Purpose: Although an apparent diffusion coefficient (ADC) value is often used for differential diagnosis of tumours, it varies with scanning parameters. The present study was performed to investigate the influence of imaging parameters, i.e., b value, repetition time (TR) and echo time (TE), on ADC value. Methods:The phantoms were scanned using diffusion weighted imaging (DWI) with changing b values (b = 0 -3000 s/mm 2 ), TR and TE to determine the influence on ADC. Moreover, ADC of the brain in normal volunteers was determined with varying b values (b = 0 -1000 s/mm 2 ).Results: Diffusion decay curves were obtained by biexponential fitting in all phantoms. The points where fast and slow components of the biexponential decay crossed were called turning points. The b values of turning points that crossed from the biexponential curve were different in each phantom. The b values of turning points depended on ADC of fast diffusion component. When ADC is calculated using two b values of front and back for the turning point, the ADC value may be different. Therefore, it was necessary to perform calculations by b value until the turning point to obtain the ADC value of the fast component. In addition, b ≥ 100 was recommended to avoid the influence of perfusion by blood. Furthermore, the choice of long TR and short TE was effective for accurate measurement of ADC. Conclusion:It is important to determine the turning point for measuring ADC.

Purpose: The development of parallel MRI has resulted in the frequent use of diffusion-weighted imaging (DWI) in clinical medicine, and it usually involves the use of contrast medium. However, the gadolinium (Gd) contrast medium may have some effect on DWI and the apparent diffusion coefficient (ADC). The present study was performed to determine whether the magnetic susceptibility of contrast medium alters the DWI signal and the value of ADC in some imaging techniques. Materials and Methods:Non-fat suppression DWI, STIR combination and CHESS combination DWI were performed to examine 10 phantoms with Gd-DTPA dissolved at concentrations from 0.0005 to 0.1 mmol in physiological saline as contrast medium. The average pixel value and ADC of each method were determined.Results: ADC showed no differences between before and after treatment with contrast medium for all imaging techniques with Gd considered to be distributed over the whole tumour. The signal intensity did not change on non-fat suppression or CHESS combination DWI but deteriorated on STIR.Therefore, ADC was not influenced by the magnetic susceptibility of contrast medium. In addition, it was suggested that the ability of tumour detection may be reduced if STIR is used as fat suppression.

The variance in the ADC values for different MR scanners is reasonably small if appropriate scanning parameters (repetition time, >3000 ms; echo time, minimum; and high enough signal-to-noise ratio of high-b diffusion-weighted image) are used.

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