Objective:To investigate the clinical value of diffusion weighted imaging(DWI) in evaluation of primary liver cancer(PLC) after high intensity focused ultrasound(HIFU) ablation.Methods:Routine magnetic resonance sequences

contrast-enhanced magnetic resonance imaging and DWI were performed on 26 patients with PLC before

within 2 weeks and 3 months after HIFU ablation.Echo planar imaging sequence was used to generate DWI.A b value of 800 s/mm2 was used to acquire imaging.Signal changes on DWI

and apparent diffusion coefficient(ADC)maps were analyzed.ADC values and signal intensity(SI)on the lesion centers

periphery

peripheral liver tissues and the muscles of back before and after HIFU ablation were measured

and compared by using analysis of variance and least significant difference(LSD) procedure for further multiple comparisons.A P value of less than 0.05 was considered statistically significant.Results:Primary liver cancer presented hyper-intensity on DWI and cold color on ADC maps before HIFU ablation

while it presented hypo-intensity on DWI and warm color on ADC maps within 2 weeks after HIFU ablation.No significant difference was found on DWI and ADC maps between 3 months and 2 weeks follow-up examination

except 1 patient presented irregular nodal hyper-intensity signal on DWI 2 weeks after HIFU ablation(diagnosed as survival tumor tissue)and 1 patient was found with irregular nodal hyper-intensity signal on DWI 3 months after HIFU ablation(diagnosed as the recurrence of primary liver cancer).The mean ADC value and SI of the remaining 24 patients were compared.Mean ADC value of PLC within 2 weeks after HIFU ablation was higher than that before HIFU ablation(P<0.01)

but no significant difference was found between mean ADC value of PLC 3 months after HIFU ablation and that of 2 weeks after HIFU ablation(P=0.50).Likewise

the same trend was found in the periphery of the PLC.Mean SI of PLC within 2 weeks after HIFU ablation was lower than that before HIFU ablation(P< 0.01)

but no significant difference was found between mean SI of PLC 3 months after HIFU ablation and that of 2 weeks after HIFU ablation(P=0.99).Likewise

the same trend was found in the periphery of the PLC.Conclusion:Coagulation necrosis of PLC after HIFU ablation can be diagnosed accurately on liver DWI;the increase of ADC value and decrease of SI can be a combined marker of coagulation necrosis of PLC after HIFU ablation.