The role of number-of-excitations (NEX) for the usability of free-breathing acquired diffusion-derived ‘vessel density’ (DDVD) and slow diffusion coefficient (SDC) maps in separating liver cysts, hemangiomas, and solid masses with a focus on small lesions

Background: Diffusion weighted imaging has been commonly used to non-invasively characterize liver space occupying lesions. This study investigates the role of number-of-excitations (NEX) for free-breathing acquired diffusion-derived ‘vessel density’ (DDVD) and slow diffusion coefficient (SDC) maps in separating liver solid mass-forming lesions (MFL), cysts, and hemangiomas (HG) with a focus on small lesions.

Methods: Liver diffusion weighted imaging data were acquired at two centers using 3.0 T scanners, applying a free-breathing single-shot spin-echo type echo-planar sequence. For Center 1 (The Fifth Affiliated Hospital of Anhui Medical University) data, DDVD was calculated with b=0 and b=10 s/mm² images (both NEX =2) and SDC was calculated with b=400 and b=600 s/mm² images (both NEX =4). For Center 2 (The Second Hospital of Nanjing) data, DDVD was calculated with b=0 and b=10 s/mm² images and SDC was calculated with b=600 and b=800 s/mm² images (all NEX =6). In total there were 148 MFLs (91 hepatocellular carcinomas, 11 intrahepatic cholangiocarcinomas, 33 metastases, 13 focal nodular hyperplasias), 22 cysts, and 45 HGs.

Results: The MFLs had a median size of 2.9 cm (range: 0.6–25 cm). Counting lesion-by-lesion, 2% of MFLs’ SDC maps were not usable, while 16.2% of MFLs’ DDVD maps were not usable. MFLs with unusable SDC map or DDVD map had mostly a size <2.0 cm. For MFLs with size ≤2.2 cm, 47.4% of the Center 1 DDVD maps (n=19, median size: 1.8 cm) were usable while 68.9% of the Center 2 maps (n=45, median size: 1.6 cm) were usable. Liquid lesions (LLs) (cysts and HGs) had a median lesion size of 1.8 cm (range: 0.5–8.1 cm). A total of 92.5% LL SDC maps were usable, and 59.7% LL DDVD maps were usable. All LLs with unusable SDC maps had a size <1.5 cm, while LLs with unusable DDVD maps had mostly a size <2.5 cm. For all MFLs and all cysts, a combination of T2 weighed imaging (T2WI), SDC, and DDVD correctly and confidently classified them as MFL or cysts. Twenty-seven HGs (27/45, 60%) were not liquid signal on T2WI, SDC and/or DDVD confirmed the diagnosis of 22.2% (6/27) of them being LL independent of T2 weighted imaging, and further supported the T2 weighted imaging-based diagnosis of 40.7% (11/27) of them being LL.

Conclusions: Liver focal lesions with unusable SDC map or unusable DDVD map have a size smaller than those with usable SDC map or usable DDVD map. Even for small lesions, DDVD map usability rate can be improved by using a higher NEX.