The rationale for proposing a magnetic resonance slow diffusion metric and its proof-of-concept testing showing spleen parenchyma and hepatocellular carcinoma have faster diffusion than liver parenchyma.

BACKGROUND

The liver and spleen have a similar amount of blood perfusion, and the spleen is waterier than the liver. The spleen tissue has a higher contrast-enhanced computed tomography (CT) extracellular volume fraction than the liver. The spleen has been reported to have a much lower apparent diffusion coefficient (ADC), intravoxel incoherent motion (IVIM)-perfusion fraction (PF), and IVIM-D than those of the liver, which appears to be unreasonable. As hepatocellular carcinomas (HCCs) are mostly associated with increased blood supply and increased proportion of arterial blood supply and with edema, HCC has been reported to have a lower ADC, lower IVIM-PF, and lower IVIM-D than liver parenchyma, which appears to be unreasonable. Cysts are known to have a longer T2 and a higher ADC than hemangiomas. Due to the 'flushing' of blood flow inside the hemangioma, we hypothesize that the actual diffusion of hemangioma liquid is faster than the more 'static' liquid of the cysts. As ADC measure is heavily affected by T2 and in order to minimize the T2 effect, we propose a new metric reflecting tissue slow diffusion which is termed 'slow diffusion coefficient (SDC)': SDC = [S( ) - S( )]/( - ), where and refers to a high -value (e.g., 400 s/mm for liver) and a higher -value (e.g., 600 s/mm) respectively, where S( ) and S( ) denote the corresponding diffusion weighted image signal intensity.

METHODS

This study utilized a random selection of authors' historical liver IVIM magnetic resonance imaging (MRI) data. For 1.5T data, SDC was calculated with =600 and 800 s/mm images. For 3.0T data, SDC was calculated with =400 and 600 s/mm images. With 1.5T data, SDC was calculated for 10 healthy volunteer cases' liver and spleen parenchyma, as well as two simple liver cysts and their corresponding liver parenchyma. With 3.0T data, SDC was calculated for 14 cases' liver and spleen parenchyma, as well as 13 HCC masses, 9 simple liver cysts, 13 hemangiomas and their corresponding liver parenchyma. As diffusion metrics can be only measured with MRI thus external validation is not possible, the measures of liver parenchyma were used to normalize the measures of spleen, HCC, cyst, and hemangioma, and the ratios were expressed in median value.

RESULTS

The median ratio of SDC/SDC was 2.47 for 1.5T data and 1.97 for 3.0T data. Two cysts at 1.5T had a median SDC/SDC ratio of 2.92. For 3.0T data, the median ratios of SDC/SDC, SDC/SDC, SDC/SDC were 2.83, 4.23, and 5.37, respectively. However, the ADC/ADC ratios were always <0.81 even when calculated with various combinations of high -values.

CONCLUSIONS

The spleen has a faster diffusion than the liver, HCCs have a faster diffusion than the adjacent liver parenchyma, and hemangiomas have a faster diffusion than simple cysts. Although it is known that cysts have a substantially longer T2 than hemangiomas, SDC of hemangioma was higher than that of cysts, suggesting 'T2 effect' is minimized for SDC.