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 DomainCommon 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)
| Imaging finding | Interpretation |
|---|---|
| Bile ducts visible | Normal hepatic function |
| Filling of the gallbladder | Patent cystic duct |
| Radiotracer is seen in the duodenum | Patent common bile duct |
| No radiotracer is seen in the gallbladder | Obstructed gallbladder (acute cholecystitis) |
| No radiotracer is seen in the duodenum | Biliary atresia |
| Radiotracer outside the biliary system | Biliary leak |
Hepatobiliary iminodiacetic acid scan
Image by Lecturio.
Normal hepatobiliary iminodiacetic acid scan showing the radioactive substance moving through the biliary system
Image: “HIDA” by Myohan. License: CC BY 3.0
Hepatobiliary iminodiacetic acid scan in a patient with gallbladder pathology:
Image: “Acute gallbladder torsion – a continued pre-operative diagnostic dilemma” by Mouawad NJ, Crofts B, Streu R, Desrochers R, Kimball BC. License: CC BY 2.0
Demonstration of uptake of tracer in liver without visualization of the gallbladder. Delayed images show excretion of tracer into the small bowel.
(a) Cholescintigraphy anterior view: left: 52–56 min after tracer injection; middle: 2 hours after tracer injection; right: 5 hours after tracer injection. Biliary atresia (BA) was suspected and the patient was referred for a technetium-99m trimethyl bromo-iminodiacetic acid hepatobiliary scintigraphy (99mTc-mebrofenin hepatobiliary scintigraphy), which showed rapid extraction of tracer by the liver with no excretion to the small bowel but with suspicion of visualization of the gallbladder (thick black arrows). The tracer was physiologically excreted in the urine and localized in a diaper (white arrow).
Image: “Potential Pitfalls on the (99m)Tc-Mebrofenin Hepatobiliary Scintigraphy in a Patient With Biliary Atresia Splenic Malformation Syndrome” by Maestri Brittain J, Borgwardt L. License: CC BY 4.0
(b) MRI, coronal view (T2 weighted), demonstrating that the liver is separated into several lobes, indicating a false positive on the hepatobiliary iminodiacetic acid scan for gallbladder visualization. This was a confirmed case of biliary atresia.
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.
- Ventilation phase:
- 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
- For evaluation of pulmonary embolism when CT pulmonary angiography is contraindicated:
- 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.
| Category | Findings |
|---|---|
| High probability of pulmonary embolism | More than 2 large mismatched segmental defects |
| Intermediate probability of pulmonary embolism |
|
| Low probability for pulmonary embolism |
|
| Normal | No matched or mismatched defects |
Normal pulmonary ventilation and perfusion (VQ) scan. The nuclear medicine VQ scan is useful in the evaluation of pulmonary embolism.
Image: “Ventperf” by Myohan. License: CC BY 3.0
High probability for pulmonary embolism:
Image: “Pulmonary embolism presenting as syncope: A case report” by Demircan A, Aygencel G, Keles A, Ozsoylar O, Bildik F. License: CC BY 3.0
Decreased perfusion is seen to the right lung and is particularly evident in the right lower lobe on the right posterior oblique (RPO) image (perfusion scan was performed using Tc-99m macroaggregated albumin).
High probability for pulmonary embolism:
Image: “Pulmonary embolism presenting as syncope: A case report” by Demircan A, Aygencel G, Keles A, Ozsoylar O, Bildik F. License: CC BY 3.0
No significant ventilation defect (ventilation scan performed using Xe-133 gas)
High probability for pulmonary embolism:
Image: “Pulmonary embolism scintigraphy PLoS” by Westgate EJ, FitzGerald GA. License: CC BY 2.0
Ventilation-perfusion scintigraphy in a woman taking oral contraceptives and valdecoxib
A: Ventilation phase: uniform ventilation to lungs
B: Perfusion phase: This and other views show decreased activity in the apical segment of right upper lobe, anterior segment of right upper lobe, superior segment of right lower lobe, posterior basal segment of right lower lobe, anteromedial basal segment of left lower lobe, and lateral basal segment of left lower lobe.
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
| Results | Description | Accuracy | Examples |
|---|---|---|---|
| Normal |
| True negative | False negative |
| Normal skeleton | Purely lytic metastasis | ||
| Abnormal | Asymmetric increased uptake | True positive | False positive |
|
|
Normal bone scan with 2 views and no asymmetric increased uptake
Image: “Nl bone scan2” by Myohan. License: CC BY 3.0
A: Bone scintigraphy during interferon-α treatment: Multiple bone metastases are visible (red arrows).
Image: “Long-term survival following multidisciplinary treatment of metastatic sarcomatoid renal cell carcinoma: A case report” by Yaegashi H, Izumi K, Konaka H, Mizokami A, Namiki M. License: CC BY 4.0
B: Bone scintigraphy at present state: Abnormal accumulations are less visible.
Oval-shaped increased osteoblastic activity on the metaphyseal region of both tibias (arrows) (stress fractures due to marathon running)
Image: “Stress fracture of bilateral tibial metaphysis due to ceremonial march training: A case report” by Kurklu M, Ozboluk S, Kilic E, Tatar O, Ozkan H, Basbozkurt M. License: CC BY 2.0
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
- Isotopes:
- 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
Normal myocardial perfusion scan with thallium-201 for the rest images (bottom rows) and technetium-sestamibi for the stress images (top rows)
Image: “Nl mpi2” by Myohan. License: CC BY 3.0
A patient with multiple perfusion defects with different degrees of severity.
Image: “Frequency and severity of myocardial perfusion abnormalities using Tc-99m MIBI SPECT in cardiac syndrome X” by Saghari, M. et al. License: CC BY 2.0
The upper images are stress phase and lower images are rest or delayed images. There is moderate ischemia in the mid anterolateral and mid inferolateral walls.
There also is severe ischemia in the basal inferior wall. Mild ischemia in the apex, apical septal, and basal anterolateral regions are also noted.
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
| Uptake | Pattern | Diagnosis |
|---|---|---|
| Increased | Nodular (hot nodule) | Toxic adenoma |
| Diffuse | Graves’ disease | |
| Normal | Symmetric uptake with no defects | |
| Decreased | Nodular (cold nodule) | Thyroid cancer |
| Diffuse | Hashimoto thyroiditis |
Normal thyroid scan
Image: “Thyroid scan” by Myohan. License: CC BY 3.0
Technetium-99 pertechnetate (99mTcO4) thyroid scan:
Image: “Thyroid Nodule Imaging, Status and Limitations” by Asia Oceania Journal of Nuclear Medicine & Biology (2015). License: CC BY 3.0
A: A large hot nodule with suppression of the rest of the thyroid
B: A large cold nodule in the left lobe of the thyroid
On cytology, the hot nodule was reported as a “follicular neoplasm” and the cold nodule was reported as a “colloid nodule.”
Initial thyroid ultrasound scanning and 99m-Technetium scintiscan
Image: “Localized subacute thyroiditis presenting as a painful hot nodule” by Li LX, Wu X, Hu B, Zhang HZ, Lu HK. License: CC BY 2.0
A: Thyroid ultrasonography reveals an ill-defined, solid, hypoechoic nodule (15 × 0.8 mm) with an irregular border in the left thyroid lobe (indicated by a white arrow).
B: Thyroid scintigraphy with 99m-Tc shows focal accumulation of radiotracer uptake in the lower lobe of the left thyroid, which represents the palpable tender nodule (indicated by a black arrow).
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
- Brandon, D.C., Thomas, A.J., Ravizzini, G.C. (2014). Introduction to nuclear medicine. https://accessmedicine.mhmedical.com/content.aspx?bookid=1562§ionid=95875470
- Elsayes, K.M., Oldham, S.A. (Eds.) (2014). Introduction to Diagnostic Radiology. McGraw-Hill.
- 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.
