Markerless attenuation correction for carotid MRI surface receiver coils in combined PET/MR imaging

Abstract

The purpose of the study was to evaluate the effect of attenuation of MR coils on quantitative carotid PET/MR exams. Additionally, an automated attenuation correction method for flexible carotid MR coils was developed and evaluated. The attenuation of the carotid coil was measured by imaging a uniform water phantom injected with 37 MBq of 18F-FDG in a combined PET/MR scanner for 24 min with and without the coil. In the same session, an ultra-short echo time (UTE) image of the coil on top of the phantom was acquired. Using a combination of rigid and non-rigid registration, a CT-based attenuation map was registered to the UTE image of the coil for attenuation and scatter correction. After phantom validation, the effect of the carotid coil attenuation and the attenuation correction method were evaluated in five subjects. Phantom studies indicated that the overall loss of PET counts due to the coil was 6.3% with local region-of-interest (ROI) errors reaching up to 18.8%. Our registration method to correct for attenuation from the coil decreased the global error and local error (ROI) to 0.8% and 3.8%, respectively. The proposed registration method accurately captured the location and shape of the coil with a maximum spatial error of 2.6 mm. Quantitative analysis in human studies correlated with the phantom findings, but was dependent on the size of the ROI used in the analysis. MR coils result in significant error in PET quantification and thus attenuation correction is needed. The proposed strategy provides an operator-free method for attenuation and scatter correction for a flexible MRI carotid surface coil for routine clinical use.

Fig 1

Images of the coil flat and bent.

Fig 2

Top panel: a) first echo UTE image and b) the corresponding CT image of the carotid coil on a cylindrical water phantom showing the correspondence between the two images. Bottom panel: c) axial and d) Sagittal views of the first echo image in a human study showing the coil highlighted by the red arrows.

Fig 3

Flow chart for the registration algorithm. After the UTE acquisition, the coil was segmented by using a mask generated by thresholding and morphological closing of the 1^st echo UTE image. The resultant image was then split at the center of mass for subsequent rigid and non-rigid registration of the attenuation map of each side of coil separately. The registration procedure was repeated for the other side of coil and then added to the system generated attenuation map to reconstruct the final fully corrected PET image.

Fig 4

A) Sample plane across the attenuation map of the carotid coil. The plastic housing around the coil is the most attenuating part of the coil followed by the metallic components and the foam. B) Corresponding image of the coil. Scale bar is 1 cm.

Fig 5

A) Image of the phantom acquisition setup on the scanner. B) A close-up image to show the placement of the markers used in the evaluation of the registration accuracy. C) Plot of the mean activity within a ROI over all axial planes that contained the phantom in the PET image.

Fig 6

Sample line profile across the neck of the patient showing the effect of attenuation (with coil- no AC) as well as after attenuation correction with the proposed method (with coil- CT AC) compared to the no coil scan, which serves as ground truth. The insert shows the location of the line profile (A). Bar plot showing measured error for the large ROI around the neck (B) and the 2 cm ROI around the left and right carotid (C).