In general, the calculation of parametric images in PET studies of neuroreceptors is based on dynamic data which have been recorded over many minutes. It is essential that the subject's head remains unmoved during the PET scan. Otherwise the data may become useless in the worst case. Although this problem is obvious, its impact on the parametric images has not been examined in detail. In the work presented here we study the degrading of parametric images caused by head movements and improvements which can be achieved by an appropriate motion correction.
With the increase of scanner resolution head motion in PET brain studies becomes an increasingly serious limitation. Methods to correct for motion have been proposed. In this work the realisation of a motion tracking system in a PET environment and the motion correction of list mode data with the MAF method is presented. In a phantom study the method is validated and the loss in image quality is documented in a phantom with simulated movements. The relevance of motion correction for patient data above the level of system resolution is studied. In a real patient study we show the effect of motion and the applicability of the presented system.
Abstract Purpose: [18F]Galacto-RGD has been developed for positron emission tomography (PET)–imaging of αvβ3 expression, a receptor involved in angiogenesis and metastasis. Our aim was to study the feasibility of PET imaging with [18F]Galacto-RGD in patients with squamous cell carcinoma of the head and neck (SCCHN). Experimental Design: Eleven patients with primary diagnosis of SCCHN were examined. After injection of 140 to 200 MBq [18F]Galacto-RGD, static emission scans 60 min post injection from the head to the abdomen (n = 11) and dynamic scans >60 min covering the tumor region (n = 6) for kinetic modeling were acquired. Standardized uptake values (SUV) were measured in tumors, muscle and oral mucosa. Immunohistochemistry was done using an αvβ3-specific antibody (n = 7). Image fusion with magnetic resonance imaging and/or computed tomography (CT) scans (n = 8) and calculation of tumor subvolumes based on SUVs was done using the iPlan software (BrainLAB). Results: [18F]Galacto-RGD PET identified 10 of 12 tumors, with SUVs ranging from 2.2 to 5.8 (mean, 3.4 ± 1.2). Two tumors <5 mm were missed. Tumor/blood and tumor/muscle ratios were 2.8 ± 1.1 and 5.5 ± 1.6, respectively. Tumor kinetics was consistent with a two-tissue compartmental model with reversible specific binding. Immunohistochemistry confirmed αvβ3 expression in all tumors with αvβ3 being located on the microvessels in all specimens and additionally on tumor cells in one specimen. Image fusion of [18F]Galacto-RGD PET with magnetic resonance imaging/multislice CT and definition of tumor subvolumes was feasible in all cases. Conclusions: [18F]Galacto-RGD PET allows for specific imaging of αvβ3 expression in SCCHN with good contrast. Image fusion and definition of tumor subvolumes is feasible. This technique might be used for the assessment of angiogenesis and for planning and response evaluation of αvβ3-targeted therapies.
Head movements during PET studies of cerebral neuroreceptors with positron emission tomography (PET), which are often recorded as dynamic studies over periods ranging from one to two hours, do not only lead to blurred images, but, by distorting pixel time-activity curves, may also seriously disturb the kinetic analysis. Here we report on the effect of head motion on parametric images of the distribution volume ratio (DVR) as well as on the elimination of artefacts, if the dynamic PET data are corrected for head movements. For this purpose we utilized six PET studies done with the 5HT2A-receptor ligand [18F]-altanserin. Prior to the tracer injection a transmission scan of 10 min was recorded for measured attenuation correction. During the PET scan, which was acquired in listmode for 1 h, the position of the head was monitored by a Polaris infrared motion tracking system. The listmode data were sorted into 42 time frames between 10 s and 2 min in duration. A time frame consists of 63 images of 128 128 voxels with a voxel size of 2 mm 2 mm 2.43 mm. The motion correction used the multiple acquisition frame (MAF) approach, which calculates individual attenuation files for each emission frame and its corresponding head position to avoid a misalignment between transmission and emission data. After reconstruction of attenuation corrected emission frames each image frame was realigned to match the head position of the first emission frame. Both the motion corrected and not corrected dynamic images were evaluated by the non-invasive Logan plot method to obtain parametric images of DVR. In addition, a dynamic [18F]-altanserin PET scan was simulated and affected by similar movements as seen in the human studies. In this way data without statistical noise could be analysed. DVR images of motion-affected [18F]-altanserin scans showed artefacts whose extent was dependent on the amount of movement. The artefacts were mainly located at the border of the cortical tissue, especially at the interior edge towards white matter. The artefacts exhibited as discontinuities and small spots, whose values exceeded the expected DVR values or were even negative. The discontinuities were found with movements of 4 mm and greater. Isolated spots were present even with movements of only 2 mm. The artefacts disappeared when the MAF based motion correction was applied. The observations obtained in human data could be confirmed in the simulated noise free [18F]-altanserin images confirming that the artefacts are due to motion and not to statistical noise. Whereas the native PET images look just blurred, if the patient has moved during the PET scan, parametric images of the Logan DVR, which are calculated by pixel-wise linear regression, contain severe discontinuities primarily at the cortical edge. At this location, the data used in the DVR calculation change between grey and white matter data because of the head motion. The MAF based head motion correction is able to avoid the described errors.
