to 13N-ammonia
- c. Amyloid deposition — 18F-NaF has high affinity for amyloid fibrils in HCM
- d. Physiological uptake — 18F-NaF always localises to cardiac muscle at the site of highest workload
Answer: A. 18F-NaF PET detects active calcification by binding to hydroxyapatite crystals at sites of active mineral deposition — the same mechanism used in bone PET. In the heart, 18F-NaF uptake reflects active microcalcification within fibrotic myocardium or within coronary atherosclerotic plaques. In HCM, fibrotic replacement of hypertrophied myocardium is a key substrate for sudden cardiac death and arrhythmia. 18F-NaF uptake at the site of maximal hypertrophy identifies active fibrotic remodelling with microcalcification — a potential marker of high-risk HCM phenotype. This is an emerging application distinct from its established use in valvular calcification and coronary plaque imaging.
Exam pearl: 18F-NaF = hydroxyapatite = active calcification. Cardiac uses: coronary plaque vulnerability, valve calcification (AS), myocardial fibrosis in HCM. Never a perfusion tracer.
JACC Cardiovasc Imaging 2021; Radiographics 2022 PMID:35245106