Nuclear Medicine

·         The most important issue regarding a PET scan is that the FDG will compete with nonradioactive glucose. Thus, if a person has recently eaten or has diabetes with a blood glucose level greater than 150 mg/dL, the sensitivity of the PET scan will be diminished. Typically, blood glucose levels greater than 200 mg/dL should exclude an individual from having a PET scan.

In general, bowel activity can be a normal finding on a PET scan, which sometimes makes the evaluation of cancer in the bowel more difficult.

·         INDICATIONS FOR PET SCANNING

1. Oncologic staging: Medicare approves PET for staging or restaging of non-small cell lung cancer, breast cancer, colorectal cancer, melanoma, lymphoma, head and neck cancer, and esophageal cancer
2. Brain tumors: differentiation of residual/recurrent brain tumor from radiation necrosis
3. Seizures: presurgical, interictal identification of refractory brain seizure foci
4. Cardiac imaging: metabolic assessment of myocardial viability

·         The classic pattern of Alzheimer's disease on an FDG PET scan is hypometabolism in the temporoparietal regions. This may also involve the posterior cingulate gyrus. The subcortical areas, sensorimotor area, visual cortex, and cerebellum are generally less affected. However, more recent studies have shown that other areas may be hypometabolic, particularly when the patient suffers specific neurocognitive deficits. Temporoparietal hypometabolism can also be observed in other conditions, including Parkinson's disease, bilateral parietal subdural hematomas, bilateral parietal stroke, and bilateral parietal radiation therapy ports.

·         FDG PET studies of depressed patients usually demonstrate decreased metabolism, which can be global or affect more specific regions, such as the frontal lobes. More recent studies have suggested that certain areas may actually have increased activity, such as the limbic regions. This more global pattern can usually be distinguished from specific neurodegenerative diseases, such as Alzheimer's or Pick's disease, which typically affect the temporoparietal and frontal lobes, respectively.

·         A "superscan" implies that so much of the methylene diphosphate (MDP) is taken up by the bones that there is no significant excretion in the kidneys and bladder or uptake in the soft tissues. The scan appears almost too good with high contrast between the bones and other tissues. The most common causes of a "superscan" are renal failure, hyperparathyroidism, metabolic bone disease, Paget's disease, or widespread metastatic disease. Thus, in a patient with cancer, a "superscan" implies widespread osseous metastases that cannot be individually distinguished but rather occupy almost the entire skeleton.

·         COMMON FINDINGS OF METASTATIC DISEASE ON BONE SCANS

1.       Solitary focal lesions

2.       Multiple focal lesions

3.       Superscan

4.       Photon-deficient lesions

5.       Normal findings (false-negative scan)

6.       Soft tissue uptake

·         CAUSES OF COLD DEFECTS ON A BONE SCAN

1. Avascular necrosis
2. Malignant bone tumors
3. Metastases
4. Prosthesis, pacemaker, jewelry, lead shield
5. Barium in colon
6. Disuse atrophy
7. External radiation therapy
8. Early osteomyelitis

·         Lung uptake is almost never normal on a bone scan and is usually associated with malignant pleural effusions, large tumors, inflammatory processes, or metastatic disease. Metastatic osteosarcoma has particularly intense uptake when involving the lungs. Lung uptake itself is usually detected by comparing the left and right hemithoraxes, observing for increased uptake in the intercostal spaces.

·         Paget's disease typically is associated with focal areas of intensely increased uptake in the flat bones and the ends of the long bones. The uptake is usually diffuse, although there can be focal areas of increased uptake. There is no definitive way of excluding metastatic disease or primary bone tumors from Paget's disease on the basis of uptake in the bones. However, the pattern of Paget's in terms of its distribution and appearance may help in the diagnosis. For example, it is less likely that an individual will have an entire hemipelvis as the only site of metastatic disease, but this can commonly be a presentation of Paget's. In the spine, the "Mickey Mouse sign" refers to foci in which there is uptake in both the entire vertebral body and the spinous process, which is almost always Paget's disease rather than metastatic disease

·         Osteoid osteomas have increased uptake on delayed bone scan images and may even have an observable photopenic center. They commonly arise in the femur or spine. Uptake is also very intense for osteochondromas and chondroblastomas. Enchondromas do not typically have significantly increased uptake on a bone scan.

·         The PIOPED study, which prospectively evaluated patients with suspected PE, helped establish the criteria by which V/Q scans are currently read. A high probability for PE, requires that the scan have the equivalent of two or more large segmental perfusion defects (75-100% involvement of the segment) that are not matched by ventilatory abnormalities. Four or more moderately sized perfusion defects (25-75% involvement of the segment) would also represent a high probability for PE. The implication of a high probability scan is that the patient has a greater than 80% chance of having a PE

·         A "stripe sign" is a rim of activity that surrounds the edge of a perfusion defect. The implication of this sign is that a perfusion defect that is bordered by the stripe sign is not the result of a PE. The physiologic basis of this interpretation is that perfusion defects related to PE should extend all the way to the periphery of the lung. If there is a stripe of perfused tissue distal to the perfusion defect, then it most likely does not represent PE.

·         Dipyridamole stress tests should not be performed in patients with severe reactive airway disease, particularly when a patient demonstrates active wheezing on physical examination. However, patients who use inhalers but who otherwise have stable airway disease can often safely undergo Dipyridamole stress tests. Any patient taking Dipyridamole or methylxanthines should not have a Dipyridamole stress test unless the medication can be stopped for at least 24-48 hours. Furthermore, all patients who are to undergo Dipyridamole studies should abstain from caffeine for 24 hours since caffeine blocks the effect of Dipyridamole and adenosine. Dobutamine stress tests have similar contraindications to those of exercise testing, except that musculoskeletal problems are not an issue.

·         Transient left ventricular cavity dilation refers to the cavity appearing larger on stress images than on resting images. This is in contrast to an individual who has fixed cavity dilation that may be associated with a cardiomyopathy. The implication of transient left ventricular cavity dilation is that during stress, the perfusion abnormalities result in worsening myocardial function such that the heart cannot maintain its usual contractile state and then dilation occurs. This is a poor prognostic sign since there is significant cardiac function and structure that is at risk for an ischemic event.

·         Stunned myocardium refers to relatively preserved myocardial perfusion as measured by SPECT or PET imaging, normal or decreased glucose metabolism on FDG PET scans, and decreased wall motion on echocardiography. The implication is that there was an ischemic event that caused a reduction in myocardial wall function, even though perfusion and metabolism are now reestablished. The stunning should resolve as long as perfusion is maintained. On the other hand, hibernating myocardium refers to decreased perfusion with relatively preserved glucose metabolism and decreased wall motion. The implication is that there is a chronic ischemia that is resulting in decreased flow and function even though the myocardium is still viable. Hibernating myocardium should respond well to attempts to revascularize the region, which would return flow and eventually reverse the functional deficits.

·         The MUGA scan is primarily used in the evaluation of myocardial wall motion and determination of left ventricular ejection fraction. Both of these can now be assessed with echocardiography, which can also be used to evaluate the structure and function of the heart's valves. For this reason, MUGAs are used relatively infrequently. The principal use is for the determination of ejection fraction in cancer patients before and after they undergo chemotherapy.

·         SCINTILLATION CAMERA (ANGER CAMERA). This instrument consists of one, two, or three large, flat, sodium iodide crystals up to 50 cm in diameter. Photons from the radioactive tracer produce luminescence in the crystal, which is then augmented many times by a large number of arrayed photo-multiplier tubes. The two-dimensional location of the source of the signal, which is determined by computing the relative intensity of luminescence emitted by the multiple photo tubes, is displayed on an oscilloscope and then recorded on film.

·         Production of Radionuclides

1. Reactor-produced Radionuclides: Mo-99/Tc-99m generator, Kr-81m generator
2. Accelerator / Cyclotron-produced Radionuclides: Ga-67, I-123, Tl-201
3. Fission-produced Radionuclides: I-131, Mo-99

·         Radionuclide Impurity: Mo-99 Breakthrough Test

·         Chemical Impurity: Aluminum Ion Breakthrough Test

·         Radiopharmaceutical Sterility and Pyrogenicity: USP XX Test, Limulus Amoebocyte Lysate Test (LAL)

·         Hot Bone Lesions

mnemonic: NATI MAN

Neoplasm

Arthropathy

Trauma

Infection

Metastasis

Aseptic Necrosis

·         "Gaussian carditis":

It is the amusing term for the phenomenon of the magnetic strip on your credit card being erased when you accidentally bring it too close to the MRI's powerful magnetic field. (Carl Gauss, a German mathematician who died in 1855, did much of the theoretical work on magnetism that ultimately led to the invention of the MRI.)