Effect of b value on monitoring therapeutic response by diffusion-weighted imaging.
OBJECTIVE
To explore the diffusion gradient b-factor that optimizes both apparent diffusion coefficient (ADC) measurement and contrast-to-noise (CNR) for assessing tumor response to transarterial chemoembolization (TACE) in a rabbit model.


METHODS
Twelve New Zealand white rabbits bearing VX2 tumors in the liver were treated with TACE. Diffusion-weighted imaging (DWI) with various b values was performed using the same protocol before and 3 d after treatment with TACE. ADC values and CNR of each tumor pre- and post-treatment with different b factors were analyzed. Correlation between ADC values and extent of necrosis in histological specimens was analyzed by a Pearson's correlation test.


RESULTS
The quality of diffusion-weighted images diminished as the b value increased. A substantial decrease in the mean lesion-to-liver CNR was observed on both pre- and post-treatment DW images, the largest difference in CNR pre- and post-treatment was manifested at a b value of 1000 s/mm(2) (P = 0.036 ). The effect of therapy on diffusion early after treatment was shown by a significant increase in ADCs (P = 0.007), especially with large b factors (>= 600 s/mm(2)). The mean percentage of necrotic cells present within the tumor was 76.3%-97.5%. A significant positive correlation was found between ADC values and the extent of necrosis with all b values except for b200, a higher relative coefficient between ADC values and percentage of necrosis was found on DWI with b1000 and b2000 (P = 0.002 and 0.006, respectively).


CONCLUSIONS
An increasing b value of up to 600 s/mm(2) would increase ADC contrast pre- and post-treatment, but decrease image quality. Taking into account both CNR and ADC measurement, diffusion-weighted imaging obtained with a b value of 1000 s/mm(2) is recommended for monitoring early hepatic tumor response to TACE.
Jiang
Zhao-Xia
ZX
Department of Radiology, Affiliated Cancer Hospital of Fudan University, Shanghai 200032, China.
Peng
Wei-Jun
WJ
Li
Wen-Tao
WT
Tang
Feng
F
Liu
Shi-Yuan
SY
Qu
Xu-Dong
XD
Wang
Jian-Hua
JH
Lu
Hong-Feng
HF
D000818:Animals; D016461:Chemoembolization, Therapeutic; D038524:Diffusion Magnetic Resonance Imaging; D007089:Image Enhancement; D007090:Image Interpretation, Computer-Assisted; D008114:Liver Neoplasms, Experimental / Q000098:blood supply / Q000473:pathology* / Q000628:therapy*; D008297:Male; D008954:Models, Biological; D009336:Necrosis; D011237:Predictive Value of Tests; D011817:Rabbits; D013997:Time Factors; D016896:Treatment Outcome