Thursday Case of the Day The arrow indicates a region of high apparent FDG uptake posterior to the bladder. What is the cause of this artifact? a) attenuation correction b) scatter correction c) reconstruction filter d) randoms correction History: A patient underwent a PET/CT study to investigate breast cancer Brad Kemp, Ph.D. Department of Radiology, Mayo Clinic, Rochester, MN Physics Figure 1: Transaxial IR CTAC image Scanning protocol: 15 mCi FDG, 65 min uptake 3D acquisition with 2 min/bed on a GE Discovery 690 PET/CT system Fully-3D iterative reconstruction (IR) with CT attenuation correction (CTAC), scatter correction and randoms from singles correction

Findings: The correct answer is a) This artifact is caused by the attenuation correction as there is a misregistration between PET and CT images. The motion of gas within the patient’s rectum between the CT and the PET images causes a mismatch in the attenuation distribution and a subsequent error in the attenuation correction. Note that this misregistration is due to internal organ motion and not gross patient motion. The motion of gas is apparent in the fused PET/CT image in Figure 2B. The PET images are overcorrected for attenuation in the region where the low-attenuation gas pocket was located for the PET acquisition. The IR NAC PET image shows symmetric perirectal activity (Figure 2C). Figure 2: (A) Transaxial IR CTAC PET image. (B) Transaxial fused PET/CT image. (C) Transaxial PET image with no attenuation correction (NAC). A B C

Lodge conducted an experiment to show that the artifact is caused by an attenuation correction error due to motion between CT and PET acquisitions [1]. See Figure 3 and 4. The results of the experimental suggest that the movement of gas within the rectum causes differences in the attenuation distribution between the CT and PET acquisitions. As such, the hot streak represents a region that is over-corrected for attenuation. Figure 4: Reconstructed PET images of the body phantom with increasing amounts of misregistration between PET and CT data. Hot and cold artifacts, as shown by the red arrows, increase as the misregistration increases. Discussion Figure 3: CT image showing body phantom with two cylindrical inserts. The upper insert contained radioactive water while the lower insert was a low attenuation medium. Red arrow shows direction of motion of PET distribution prior to attenuation correction.

Figure 5 shows images from a PET/CT exam in which the gas pocket within the rectum is stationary; the PET and CT images are properly registered and there is no error in the attenuation correction of the PET images. Discussion Figure 5: (A) Transaxial IR CTAC PET image. (B) Transaxial fused PET/CT image. (C) Transaxial IR NAC PET image. Reference Lodge MA, Muhammad AC, Udall DN, Wahl RL. Characterization of a perirectal artifact in 18 F-FDG PET/CT. J Nucl Med 2010;51: A B C