Alternative method to generate a complete µ-map in PET-MR breast imaging.

Objectives: Attenuation correction (AC) is necessary for data quantification in PET imaging. PET reconstructions without AC appear to show areas of low attenuation (patient surface and lungs) as having increased uptake. Transitionally, a transmission-based AC map (µ-map), obtained from rotating rod sources or through CT, is created and utilized during the PET emission data reconstruction. These methods are unavailable in PET/MR imaging. Instead, MR imaging sequences (DIXON) are utilized to generate the required µ-map. In MR imaging voxel intensities decrease as their distance from the radio frequency coil increases. Previous experiments demonstrated the creation of an incomplete µ-map if only the breast coils was utilized (per clinical diagnostic breast MR protocol). A complete µ-map was generated when an additional body matrix coil was placed on the subject’s back. But, the additional coil limits the size of the subject that can be imaged due MR bore clearance issues. Siemens mMR scientists proposed and modified the DIXON based MRAC sequence (permit multiple averages) to allow for creation of complete µ-maps utilizing only the internal body coil. The aim of this project is to validate the creation and application of µ-maps generated with only the internal body coil in breast PET-MR imaging.

Methods: A volunteer was initially imaged utilizing a DIXON sequence with only a breast coil. A second scan was then completed utilizing both the breast coil and a surface matrix on the subject’s back. A third scan was completed utilizing the internal body coil only. Saline bags were utilized to simulate breast tissue. To simulate lesions in a patient, conical 1.5 mL vials containing a clinically relevant concentration of F-18 FDG were placed in the mediastinal and central breast regions of the volunteer. Standardized uptake values (SUVs) were obtained from regions of interest drawn around the mediastinal and central breast sources for each generated µ-map and PET reconstruction - breast coil only (incomplete), breast and body matrix coils(complete) and internal body coil (complete).

Results: Please see Table 1.

Conclusions: In PET/MR imaging, care must be taken to ensure that the DIXON sequence has adequate MR signal to generate complete µ-maps. Incomplete µ-maps result in PET reconstructions having uncompensated PET counting losses, inaccurate quantification, and underestimated SUVs. Although the internal body coil only sequence generated a complete µ-map, the approximate 20% lower SUV_Max compared to the breast coil and body matrix coil µ-maps reconstructions, it does not to appear to be a viable alternative to the latter and requires further investigation. Inspection of the internal body coil µ-map reveals a shift in the segmented attenuation coefficients across the µ-map, most notably the shift of simulated breast tissue from water to fat. Further investigation and refinement of internal body coil derived µ-map technique will be required.