Computed diffusion weighted imaging (cDWI) and voxelwise-computed diffusion weighted imaging (vcDWI) for oncologic liver imaging: A pilot study

Abstract

Objective: Aim of the study was to evaluate the influence of the selection of measured b-values on the precision of cDWI in the upper abdomen as well as on the lesion contrast of PET-positive liver metastases in cDWI and vcDWI.

Methods: We performed a retrospective analysis of 10 patients (4 m, 63.5 ± 12.9 y/o) with PET-positive liver metastases examined in 3 T-PET/MRI with b = 100,600,800,1000 and 1500s/mm². cDWI (cb1000/cb1500) and vcDWI were computed based on following combinations: i) b = 100/600 s/mm², ii) b = 100/800 s/mm², iii) b = 100/1000s/mm², iv) b = 100/600/1000s/mm² v) all measured b-values. Mean signal intensity (SI) and standard deviation (SD) in the liver, spleen, kidney, bone marrow and in liver lesions were acquired. The coefficient of variation (CV = SD/SI), the differences of SI between measured and calculated high b-value images and the lesion contrast (SI lesion/liver) were computed.

Results: With increasing upper measured b-values, the CV in cDWI and vcDWI decreased (CV in the liver in cb1500: 0.42 with b100/600 s/mm² and 0.28 with b100/b1000s/mm²) while the differences of measured and calculated b-value images decreased (in the liver in cb1500: 30.7% with b = 100/600 s/mm², 19.7% with b100/b1000s/mm²). In diffusion-restricted lesions, lesion contrast was at least 1.6 in cb1000 and 1.4 in cb1500, respectively, with an upper measured b-value of b = 800 s/mm² and 2.1 for vcDWI with an upper measured b-value of b = 1000s/mm². Overall, the lesion contrast was superior in cb1500 and vcDWI compared to cb1000 (15% and 11%, respectively).

Conclusion: Measuring higher upper b-values seems to lead to more precise computed high b-value images and a decrease of CV. vcDWI provides a comparable lesion contrast to b = 1500s/mm² and offers additionally the reduction of T2 shine-through effects. For vcDWI, measuring b = 1000s/mm² as upper b-value seems to be necessary to guarantee good lesion visibility in the liver based on our preliminary results.

Fig. 1

Whisker box plots including median, first and third quartile, maximum and minimum of lesion contrast of measured and calculated b-values as well as in vcDWI. The mean lesion contrasts of cb1500 and vcDWI seem to be on a comparable level. However, highest variances in computed b-value images as well as in vcDWI are seen with b = 100/600, resulting in values partially close to zero.

Fig. 2

Mean signal intensities of measured and computed b-values as well as vcDWI in lesions and organs. Note the superior signal intensity in lesions in vcDWI compared to measured and calculated b1500. Physiological spleen tissue also shows high signal intensity caused by high cellularity.

Fig. 3

Coefficient of variation in physiological tissue and image background. Taking higher b-value images for the calculation of cb1000 and cb1500 decreases the covariance. Overall, vcDWI shows higher CV in physiological tissue; the high CV in vcDWI in kidneys and background are mainly caused by the very low signal intensity in those ROIs (see Fig. 2).

Fig. 4

Relative differences of the mean signal intensities between measured and computed b-value images in %. A slight trend towards decreasing deviations with increasing upper b-values is present while b = 100/1000 provides overall best results for most organs in cb1500 from the b-value combinations tested.

Fig. 5

Example of a 60 y/o male patient with melanoma and metastatic liver lesions. The larger lesion (white arrow) can clearly be seen in vcDWI as well as in computed and measured b-values (cb and mb, respectively); note the differences in lesion-to-liver contrast and image noise. One small lesion (dotted arrow) is visible in measured b-values (mb1000 and mb1500) but not in the calculated b-values (cb1000 and cb1500) based on b = 100/800. In vcDWI, this small lesion is clearly visible if the calculation is based on b = 100/1000; In contrast, the lesion is masked if the calculation is based on b = 100/800.

Fig. 6

Example of a 71 y/o female patient with 68Ga-PET-positive liver metastases of a neuroendocrine tumor. The patient had undergone a partial liver resection and chemotherapy. The lesion shows a distinct PET tracer uptake (top row, left hand side: PET overlaid with T2 TSE) which stands for a high expression of the somatostatin receptor (SSR). The lesion does not provide a considerable diffusion restriction which is a common finding in metastases of NET under therapy. Thus, the lesion is not distinguishable in b = 1500 images (mb1500/cb1500) or vcDWI. However, in b = 1000 images (mb1000/cb1000), the lesion is visible which can be explained by a T2-shine-through effect. All calculated b-values as well as the vcDWI were based on b = 100/800.