"Picture-in-Picture" Artifact: Introduction and Characterization of a Hitherto Unrecognized Imaging Artifact in Creating Perfusion Defects in Myocardial Perfusion Single-Photon Emission Computed Tomography

Abstract

Following a moving hot spot in the projections of raw images and profound perfusion defects in myocardial perfusion single-photon emission computed tomography (SPECT) imaging of a patient, a hypothesis was postulated that the perfusion defects were artifactual, and the high activity concentration of the gallbladder may be a culprit for this phenomenon, owing to flawed event positioning function of the gamma camera due to a malfunctioning digital event processor electronics board. To depict the characteristics of this artifact, a point source containing an activity of 3 mCi of pertechnetate is placed on the scanning table with the detector facing the table (at a distance of 30 cm), and then, in other detector positions and 1-min static images are acquired accordingly. The ratio is calculated as follows: count of the artifactual focus: 1860, count of the index focus: 705,727, and artifactual-to-index focus ratio: 0.003. In testing the uniformity of gamma camera based on the National Electrical Manufacturers Association protocol, a nonuniform response was detected, seemingly, a smaller field of view (FOV) is reproduced in the main FOV causing nonuniformity more than the acceptable level. The smaller flood image lies in the upper right corner of the main flood image. In essence, the extremely bright gallbladder was the source of error, and its image was reproduced in the FOV, which was superimposed on the left ventricular myocardium in some of the projections and was propagated to SPECT images.

Keywords: Gamma camera; imaging artifact; myocardial perfusion single-photon emission computed tomography; perfusion defects; picture-in-picture artifact.

Figure 1

Cinematic raw images (32 projections from the left posterior oblique to the right anterior oblique) (a) and corresponding single-photon emission computed tomography image (b) of the initial rest image. A bright hotspot (indicated by arrows) is visible in the lateral wall of the left ventricular myocardium roughly from projection 17–21. Then, over later projections, the hotspot moves gradually to eventually reside in the septal wall (projections 25–32). Single-photon emission computed tomography image shows some deformation of the left ventricular with the downscaling artifact as a result of the intense foci of uptake in the septum, more noticeably, and also basal lateral wall

Figure 2

Cinematic raw image (32 projections from the left posterior oblique to the right anterior oblique) (a) and corresponding single-photon emission computed tomography image (b) of the repeat rest image with the same acquisition and processing parameters after ingestion of a fatty meal to empty gallbladder. No hotspot is observed in the left ventricular myocardium or around the heart in raw images, and no perfusion defect is seen in single-photon emission computed tomography image

Figure 3

A static image of a point source placed on the scanning table. (a) A smaller spot with much lower brightness is visible above and right to the source. As a negative control, when the source is removed from the field of view, no artifact is detected (b). (c) Lateral views of the source obtained with the detector rotation (d) ROI drawn on point sources (e) count density profile. (f) Three-point sources are arranged in a triangular configuration and then imaged

Figure 4

(a) In the uniformity flood images, a frame-shaped pattern is seen, superimposed on the main image, which is shifted to the upper right corner, the characteristic pattern of this artifact or “picture-in-picture artifact.” As it can be seen, the results of integral uniformity and differential uniformity are far higher than the acceptable levels. (b) After replacing with a new properly functioning board, the results of uniformity tests (visual and quantitative) are acceptable