Quantification of repositioning errors in PET-studies through superposition of emission and transmission scan - A comparison between one-step and two-step acquisitions

Aim: The purpose of this study was to quantify positioning discrepancies be tween emission (E) and transmission (T) scan using image fusion. A direct c omparison of one-step and two-step acquisitions was performed where ail stu dies were analyzed in respect to artifacts caused by inaccurate positioning . In addition, phantom measurements were conducted to estimate the conseque nces of repositioning errors on standardized uptake value calculations (SUV ). Methods: 40 patients were examined by two-step whole-body scans using PE T and 15 patients were subject to one-step examinations in the head/neck ar ea. Repositioning between the scans was achieved by a laser matrix position ing system in combination with external body markings, After reconstruction and image fusion of the scans, the positioning discrepancies were measured as the distances between the outer boundaries of E and T in four different body regions. Additional evaluations of the SUV by increasing E-T dislocat ion were performed using a Jaszczak phantom containing hollow spheres. Resu lts: For the two-step acquisitions, the mean spatial deviations along the t hree orthogonal axes x, y, and z were between 8.9 mm and 13.8 mm, whereas f or the one-step examinations mean values between 3.5 mm and 4.3 mm were det ermined (level of significance in each direction p <0.0001). Artifacts were found in 47.5% of the whole body scans, but in none of the head/neck studi es. The development of image artifacts was simulated by phantom studies. In contrast, the deviations of the computed SUV caused by increasing position ing discrepancies were minimal because of the minimal differences between t he attenuation coefficients of the media involved. Conclusion: The presente d data show that an artifactfree reconstruction of attenuation-corrected st udies requires a precise positioning of the patient. One-step examination p rotocols without repositioning are advantageous due to the significantly lo wer positioning discrepancies. The additional reconstruction of nonattenuat ion-corrected studies has proven to be useful in discovering image artifact s and is therefore recommended.