Technical noteImaging parameter effects in apparent diffusion coefficient determination of magnetic resonance imaging
Introduction
Diffusion weighted images (DWI) obtained on magnetic resonance imaging (MRI) are useful for diagnosis of acute cerebral stroke. DWI is often used for examination of the human body because an echo planar imaging sequence with parallel imaging yields excellent image quality [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Body DWI may allow rough differential diagnosis of tumours and is useful in examinations to find tumours. The apparent diffusion coefficient (ADC) is used frequently in differential diagnosis. It has been reported that DWI has the potential to differentiate between benign and malignant tumours [14], [15], [16], [17], [18], [19], [20], [21], [22]. On the other hand, some investigators have reported that because ADC values of benign tumours and malignant tumours overlap, they are therefore not useful to differentiate between bulk benign and malignant tumours [23]. In these cases, it is problematic that the ADC value varies with the scanning parameters. This leads to serious problems when ADC is used to distinguish a tumour. In addition, use of ADC value with an error may lead to wrong conclusions. Therefore, we reviewed how ADC value varies with scanning parameters, how to obtain ADC with high precision and points to which attention should be paid.
The purpose of this study was to identify the scanning parameters to obtain ADC value measurement with a high degree of precision.
Section snippets
Influence of b value
The phantoms were scanned with various b values (b = 0–3000 s/mm2) to determine the influence of the b value of the motion probing gradient (MPG) on signal intensity of DWI. Five types of phantom (agarose 1 g, 3 g, 4 g and 5 g in 100 mL of hot water and 100 mL of liquid detergent in bottles) were fixed in a water tank.
T1 and T2 values of each phantom are shown in Table 1.
Scanning parameters were TR/TE with 8000/100 ms with spin-echo type single-shot echo planar imaging (EPI), matrix size, 128 × 128; field
Results
The logarithms of DWI signal intensities with b = 0–3000 s/mm2 are plotted in Fig. 2 In all phantoms, the signal intensity deteriorated as b value increased, indicated by two straight lines on the logarithmic scale. That is, diffusion decay curves were obtained from the biexponential in all phantoms. The points where fast and slow components of the biexponential decay crossed were called turning points (Fig. 3).
The positions of the turning points were different in each phantom in Fig. 2. Fig. 4
Discussion
In all phantoms, the signal intensity deteriorated with increases in b value as shown in Fig. 2. The decay curves were best fit with the biexponential function. In general, biexponential decay occurs due to intra- and extracellular diffusion [24], [25]. Other investigators reported that the fast and slow diffusion components responsible for the biexponential decay are attributable to water and lipid protons using dairy cream [26], [27]. In this study, the biexponential decay was seen in the
Conclusions
Recently, DWI and ADC have become important tools in surveying and discrimination of tumours. Many studies are now performed using ADC value to distinguish tumours. On the other hand, the ADC value varies with scanning parameters of DWI. Use of an ADC value with an error may lead to wrong conclusions.
Therefore, the present study was performed to investigate the scanning parameters to obtain high precision in ADC measurement. It is necessary to use b values smaller than the turning point so that
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