Unusual Artifacts Encountered during Routine Studies on Positron Emission Tomography-Computed Tomography and Gamma Camera

Case 1

Our first case shows a PET-CT scan performed for oncologic indication. On viewing the reconstructed images, we identified two unusual artifacts on the PET image. One appeared as multiple equidistant hot spots in a linear fashion along the length of the patient’s body [Figure 1 black arrow], and the other artifact noted were multiple horizontal photopenic lines perpendicular to the body plane at the level of these hot spots seen on PET image [Figure 1 red arrow].

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Figure 1 Positron emission tomography image in sagittal plane (a) and maximum intensity projection image (b) Artifacts observed are the multiple equidistance hot spots in a linear fashion along the length of patient’s body (black arrow) and multiple horizontal photopenic lines perpendicular to body plane at the level of these hot spots (red arrow)

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Preliminary, we thought it was due to normalization error, but all other scans of the day were fine, so we ruled out any possibility of contamination on the scanner bed. However, on probing further, these hot spots appeared to be originating posterior to the scanner bed. We then examined the PET gantry thoroughly and found an isolated cap of an intravenous cannula accidentally dislodged on the camera gantry near the protective covering of the collimator. The multiple equidistant hot spots in a linear fashion appeared to be caused by this cap of the intravenous cannula, which was confirmed on reacquiring the scan after removing the cap, hence suggesting faulty normalization. Thus, we need to ensure an absolutely clean surface over the gantry, as even the tiniest of objects can create large interferences in interpretation.

As is known normalization is a correction done to eliminate differences in sensitivity between detectors (or detector pairs). This correction in PET is obtained from a recording that provides a uniform irradiation of all detectors, which could be by a rotating pin source or a uniform cylinder phantom. Normalization scans should reflect the underlying performance of the detector elements of the PET scanner. When a normalization scan is being performed, one must ensure that no objects other than the recommended source of radiation are present in the field of view (FOV) of the scanner. Normalization scans should be performed on a regular basis (preferably every quarter) or when detectors or signal processing boards are tuned or replaced.[5]

Case 2

In our second case, on the acquired PET-CT scan images, we observed on the CT images linear dotted artifacts on sagittal and coronal sections [Figure 2a and 2b], which appeared as concentric thin ring shape densities on axial images [Figure 2c].

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Figure 2 Computed tomography (CT) scan images of positron emission tomography-CT scan: Artifact (marked by yellow arrows) seen as linear dotted artifacts on coronal (a) and sagittal sections (b), which appeared as concentric ring shape densities on axial sections (c)

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After analyzing the artifact, we probed into the gantry and software. Preliminarily, it was thought to be a fault in the hardware. However, after careful assessment, we found a spillage of intravenous contrast on the Mylar window and a small amount even over the CT tube aperture. We cleaned the entire spillage area and reacquired the scan to see if all the artifacts had disappeared; hence, we concluded that the spillage of contrast was the cause of this artifact. Proper fitting of the Mylar window and proper care while handling contrast to avoid any spillage can prevent this occurrence.

It has been seen in literature that rings artifacts could appear in transaxial images of computer tomography due to a defective detector element or set of detector elements, which can be eliminated by replacement of the defective CT detector module followed by CT number calibration.[6]

Another cause reported for ring artifacts on transaxial CT images can be due to loose electronic contact, which could be avoided by maintaining strict environmental conditions prescribed by the manufacturer.[7]

Case 3

In the third case, we have two different scan images, of a planar three-phase bone scan and myocardial perfusion single-photon emission computed tomography (SPECT) scan, on a gamma camera where we observe a similar artifact, an irregular significantly hot area within FOV which was seen intermittently during the scan [Figure 3a and b]. After ruling out all possibilities of contamination, we further explored the possibility of a fault with gamma camera hardware or software.

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Figure 3 Artifact appears as intermittent hot spots (blue arrow) in Three-phase bone scan perfusion phase image (a) and Artifacts appears as intermittent hot spots (white arrow) in myocardial perfusion single-photon emission computed tomography image (b)

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Further analysis revealed a fault in the power distribution and organization control (PDOC) board [Figure 4]. The PDOC board includes positioning and summing circuits, and its function is to sort out the data from Photomultiplier tubes (PMT) and provide the x and y events and z energy information. Unfortunately, this artifact, although rare, is unavoidable, but whenever identified, it must raise suspicion of PDOC board flaw and be reported so that it can be repaired/replaced.

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Figure 4 Power distribution and organization control board

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Ideally, artifacts should be prevented and, if they are not, should be identified at the time of imaging or reporting. Artifacts in nuclear imaging can be identified in many ways, not only by the reporting specialist physician but also by other staff involved. Few artifacts can be completely avoided with the help of proper precautions, awareness, and routine mandatory check-ups, while a few others may not be under the user’s control.

In PET-CT and SPECT-CT scans, we come across multiple artifacts due to high attenuation bodies, truncation, respiratory motion, and misregistration, for which multiple software is now available to decrease these artifacts.[89] Artifacts due to intravenous cannulas can occur due to radiopharmaceutical residues in the catheter lumen or venous system, and proper flushing with saline after administration is good practice to prevent them. Similarly, urinary catheter tubes and bags, and nephrostomy bags should be properly positioned away from the interpretable FOV. Metallic objects such as buttons, coins, keys, and even internal metallic devices/prostheses such as a pacemaker, fixator rods and screws, metallic plates, and previously ingested barium contrast can cause attenuation artifacts. Thus, proper instructions to remove all possible metallic objects should be given to the patient, and reporting physician should be informed about the presence of other interfering metallic objects that cannot be removed. Contamination artifacts, most commonly urine contamination artifacts, are seen on clothing and skin, which can easily be confirmed and eliminated or corrected by washing skin, removing contaminated clothing, or different image views when required.[2]

Physicians and technologists, as well as paramedical staff, together share the responsibility of providing quality nuclear medicine service to our patients. Good compliance with guidelines and protocols and good communication among medical team members make for the best outcomes.[1]

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.