Toward implementing an MRI-based PET attenuation-correction method for neurologic studies on the MR-PET brain prototype

Abstract

Several factors have to be considered for implementing an accurate attenuation-correction (AC) method in a combined MR-PET scanner. In this work, some of these challenges were investigated, and an AC method based entirely on the MRI data obtained with a single dedicated sequence was developed and used for neurologic studies performed with the MR-PET human brain scanner prototype.

Methods: The focus was on the problem of bone-air segmentation, selection of the linear attenuation coefficient for bone, and positioning of the radiofrequency coil. The impact of these factors on PET data quantification was studied in simulations and experimental measurements performed on the combined MR-PET scanner. A novel dual-echo ultrashort echo time (DUTE) MRI sequence was proposed for head imaging. Simultaneous MR-PET data were acquired, and the PET images reconstructed using the proposed DUTE MRI-based AC method were compared with the PET images that had been reconstructed using a CT-based AC method.

Results: Our data suggest that incorrectly accounting for the bone tissue attenuation can lead to large underestimations (>20%) of the radiotracer concentration in the cortex. Assigning a linear attenuation coefficient of 0.143 or 0.151 cm(-1) to bone tissue appears to give the best trade-off between bias and variability in the resulting images. Not identifying the internal air cavities introduces large overestimations (>20%) in adjacent structures. On the basis of these results, the segmented CT AC method was established as the silver standard for the segmented MRI-based AC method. For an integrated MR-PET scanner, in particular, ignoring the radiofrequency coil attenuation can cause large underestimations (i.e., <or=50%) in the reconstructed images. Furthermore, the coil location in the PET field of view has to be accurately known. High-quality bone-air segmentation can be performed using the DUTE data. The PET images obtained using the DUTE MRI- and CT-based AC methods compare favorably in most of the brain structures.

Conclusion: A DUTE MRI-based AC method considering all these factors was implemented. Preliminary results suggest that this method could potentially be as accurate as the segmented CT method and could be used for quantitative neurologic MR-PET studies.

FIGURE 1

Representative slices from the simulated datasets obtained from co-registered MR-CT data – left to right: segmented head structures (brain structures and bone tissues obtained from MR and CT, respectively), simulated emission data and μ-maps (CT_scaled, CT_segmented, MR_first and MR_second, see text for description).

FIGURE 2

Subject related factors affecting the PET data quantification: (A) Effects of bone tissue segmentationand (B) bone LAC selection. Representative images of the RCs are presented for each case.

FIGURE 3

Coil related factors affecting the PET data quantification of the simulated cylinder: CT of the RF coil (top left); Representative images of the RCs observed when the coil attenuation is ignored (top right) or inaccurately accounted for (bottom row).

FIGURE 4

Bone MR imaging using single echo UTE sequences: (A) Representative images obtained in a healthy volunteer: UTE₁ (left) and UTE₂ (right); (B) Human skull imaging - MR DUTE₁ image (approximately1 mm isotropic) (left) and CT scan (200 μm isotropic) (right). The MR image also shows the location of RF coil elements relative to skull.

FIGURE 5

Proof-of-principle DUTE-based AC in the integrated MR-PET scanner: (A) μ-maps obtained from the three tissue classes segmentation based on the CT (left) and DUTE (right) data; (B) PET images reconstructed using the CT- (left) and DUTE-based (middle) AC methods and RC images between the two methods (right). The images at two representative locations are shown in each case.

FIGURE 6

PET images reconstructed ignoring (middle column) and including (right column) the RF coil attenuation. Representative slices in two orientations are shown in each case. The corresponding slices of the complete attenuation map including the coil are shown in the left column.