Integrated PET/MR imaging: automatic attenuation correction of flexible RF coils.

PURPOSE: Flexible radiofrequency (RF) surface coils used in simultaneous PET/MR imaging are currently disregarded in PET attenuation correction (AC) since their position and individual geometry are unknown in whole-body patient scans. The attenuation of PET emission data due to the presence of RF surface coils has been investigated by several research groups but so far no automatic approach for the incorporation of RF surface coils into PET AC has been described. In this work, an algorithm is presented and evaluated which automatically determines the position of multiple RF surface coils and corrects for their attenuation of the PET emission data. METHODS: The presented algorithm nonrigidly registers pre-acquired CT-based three-dimensional attenuation templates of RF surface coils into attenuation maps used for PET AC. Transformation parameters are obtained by nonrigid B-spline landmark registration of marker positions in the CT-based attenuation templates of the RF surface coils to marker positions in the current MR images of the patient. The use of different marker patterns enables the registration algorithm to distinguish multiple partly overlapping RF surface coils. To evaluate the registration algorithm, two different PET emission scans of a NEMA standard body phantom with six active lesions and of a large rectangular body phantom were performed on an integrated whole-body PET/MR scanner. The phantoms were scanned with and without one (NEMA phantom scan) or three (large body phantom scan) flexible six-channel RF surface coils placed on top. Additionally, the accuracy and performance of the algorithm were evaluated on volunteer scans (n=5) and on a patient scan using a typical clinical setup of three RF surface coils. RESULTS: Overall loss of true counts due to the presence of the RF surface coils was 5.1% for the NEMA phantom, 3.6% for the large body phantom, and 2.1% for the patient scan. Considerable local underestimation of measured activity concentration up to 15.4% in the top part of the phantoms and 15.5% for a lesion near the body surface of the patient was measured close to the high attenuating hardware components of the RF coils. The attenuation maps generated by the registration algorithm reduced the quantification errors due to the RF surface coils to values ranging from -3.9% to 4.3%. Concerning the volunteer examinations, the attenuation templates of the three RF surface coils were registered to their correct positions with an overall accuracy of about 3 mm. CONCLUSIONS: The presence of flexible RF surface coils leads to considerable local errors in the simultaneously measured PET activity concentration up to 15.5% especially in regions close to the coils. The presented automatic algorithm accurately and reliably reduces the PET quantification errors caused by multiple partly overlapping flexible RF surface coils to values of 4.3% or better.

Toward simultaneous PET/MR breast imaging: systematic evaluation and integration of a radiofrequency breast coil.

PURPOSE: With the recent introduction of integrated whole-body hybrid positron emission tomography/magnetic resonance (PET/MR) scanners, simultaneous PET/MR breast imaging appears to be a potentially attractive new clinical application. In this study, the technical groundwork toward performing simultaneous PET/MR breast imaging was developed and systematically evaluated in phantom experiments and breast cancer patient hybrid imaging. METHODS: Measurements were performed on a state-of-the-art whole-body simultaneous PET/MR system (Biograph mMR, Siemens AG, Erlangen, Germany). The PET signal attenuating effects of a MR-only four-channel radiofrequency (RF) breast coil that is present in the PET field-of-view (FoV) during a simultaneous PET/MR data acquisition has been investigated and quantified. For this purpose, a dedicated PET/MR visible breast phantom featuring four modular inserts with various structures (no insert, MR insert, PET insert, and PET/MR insert) was developed. In addition to a systematic evaluation of MR-only image quality, the following phantom scans were performed using (18)F radio tracer: (1) PET emission scan with only the homogeneous breast phantom; (2) PET emission scan additionally with the RF breast coil in the PET FoV. Attenuation correction (AC) of PET data was performed with CT-based three-dimensional (3D) hardware attenuation maps (µ-maps) of the RF coil and breast phantom. Finally, a simultaneous PET/MR breast imaging was performed in two breast cancer patients. RESULTS: The modular breast phantom allowed for systematic evaluation of various MR, PET, and PET/MR image quality parameters. The RF breast coil provided MR images of good image quality, unaffected by PET imaging. The global attenuation of the RF breast coil on the PET emission data was approximately 11%. This hardware attributed PET signal attenuation was successfully corrected by using an appropriate CT-based 3D µ-map of the RF breast coil. Imaging of two breast cancer patients confirmed the successful integration of the RF breast coil into the concept of simultaneous PET/MR breast imaging. CONCLUSIONS: The successful integration of a four-channel RF breast coil with a defined table position together with the CT-based µ-maps provides a technical basis for future clinical PET/MR breast imaging applications.

Magnetic resonance-based attenuation correction for PET/MR hybrid imaging using continuous valued attenuation maps.

OBJECTIVES: Attenuation correction of positron emission tomographic (PET) data is critical in providing accurate and quantitative PET volumes. Deriving an attenuation map (µ-map) from magnetic resonance (MR) volumes is a challenge in PET/MR hybrid imaging. The difficulty lies in differentiating cortical bone from air from standard MR sequences because both these classes yield little to no MR signal and thus shows no distinguishable information. The objective of this contribution is 2-fold: (1) to generate and evaluate a continuous valued computed tomography (CT)-like attenuation map (µ-map) with continuous density values from dedicated MR sequences and (2) to compare its PET quantification accuracy with respect to a CT-based attenuation map as the criterion standard and other segmentation-based attenuation maps for studies of the head. MATERIALS AND METHODS: Three-dimensional Dixon-volume interpolated breath-hold examination and ultrashort echo time sequences were acquired for each patient on a Siemens 3-T Biograph mMR PET/MR hybrid system and the corresponding patient CT on a Siemens Biograph 64. A pseudo-CT training was done using the epsilon-insensitive support vector regression ([Latin Small Letter Open E]-SVR) technique on 5 patients who had CT/MR/PET triplets, and the generated model was evaluated on 5 additional patients who were not included in the training process. Four µ-maps were compared, and 3 of them derived from CT: scaled CT (µ-map CT), 3-class segmented CT without cortical bone (µ-map no bone), 4-class segmented CT with cortical bone (µ-map bone), and 1 from MR sequences via [Latin Small Letter Open E]-SVR technique previously mentioned (ie, MR predicted [µ-map MR]). Positron emission tomographic volumes with each of the previously mentioned µ-maps were reconstructed, and relative difference images were calculated with respect to µ-map CT as the criterion standard. RESULTS: For PET quantification, the proposed method yields a mean (SD) absolute error of 2.40% (3.69%) and 2.16% (1.77%) for the complete brain and the regions close to the cortical bone, respectively. In contrast, PET using µ-map no bone yielded 10.15% (3.31%) and 11.03 (2.26%) for the same, although PET using µ-map bone resulted in errors of 3.96% (3.71%) and 4.22% (3.91%). Furthermore, it is shown that the model can be extended to predict pseudo-CTs for other anatomical regions on the basis of only MR information. CONCLUSIONS: In this study, the generation of continuous valued attenuation maps from MR sequences is demonstrated and its effect on PET quantification is evaluated in comparison with segmentation-based µ-maps. A less-than-2-minute acquisition time makes the proposed approach promising for a clinical application for studies of the head. However, further experiments are required to validate and evaluate this technique for attenuation correction in other regions of the body.

Comparison of lesion detection and quantitation of tracer uptake between PET from a simultaneously acquiring whole-body PET/MR hybrid scanner and PET from PET/CT.

PURPOSE: PET/MR hybrid scanners have recently been introduced, but not yet validated. The aim of this study was to compare the PET components of a PET/CT hybrid system and of a simultaneous whole-body PET/MR hybrid system with regard to reproducibility of lesion detection and quantitation of tracer uptake. METHODS: A total of 46 patients underwent a whole-body PET/CT scan 1 h after injection and an average of 88 min later a second scan using a hybrid PET/MR system. The radioactive tracers used were (18)F-deoxyglucose (FDG), (18)F-ethylcholine (FEC) and (68)Ga-DOTATATE (Ga-DOTATATE). The PET images from PET/CT (PET(CT)) and from PET/MR (PET(MR)) were analysed for tracer-positive lesions. Regional tracer uptake in these foci was quantified using volumes of interest, and maximal and average standardized uptake values (SUV(max) and SUV(avg), respectively) were calculated. RESULTS: Of the 46 patients, 43 were eligible for comparison and statistical analysis. All lesions except one identified by PET(CT) were identified by PET(MR) (99.2 %). In 38 patients (88.4 %), the same number of foci were identified by PET(CT) and by PET(MR). In four patients, more lesions were identified by PET(MR) than by PET(CT), in one patient PET(CT) revealed an additional focus compared to PET(MR). The mean SUV(max) and SUV(avg) of all lesions determined by PET(MR) were by 21 % and 11 % lower, respectively, than the values determined by PET(CT) (p < 0.05), and a strong correlation between these variables was identified (Spearman rho 0.835; p < 0.01). CONCLUSION: PET/MR showed equivalent performance in terms of qualitative lesion detection to PET/CT. The differences demonstrated in quantitation of tracer uptake between PET(CT) and PET(MR) were minor, but statistically significant. Nevertheless, a more detailed study of the quantitative accuracy of PET(MR) and the factors governing it is needed to ultimately assess its accuracy in measuring tissue tracer concentrations.