Nuclear Imaging

Terminology and Technological Aspects

Mechanism

• Nuclear medicine differs from the rest of radiology because it involves functional rather than structural imaging.
• Radioisotopes: unstable forms of an element that emit detectable particles as they decay into more stable forms
• Radiopharmaceuticals:
  • Artificially produced isotopes bound to pharmaceuticals (radioisotope + an organic molecule) 
  • Administered to patients and used in nuclear medicine
  • The organic molecule allows isotopes to concentrate within a specific target organ.
  • The radioisotope emits detectable ionizing radiation (high-energy rays) when it decays, which is visualized during imaging.

Image creation

Equipment:

• The machine is equipped with a gamma camera that detects radiation and forms an image.
• Components of the gamma camera:
  • Collimator: 1st layer between the patient and the crystal. The collimator is made of lead with holes to reduce scatter.
  • Crystal: emits faint light after interacting with gamma rays
  • Photomultiplier tubes: detect and convert light from the crystal into electrical signals
  • Electronics for processing data: analyze signals and produce viewable images 

Imaging techniques:

• Single-photon emission computed tomography (SPECT):
  • Uses gamma-emitting radioisotopes
  • A 3D nuclear imaging made by multiple 2D images acquired at different angles
• PET:
  • Uses positron-emitting radioisotopes
  • Superior image quality (contrast and resolution) but more expensive
• Combined imaging techniques:
  • CT (PET-CT or SPECT-CT) or MRI is integrated.
  • Improved localization of lesions

Common nuclear medicine exams

• Hepatobiliary iminodiacetic acid (HIDA) scan
• Ventilation and perfusion (VQ) scan
• Bone scan
• Cardiac scan
• Thyroid scan

Hepatobiliary Iminodiacetic Acid Scan

• HIDA scan (cholescintigraphy):
  • Examination of the gallbladder
  • The radiopharmaceutical is normally taken up by the liver and excreted through the biliary system as bile.
  • Radiopharmaceutical: technetium-99m iminodiacetic acid
  • Isotope: technetium-99m
• Indications: 
  • Acute cholecystitis with equivocal ultrasound findings
  • Biliary atresia
  • Biliary leak
  • Biliary dyskinesia: Cholecystokinin is administered and gallbladder ejection fraction is calculated.
• Contraindication: allergy to the radiotracer (anaphylaxis) 

Ventilation and Perfusion Scan

• Phases:
  • Ventilation phase:
    • The patient breathes in the radiopharmaceutical, typically Xenon-133, in the form of an aerosol (other option: technetium-99m diethylenetriaminepentaacetate (DTPA)).
    • Small particles then are deposited into the alveoli and images are acquired.
  • Perfusion phase:
    • An injectable radiopharmaceutical (technetium-99m macroaggregated albumin (MAA)) is administered. 
    • Technetium-99m MAA enters the pulmonary vessels and images are acquired.
• Indications: 
  • For evaluation of pulmonary embolism when CT pulmonary angiography is contraindicated:
    • IV contrast allergy
    • Pregnancy 
    • Renal failure
  • Preoperative estimates of lung function: preparation for surgical excision
• Contraindication: allergy to radiotracers (anaphylaxis) 
• Normal VQ scan findings:
  • Normal perfusion: uniform uptake in the lungs with areas of photopenia in the region of the heart and hila 
  • Normal ventilation: radiotracer homogeneously washing into the lung with quick washout and no evidence of air trapping
  • Always obtain a chest radiograph prior to a VQ scan to exclude consolidation, which gives a false-positive result.

Bone Scan

• The skeletal system can be assessed using nuclear medicine.
  • The radiopharmaceutical that is used chemoadsorbs to the hydroxyapatite crystals in the bone matrix; thus, areas of bone turnover can be identified.
  • Increased bone turnover is seen in fractures, tumors, and acute infection.
  • Isotope: technetium-99m
  • Radiopharmaceutical: methylene diphosphonate
• Imaging:
  • Images are obtained 4 hours after injection, allowing the radiotracer to be taken up by the bone.
  • Anterior and posterior images are obtained.
• Indications: 
  • Bony metastases screening
  • Evaluation of fractures (e.g., stress fracture) that are not visible on a radiograph
  • Infections

Cardiac Scan

• Also known as myocardial perfusion imaging
• Detects variation in blood flow and myocardial extraction of radiotracers 
• Materials: 
  • Isotopes: 
    • Technetium-99m
    • Thallium-201 
  • Radiopharmaceuticals:
    • Sestamibi 
    • Teboroxime
• Procedure:
  • In normal coronary arteries, significant arterial dilatation is seen in response to exercise/stress.
  • Stenotic areas do not show dilatation; thus, ischemia and ECG changes occur. 
  • In a cardiac scan, stress is either:
    • Induced by exercise by running on a treadmill
    • Pharmacologically induced by giving adenosine or dobutamine to those who cannot run 
• Imaging is obtained at both stress and rest.
• Radiopharmaceuticals are injected when 85% of the maximum predicted heart rate (MPHR) is reached.
• Indications:
  • Myocardial ischemia or infarction evaluation
  • Wall-motion abnormalities: performed using ECG-gated SPECT scan
  • Calculate left ventricular ejection fraction
• Normal cardiac scan obtained in 3 different planes:
  • The top row of each set is performed under cardiac stress.
  • Bottom row of each set is performed at rest.
  • Normal flow to all aspects of the heart at both rest and stress
• Abnormal cardiac scans:
  • Myocardial ischemia: areas of photopenia (i.e., decreased uptake) under stress, which improve at rest
  • Myocardial infarction: persistent photopenia (i.e., decreased uptake) despite rest state

Thyroid Scan

• The thyroid gland transports iodine; thus, radioiodine is used to detect the function of the whole gland or nodules.
• Radiopharmaceuticals:
  • Radioactive iodine (iodine-123)
  • Technetium-99 pertechnetate
• Indications: 
  • Thyroid nodules
  • Patients with thyrotoxicosis
  • Patients with thyroid cancer:
    • Distant metastasis (whole body scan)
    • Local residual disease
  • Therapeutic use: radioactive thyroid ablation (in Graves’ disease or thyroid carcinoma) using higher doses of radioactive iodine (iodine-131 destroys thyroid cells)
• Contraindications:
  • Pregnancy 
  • Lactation

Other Imaging Modalities by System

• Imaging of the CNS (brain, spinal cord, and vertebral column): 
  • Radiography is often used to evaluate fractures of the vertebral column. 
  • CT is a good choice to determine head trauma and exclude intracranial hemorrhage. 
  • MRI provides more detailed images of the brain and spinal cord, allowing identification of infarction, tumors, disc herniation, and demyelinating disease.
• Pulmonary radiology and imaging of the mediastinum: 
  • Radiography is the preferred initial imaging study for viewing lung pathology. 
  • CT provides more detailed views of the lung parenchyma, mediastinal structures, and vasculature. 
  • MRI is not often used, but may be employed for evaluating malignancies and cardiac disease. 
  • Ultrasound can be used for rapid bedside-trauma assessment and for guiding procedures (thoracentesis).
• Breast imaging: 
  • Mammography is often the initial choice for breast cancer screening. 
  • MRI can be used to further evaluate and stage breast cancers. 
  • Ultrasound is helpful in the evaluation of lymph nodes and in guiding biopsy.
• Imaging of the abdomen and renal imaging: 
  • Radiography is often used to evaluate for kidney stones, bowel obstruction, and pneumoperitoneum. In addition, barium may be used to assess swallowing and bowel function. 
  • CT and MRI provide more detailed assessments of the abdominal viscera and vasculature. 
  • Nuclear medicine can be used to assess gallbladder function, gastric emptying, and GI bleeding.
• Imaging of the uterus and ovaries: 
  • Ultrasound is the most commonly used modality to evaluate the ovary and uterus, including assessing pregnancies and determining the cause of abnormal uterine bleeding. 
  • CT and MRI provide more detailed views and are often useful in assessing cysts, malignancies, and benign masses.
• Imaging of the musculoskeletal system: 
  • Radiography is often used to exclude fractures. 
  • CT is more sensitive to bone pathology, including osteomyelitis. 
  • MRI is preferred for soft-tissue evaluation, such as assessing for malignancy and myositis. 
  • Bone scan can be useful in determining occult fractures, osteomyelitis, and metabolic bone disease.