This is one of the most common pitfalls in nuclear medicine imaging, causing scan artifacts that resemble focal lesions. Deposition of tracer can occur due to inadvertent spray of tracer during injection at an improperly secured injection port or due to urinary contamination or salivary secretions. A common occurrence is the appearance of a focal uptake seen in the chest in a radioiodine (I-123 or I-131) whole body scan, in which the abnormality is not to any structural lesion but from a small collection of swallowed activity in the esophagus. Tracer contamination has been observed in the hair, ears, chest wall, abdominal wall, and pelvic regions as well as in the extremities (Fig. 4.2). Occasionally, these cause interpretive challenges that may have to be resolved by re-scanning the patient after the tracer contamination is removed (e.g., by washing). A very common occurrence is the presence of a focus of tracer activity in the extremities, due to slight tracer extravasation at the injection site or a collection in a tortuous vessel. Another scan artifact is due to tracer in a long peripheral line or central line that had not been adequately flushed with normal saline after tracer injection. This may occasionally cause confusion in scan interpretation or obscure structures.

Photopenic (photon-deficient) defects can be seen on the various kinds of nuclear medicine scans, more commonly in bone scans, due to attenuation from metallic or dense objects. These include pacemakers (Fig. 4.3), buttons, buckles, jewelry bangles, coins in pockets, breast prostheses, and orthopedic prostheses. Patients should be informed to remove these objects if clinically possible before the scan is started. Occasionally, they cause confusion with osteolytic lesions or obscure parts of the scan.

Patient motion may introduce loss of resolution of the image and scan artifacts. This is particularly important in myocardial perfusion imaging and renography imaging. Patient motion in a renography study will typically cause artifactual abnormalities on the time-activity curves, and care must be exercised in reducing this possibility. In myocardial perfusion imaging, patient motion may give rise to apparent perfusion defects. Patient motion can be detected by cine displays, sinograms, and summed planar images. There are a number of software algorithms for correcting patient motion, but the best way to avoid this pitfall is to be aware of its possible occurrence and minimize patient motion or to repeat the study if suspected.

Occasionally, a patient may have performed two nuclear medicine imaging studies within a short period of time, such that the images superimpose. This produces the occasional unusual scan appearance that can be misleading. For example, a Tc-99m diphosphonate bone scan that is performed within 24 h of a myocardial perfusion imaging using Tc-99m sestamibi will show apparent cardiac, hepatic, thyroid, and bowel uptake of “bone” tracer. Sometimes, when a bone scan is performed 1 week after a patient has received therapeutic activity of I-131 for thyroid remnant ablation, the high energy photons of iodine-131 will cause septal penetration and obscure the Tc-99m bone scan image.

Center of rotation error occurs when the point of reference used by the scanner in reconstructing images is not aligned with the mechanical axis of the SPECT scanner. If the center of rotation is properly calibrated, a radioactive point source placed at the center of the camera orbit should project to the center of the reconstructed image. Errors of misalignment of the center of rotation may manifest as areas of reduced counts or ring artifacts which may mimic true defects. It is important for nuclear medicine technologists to properly calibrate the center of rotation for the scanner on a routine basis as a quality control procedure.

Nuclear medicine imaging in the young can be fraught with subtleties. For example, bone scans in the young up to 20 years old typically demonstrate increased epiphyseal plate activity, and this requires care in interpretation as some of these may mimic trauma or conceal subtle changes of trauma. In younger subjects, increased tracer uptake at the manubriosternal junction is often a normal variant. In some patients, a small photopenic defect in the inferior aspect of the sternal body due to incomplete fusion of the cartilaginous bars can be seen. This photopenic area, called the sternal foramina, is typically surrounded by uniform radioactivity and should not be mistaken for an osteolytic lesion. Horizontal lines of uptake in the sternal body can also be seen in younger subjects, and these are often very uniform, regular spaced, and are in keeping with bone fusion centers in a growing sternum.

Radiotracer changes due to degeneration can often be seen in the older subjects. Focal irregularities in uptake of bone tracer can be seen at the knees due to osteoarthritis or in the lumbar spine due to spondylosis. They tend to have usual appearance of distribution at the joint surfaces and at the margins of the vertebrae or facet joints. Most of these pattern if equivocal on planar imaging can be further characterized with SPECT-CT.

The uptake of FDG or radiotracers for cerebral amyloid imaging by the cerebral hemispheres and deep cerebral structures may show age-related changes, and experience and care should be exercised. Cortical FDG uptake decreases with age, particularly in the frontal lobes. In contrast, metabolic activity of the basal ganglia, visual cortices, and cerebellum remains unchanged. The normal thymus may be visualized on a routine FDG PET-CT as an FDG-avid anterior mediastinal mass in a young person. Occasionally, the normal thymus or lactating breasts can be visualized on the I-123/I-131 whole body scan (Fig. 4.4).

In V/Q lung scanning, the radiotracers used are typically Xe-133 gas, Tc-99m Technegas, or Tc-99m DTPA aerosol for ventilation and Tc-99m MAA for perfusion. The particle size for Tc-99m MAA is usually 20–80 μm and largely trapped in the pulmonary capillary vasculature. Only less than 4 % of administered MAA activity escapes into the systemic circulation. This small amount of radioactivity is usually not visually detectable. Routinely, minimal or no kidney activity is visualized in both the ventilation and perfusion images. The visualization of the kidney, thyroid, and brain cerebral hemispheres after the injection of Tc-99m MAA is thus highly indicative of a right–left systemic shunt. Care must be taken in attributing thyroid and kidney uptake to systemic shunting as these may be seen if free pertechnetate is present.

Ectopic organs are potential sources of interpretation errors. Examples include ectopic thyroid gland in the lingual or hyoid position (Fig. 4.5), ectopic kidney in the pelvis, and ectopic parathyroid adenomas in the mediastinum. A transplanted kidney seen in the iliac fossa can be missed or mistaken for bone pathology (Fig. 4.6).