Attenuation correction for flexible magnetic resonance coils in combined magnetic resonance/positron emission tomography imaging

Abstract

Introduction: Attenuation correction for magnetic resonance (MR) coils is a new challenge that came about with the development of combined MR and positron emission tomography (PET) imaging. This task is difficult because such coils are not directly visible on either PET or MR acquisitions with current combined scanners and are therefore not easily localized in the field of view. This issue becomes more evident when trying to localize flexible MR coils (eg, cardiac or body matrix coil) that change position and shape from patient to patient and from one imaging session to another. In this study, we proposed a novel method to localize and correct for the attenuation and scatter of a flexible MR cardiac coil, using MR fiducial markers placed on the surface of the coil to allow for accurate registration of a template computed tomography (CT)-based attenuation map.

Materials and methods: To quantify the attenuation properties of the cardiac coil, a uniform cylindrical water phantom injected with 18F-fluorodeoxyglucose (18F-FDG) was imaged on a sequential MR/PET system with and without the flexible cardiac coil. After establishing the need to correct for the attenuation of the coil, we tested the feasibility of several methods to register a precomputed attenuation map to correct for the attenuation. To accomplish this, MR and CT visible markers were placed on the surface of the cardiac flexible coil. Using only the markers as a driver for registration, the CT image was registered to the reference image through a combination of rigid and deformable registration. The accuracy of several methods was compared for the deformable registration, including B-spline, thin-plate spline, elastic body spline, and volume spline. Finally, we validated our novel approach both in phantom and patient studies.

Results: The findings from the phantom experiments indicated that the presence of the coil resulted in a 10% reduction in measured 18F-FDG activity when compared with the phantom-only scan. Local underestimation reached 22% in regions of interest close to the coil. Various registration methods were tested, and the volume spline was deemed to be the most accurate, as measured by the Dice similarity metric. The results of our phantom experiments showed that the bias in the 18F-FDG quantification introduced by the presence of the coil could be reduced by using our registration method. An overestimation of only 1.9% of the overall activity for the phantom scan with the coil attenuation map was measured when compared with the baseline phantom scan without coil. A local overestimation of less than 3% was observed in the ROI analysis when using the proposed method to correct for the attenuation of the flexible cardiac coil. Quantitative results from the patient study agreed well with the phantom findings.

Conclusions: We presented and validated an accurate method to localize and register a CT-based attenuation map to correct for the attenuation and scatter of flexible MR coils. This method may be translated to clinical use to produce quantitatively accurate measurements with the use of flexible MR coils during MR/PET imaging.

FIGURE 1

A, Magnetic resonance image of the markers. B, Computed tomography scan of the coil showing the placement of the markers. C, Photograph of the bent coil. D, Line profile across the coil CT attenuation map before clipping the attenuation maps.

FIGURE 2

Region-of-interest analysis in PET images of the phantom-only scan (Phanton Only) or phantom with the coil scan without correction (With Coil -No AC) or with correction using the template attenuation map (With Coil - CT AC) plotted over all the axial planes. A, Large reduction in the activity of more than 20% in a 2.5-cm ROI placed at the top of the phantom. B, More than 5% reduction in activity was measured at the ROI placed at the bottom of the phantom. CT–based attenuation correction resulted in accurate correction.

FIGURE 3

Comparison between deformable registration methods. Overlap between the reference image (hot metal color map) and the maximum bend (A), B-spline (B), elastic body spline (C), thin-plate spline (D), V-spline (E), all in rainbow color map.

FIGURE 4

A, Attenuation map of the coil over the phantom after the rigid registration only. B, Plot of a profile across the phantom in emission images reconstructed using the attenuation map from (A).

FIGURE 5

Region-of-interest analysis in PET images of the phantom-only scan (Phantom Only) compared with that acquired with the coil present using the template CT attenuation map (With Coil - CT AC) or a registered coil attenuation map (With Coil - V-Spline). Using our registration technique can accurately correct for the coil’s attenuation as shown in the ROI analysis at the top ROI (A) and the bottom ROI (B).

FIGURE 6

Overlap between the MR image of a patient including the MR markers attached to the coil and the coil attenuation map. After the V-spline registration, the MR marker aligned with the markers on the coil attenuation map.

FIGURE 7

A profile was drawn 8 cm under the anterior surface of a patient comparing the acquisition done without the coil (No Coil Scan), the scan done with the coil but without attenuation correction (With Coil-No AC), or with attenuation correction using our registration technique (With Coil - V-Spline).