Nuclear Imaging

Nuclear imaging is the radiologic examination using radiopharmaceuticals, which are radioactive substances taken up by specific types of cells. Nuclear medicine is more concerned with the functional and molecular aspects of the organ or pathology being investigated rather than the structure. Radiopharmaceuticals are administered to the patient and in vivo distribution is recorded. Nuclear imaging has been widely used to diagnose and follow up certain diseases. Multiple organ systems can be evaluated, including the cardiovascular system, where ischemia is detected; endocrine system, where thyroid activity is evaluated; hepatobiliary system, where cystic duct obstruction can be seen; and skeletal system where tumors/metastasis or fractures are localized.

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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

A SPECT/CT system with relevant components labeled in the photograph on the right

Image: “NM19 290” by Kieran Maher. License: Public Domain

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) 
Table: Interpretation of HIDA scans
Imaging findingInterpretation
Bile ducts visibleNormal hepatic function
Filling of the gallbladderPatent cystic duct
Radiotracer is seen in the duodenumPatent common bile duct
No radiotracer is seen in the gallbladderObstructed gallbladder (acute cholecystitis)
No radiotracer is seen in the duodenumBiliary atresia
Radiotracer outside the biliary systemBiliary leak

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.
Table: VQ scan interpretation based on Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) revised criteria
CategoryFindings
High probability of pulmonary embolismMore than 2 large mismatched segmental defects
Intermediate probability of pulmonary embolism
  1. 2 large, mismatched segmental defects (borderline high probability)
  2. 1 moderate to 2 large, segmental defects or any other pattern difficult to characterize as high or low probability (true intermediate probability)
  3. Single matched defect with a normal chest radiograph (borderline low probability)
Low probability for pulmonary embolism
  1. Non-segmental perfusion defects
  2. Any perfusion defect with a substantially larger radiographic finding
  3. Matched ventilation/perfusion defect with a normal chest radiograph
  4. Any amount of small perfusion defects with a normal chest radiograph
NormalNo matched or mismatched defects

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
Table: Interpretation of bone scan
ResultsDescriptionAccuracyExamples
Normal
  • Symmetric uptake
  • Urinary bladder and skull are normally bright
True negativeFalse negative
Normal skeletonPurely lytic metastasis
AbnormalAsymmetric increased uptakeTrue positiveFalse positive
  • Osteoblastic metastasis
  • Fractures
  • Degenerative changes
  • Osteomyelitis

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
Table: Interpretation of thyroid scan
UptakePatternDiagnosis
IncreasedNodular (hot nodule)Toxic adenoma
DiffuseGraves’ disease
NormalSymmetric uptake with no defects
DecreasedNodular (cold nodule)Thyroid cancer
DiffuseHashimoto thyroiditis

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.

References

  1. Brandon, D.C., Thomas, A.J., Ravizzini, G.C. (2014). Introduction to nuclear medicine. https://accessmedicine.mhmedical.com/content.aspx?bookid=1562&sectionid=95875470
  2. Elsayes, K.M., Oldham, S.A. (Eds.) (2014). Introduction to Diagnostic Radiology. McGraw-Hill. 
  3. Chen, M.M., Whitlow, C.T. (2011). Chapter 1. Scope of Diagnostic Imaging. In Chen M.M., Pope T.L., Ott D.J.(Eds.). Basic Radiology, 2e. McGraw-Hill.

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