Abstract
ObjectivesCombined positron emission tomography (PET) and magnetic resonance imaging (MRI) targeting the prostate-specific membrane antigen (PSMA) with a 68Ga-labelled PSMA-analog (68Ga-PSMA-11) is discussed as a promising diagnostic method for patients with suspicion or history of prostate cancer. One potential drawback of this method are severe photopenic (halo-) artifacts surrounding the bladder and the kidneys in the scatter-corrected PET images, which have been reported to occur frequently in clinical practice. The goal of this work was to investigate the occurrence and impact of these artifacts and, secondly, to evaluate variants of the standard scatter correction method with regard to halo-artifact suppression.MethodsExperiments using a dedicated pelvis phantom were conducted to investigate whether the halo-artifact is modality-, tracer-, and/or concentration-dependent. Furthermore, 31 patients with history of prostate cancer were selected from an ongoing 68Ga-PSMA-11-PET/MRI study. For each patient, PET raw data were reconstructed employing six different variants of PET scatter correction: absolute scatter scaling, relative scatter scaling, and relative scatter scaling combined with prompt gamma correction, each of which was combined with a maximum scatter fraction (MaxSF) of MaxSF = 75% or MaxSF = 40%. Evaluation of the reconstructed images with regard to halo-artifact suppression was performed both quantitatively using statistical analysis and qualitatively by two independent readers.ResultsThe phantom experiments did not reveal any modality-dependency (PET/MRI vs. PET/CT) or tracer-dependency (68Ga vs. 18F-FDG). Patient- and phantom-based data indicated that halo-artifacts derive from high organ-to-background activity ratios (OBR) between bladder/kidneys and surrounding soft tissue, with a positive correlation between OBR and halo size. Comparing different variants of scatter correction, reducing the maximum scatter fraction from the default value MaxSF = 75% to MaxSF = 40% was found to efficiently suppress halo-artifacts in both phantom and patient data. In 1 of 31 patients, reducing the maximum scatter fraction provided new PET-based information changing the patient’s diagnosis.ConclusionHalo-artifacts are particularly observed for 68Ga-PSMA-11-PET/MRI due to 1) the biodistribution of the PSMA-11-tracer resulting in large OBRs for bladder and kidneys and 2) inaccurate scatter correction methods currently used in clinical routine, which tend to overestimate the scatter contribution. If not compensated for, 68Ga-PSMA-11 uptake pathologies may be masked by halo-artifacts leading to false-negative diagnoses. Reducing the maximum scatter fraction was found to efficiently suppress halo-artifacts.
Highlights
Since the prostate-specific membrane antigen (PSMA) shows substantially increased expression in primary and recurrent prostate cancer cells [1,2], positron emission tomography (PET) targeting PSMA has been proposed for sensitive imaging of recurrent prostate cancer [3]
Halo-artifacts are observed for 68Ga-PSMA-11-PET/magnetic resonance imaging (MRI) due to 1) the biodistribution of the PSMA-11-tracer resulting in large organ-to-background activity ratios (OBRs) for bladder and kidneys and 2) inaccurate scatter correction methods currently used in clinical routine, which tend to overestimate the scatter contribution
If not compensated for, 68Ga-PSMA-11 uptake pathologies may be masked by halo-artifacts leading to false-negative diagnoses
Summary
Since the prostate-specific membrane antigen (PSMA) shows substantially increased expression in primary and recurrent prostate cancer cells [1,2], positron emission tomography (PET) targeting PSMA has been proposed for sensitive imaging of recurrent prostate cancer [3]. Large organ-to-background activity ratios (OBRs) sometimes result in photopenic artifacts surrounding the kidneys and the bladder in both PET/CT [8,15,16] and, in particular, PET/MRI [11,13,17] These so-called halo-artifacts pose one potential drawback of the clinical use of 68Ga-PSMA-11-PET/MRI for prostate cancer detection and staging. Both primary prostate cancer and local recurrences after radical prostatectomy are typically located very close to the bladder and any photopenic artifact surrounding the bladder could potentially mask findings, falsify uptake values and influence the patients’ diagnosis. Retroperitoneally between the kidneys, the halo may, potentially, mask metastases and impair tumor detectability and staging
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