elective 75

What biological process does bone scintigraphy primarily detect?

Osteoblastic activity and bone remodeling, rather than tumor cells directly.

Which radiopharmaceutical is commonly used for bone scintigraphy?

Tc-99m-MDP (methylene diphosphonate), a bone-seeking diphosphonate.

What is the physical half-life of Tc-99m?

Approximately 6 hours.

What preparation is required for a routine Tc-99m-MDP bone scan?

No fasting; intravenous tracer injection, hydration, frequent urination, and delayed imaging approximately 2–4 hours later.

Why are hydration and frequent urination encouraged after Tc-99m-MDP injection?

The tracer is predominantly renally eliminated; these measures improve background clearance and reduce bladder radiation exposure.

What do the three phases of bone scintigraphy assess, and when are they acquired?

Perfusion: blood flow immediately after injection; blood pool: soft-tissue vascularity within minutes; delayed bone phase: osteoblastic activity at approximately 2–4 hours.

Which bone-scan phase is the principal component for routine skeletal metastasis assessment?

Delayed skeletal imaging.

Why is bone scintigraphy highly sensitive but relatively nonspecific?

Many causes of increased bone turnover produce uptake, including metastases, fractures, arthritis, and infection.

Why can a normal bone scan fail to exclude skeletal metastases?

Purely osteolytic metastases may produce little reactive osteoblastic activity and therefore little or no tracer uptake.

What are the principal non-neoplastic indications for bone scintigraphy?

Occult fractures or trauma, degenerative disease, arthritis, avascular necrosis, and metabolic bone disease.

What are the main roles of nuclear medicine in oncology?

Detection, staging, treatment-response assessment, follow-up, and detection of recurrence or metastases.

Why is increased F-18-FDG uptake not specific for malignancy?

FDG reflects glucose utilization; infection and inflammation can also show increased uptake.

How does FDG become trapped in many tumor cells?

FDG enters through glucose transporters, including GLUT1; hexokinase converts it to FDG-6-phosphate, which is relatively trapped and does not proceed through glycolysis.

What preparation is generally required for routine oncological FDG PET?

Fast for at least 4–6 hours and assess blood glucose before injection according to protocol.

Why can eating before oncological FDG PET impair tumor visualization?

Increased insulin promotes skeletal muscle FDG uptake, altering tracer distribution and reducing tumor-to-background contrast.

How does FDG preparation for myocardial viability differ from oncological imaging?

Viability protocols may use glucose loading with or without insulin to promote myocardial uptake; oncological protocols use fasting to limit insulin-stimulated uptake.

What are the physical half-lives of F-18, Ga-68, and I-131?

F-18: approximately 110 minutes; Ga-68: approximately 68 minutes; I-131: approximately 8 days.

How do Ga-67 and Ga-68 differ in imaging modality and typical applications?

Ga-67 uses gamma-camera/SPECT imaging, historically for lymphoma and inflammation; Ga-68 is a positron emitter used in targeted PET tracers.

What does Ga-68-DOTATATE target?

Somatostatin receptors in receptor-positive neuroendocrine tumors.

What is the main application of Ga-68-PSMA PET?

Imaging PSMA-expressing prostate cancer.

What is the principal tumor application of radioiodinated MIBG?

Selected catecholamine-producing and neuroendocrine tumors.

Why is thyroid blockade used with certain radioiodinated tracers such as I-131-MIBG?

Potassium iodide or Lugol's iodine reduces unwanted thyroid uptake of free radioactive iodine when the thyroid is not the target.

Which emissions allow I-131 to provide both therapy and imaging?

Beta emissions provide therapeutic radiation; gamma emissions permit imaging.

Which tracer is commonly used for thyroid scintigraphy, and how does its handling differ from iodine?

Tc-99m-pertechnetate enters follicular cells through the sodium-iodide symporter but is not organified into thyroid hormones.

What is the usual procedure for Tc-99m-pertechnetate thyroid scintigraphy?

Fasting is generally unnecessary; inject intravenously and acquire gamma-camera images after approximately 15–20 minutes.

What is the key difference between thyroid scintigraphy and radioactive iodine uptake (RAIU) testing?

Scintigraphy maps the distribution of functioning thyroid tissue; RAIU quantifies the percentage of administered radioiodine taken up by the thyroid.

What thyroid scan pattern is characteristic of Graves' disease?

Diffuse increased tracer uptake, often with diffuse enlargement, due to TSH-receptor stimulation and increased hormone synthesis.

What thyroid scan pattern is characteristic of a toxic adenoma?

A single focal hot nodule with suppression of uptake in the surrounding thyroid tissue.

What thyroid scan pattern is characteristic of toxic multinodular goiter?

Heterogeneous or patchy increased uptake in multiple autonomous nodules, with suppression of less active tissue.

Why is RAIU low during the thyrotoxic phase of destructive thyroiditis?

Follicular destruction releases stored hormones without increased new hormone synthesis.

What does elevated T3/T4 with suppressed TSH and low RAIU suggest?

Destructive thyroiditis, while also considering exogenous thyroid hormone and iodine-related interference.

How do hot and cold thyroid nodules differ in uptake and malignancy risk?

Hot: increased uptake, hyperfunctioning, and very low malignancy risk; cold: reduced or absent uptake, with malignancy possible.

What further assessment may a cold thyroid nodule require?

Ultrasound risk assessment and possible fine-needle aspiration based on sonographic features and clinical risk.

For which thyroid cancers is radioiodine whole-body scintigraphy used?

Selected iodine-avid differentiated thyroid cancers, particularly papillary and follicular carcinomas.

What are the main uses of postoperative radioiodine whole-body imaging?

Detect residual iodine-avid tissue or tumor, local recurrence, distant metastases, and assess findings after radioiodine therapy.

Does iodine uptake in the neck after thyroidectomy prove residual cancer?

No. It may represent benign thyroid remnant tissue, persistent malignancy, or another source of uptake; localization and clinical correlation are needed.

How does SPECT/CT improve interpretation of abnormal radioiodine uptake?

It combines functional uptake with CT anatomy, helping localize thyroid-bed remnants, lymph nodes, and other lesions.

Which thyroid carcinoma is associated with calcitonin and generally does not respond to conventional radioiodine therapy?

Medullary thyroid carcinoma.

Which tracer is commonly used to localize hyperfunctioning parathyroid tissue?

Tc-99m-sestamibi (MIBI).

What finding suggests a parathyroid adenoma on dual-phase sestamibi imaging?

Persistent focal uptake on delayed imaging, approximately 2 hours after injection, while surrounding thyroid activity washes out.

What is the clinical role and key limitation of parathyroid sestamibi scintigraphy?

It localizes hyperfunctioning tissue after biochemical diagnosis of hyperparathyroidism; a negative scan does not exclude parathyroid disease.

Which tracers are commonly used for myocardial perfusion scintigraphy?

Tc-99m-sestamibi and Tc-99m-tetrofosmin; Tl-201 is an older perfusion tracer with redistribution.

Why can significant coronary stenosis produce a perfusion defect during stress?

The stenosed artery has reduced coronary flow reserve, so its territory receives relatively less blood flow and tracer during stress.

How is stress induced for myocardial perfusion imaging?

Exercise using a treadmill or bicycle, or pharmacological vasodilation using agents such as adenosine, dipyridamole, or regadenoson.

What does reduced myocardial tracer uptake during stress that improves at rest indicate?

A reversible perfusion defect, indicating inducible myocardial ischemia.

What does reduced myocardial uptake during both stress and rest indicate?

A fixed defect, which may reflect scar, hibernating myocardium, or attenuation artifact; it does not alone prove nonviability.

What are the typical coronary arterial territories of the anterior/septal, lateral, and inferior walls?

Anterior/septal: LAD; lateral: LCx, sometimes diagonal branches; inferior: usually RCA, depending on coronary dominance.

What additional information does ECG-gated myocardial perfusion SPECT provide?

Left ventricular EDV, ESV, ejection fraction, regional wall motion, and wall thickening in addition to perfusion.

Which radiopharmaceutical is used for MUGA, and what does the test assess?

Tc-99m-labeled red blood cells; ECG-gated blood-pool imaging assesses ventricular function, particularly LVEF and wall motion.

What does decreased myocardial perfusion with preserved FDG uptake indicate?

A perfusion–metabolism mismatch, supporting viable myocardium that may recover after revascularization.

What does decreased myocardial perfusion with decreased FDG uptake suggest?

A matched perfusion–metabolism defect, suggesting nonviable myocardial scar.

Why may cardiac inflammation FDG PET require a high-fat, low-carbohydrate diet and prolonged fasting?

To suppress physiological myocardial glucose uptake so inflammatory FDG uptake can be assessed.

How does the radiation source differ between nuclear medicine and conventional X-ray/CT imaging?

Nuclear medicine detects emissions from an administered radiopharmaceutical; X-ray/CT uses an external radiation source.

What are the two components of many radiopharmaceuticals?

A radiation-emitting radionuclide and a targeting molecule that determines biological distribution.

How do planar imaging, SPECT, and PET differ?

Planar imaging is two-dimensional; SPECT is three-dimensional gamma-camera imaging; PET is three-dimensional positron-emission imaging.

What is the difference between static and dynamic scintigraphy?

Static imaging captures selected time points; dynamic imaging acquires sequential images and can generate time–activity curves.

What is the main advantage of hybrid imaging such as SPECT/CT, PET/CT, or PET/MRI?

It combines functional or metabolic information with anatomy, improving localization and diagnostic interpretation.

How do deterministic radiation effects differ from stochastic effects?

Deterministic effects generally have a threshold and increasing severity above it; stochastic effects have dose-related probability but dose-independent severity, with no established threshold in the standard protection model.

What are examples of deterministic and stochastic radiation effects?

Deterministic: skin injury, cataracts, and tissue damage; stochastic: cancer and heritable effects.

What are the three fundamental principles of radiation protection?

Justification, optimization using ALARA, and dose limitation for occupational/public exposure; patient medical exposures rely on justification and optimization rather than fixed diagnostic dose limits.

What does ALARA mean?

As Low As Reasonably Achievable, while maintaining clinically useful image quality.

What are the three practical methods for reducing radiation exposure?

Minimize time near the source, increase distance, and use appropriate shielding.

What happens to radiation intensity if distance from an ideal point source doubles?

It falls to approximately one-quarter, according to the inverse-square law.

What is the purpose of a personal dosimeter?

To measure accumulated radiation exposure; it does not shield or protect the wearer.

Is pregnancy an absolute contraindication to diagnostic scintigraphy?

No. Carefully justified imaging may be appropriate, including lung scintigraphy for suspected PE; test selection depends on clinical circumstances and maternal–fetal considerations.

Which radiopharmaceuticals are used for ventilation and perfusion lung scans?

Ventilation: inhaled radioactive gas or aerosol; perfusion: intravenous Tc-99m-MAA (macroaggregated albumin).

What V/Q scan pattern classically suggests pulmonary embolism?

Preserved ventilation with decreased regional perfusion: a V/Q mismatch.

What does a matched ventilation–perfusion defect often suggest?

Parenchymal lung disease, such as pneumonia or COPD, although patterns can be complex.

Which tracer is used for static renal cortical scintigraphy, and what are its main indications?

Tc-99m-DMSA; renal cortical scarring, selected acute pyelonephritis assessment, ectopic/congenital renal abnormalities, and differential cortical function.

How can DMSA follow-up distinguish acute pyelonephritis from permanent renal scarring?

An acute cortical defect may resolve; persistence several months later supports scarring, especially with cortical thinning or contour deformity.

How do Tc-99m-MAG3 and Tc-99m-DTPA differ in renal clearance?

MAG3 is cleared primarily by tubular secretion; DTPA is cleared by glomerular filtration.

What does dynamic renal scintigraphy evaluate, and what curve does it produce?

Renal perfusion, uptake, function, and urinary drainage; it produces a renogram, a renal time–activity curve.

What is the role of furosemide in diuretic renography?

It increases urine flow to help distinguish obstructive from non-obstructive collecting-system dilation.

How are rapid clearance and persistent retention after furosemide interpreted?

Rapid clearance favors non-obstructive dilation; persistent poor drainage raises concern for obstruction and must be interpreted with renal function, hydration, and bladder status.

What is the physiological basis of ACE inhibitor renography in renal artery stenosis?

ACE inhibition reduces angiotensin II-mediated efferent arteriolar constriction, potentially lowering filtration in a stenotic kidney and altering tracer uptake, transit, or excretion.