Apparent diffusion coefficient measurement of the parotid gland parenchyma

Abstract

The measurements of apparent diffusion coefficient (ADC) with diffusion weighted magnetic resonance imaging (DW-MRI) is becoming a popular diagnostic and research tool for examination of parotid glands. However, there is little agreement between the reported ADC values of the parotid gland in published literature. In this review 43 studies on ADC measurement of the parotid glands were included. The analyses indicated several possible culprits of the observed ADC discrepancies. For example, DW-MRI examinations under gustatory stimulation gives higher ADC values compared to the unstimulated parotid gland (P=0.003). The diffusion weighting factors (b-values) can either increase (b-value <200 s/mm²) or decrease ADC values (b-values >1,000 s/mm²). The timing of follow-up DW-MRI after radiotherapy (RT) indicates correlation to the found ADC values (R² =0.39). Interestingly, the choice of regions of interest (ROI) appears not to affect the measurements of ADC (P=0.75). It can be concluded that there is a critical need for standardization of ADC measurement of the parotid glands to allow valid inter-study comparisons and eventually to reach consensus on the use of ADC as biomarker.

Keywords: Diffusion weighted magnetic resonance imaging (DW-MRI); apparent diffusion coefficient; parotid gland parenchyma.

Figure 1

Diagram showing how apparent diffusion coefficient (ADC) is related to the MRI signal at different b-values. Modified from Mahmood and Hansen, 2015 (15). In the low b-values region the signal from capillary blood flow contributes to the total signal. In the high b-value region the noise floor can constrain the recorded signal to stay higher than can be attributed to the underlying diffusion process. ADC should be measured from mid-range b-values.

Figure 2

Mean apparent diffusion coefficient (ADC) of normal parotid parenchyma. Numbers indicate study id.

Figure 3

Mean apparent diffusion coefficient (ADC) of normal parotid parenchyma under gustatory stimulation. Numbers indicate study id.

Figure 4

Mean apparent diffusion coefficient (ADC) of parotid glands from nine studies using gustatory stimulation under DWI-MR examinations, first at rest and then under stimulation. T-test-Matched pairs t-test.

Figure 5

Overview of the variation in b-values in all 43 included studies.

Figure 6

The distribution of reported b-values. The most popular individual choices of b-values: b=0 s/mm² (35 studies), b=500 s/mm² (16 studies), b=800 s/mm² (12 studies), b=1,000 s/mm² (32 studies).

Figure 7

Reported mean apparent diffusion coefficient (ADC) values of the normal parotid parenchyma and the corresponding b-values configurations. Horizontal lines indicate that b-values belong to the same acquisition.

Figure 8

Number of studies divided into the different delineation strategies.

Figure 9

The change in apparent diffusion coefficient (ADC) from pre-RT to post-RT as a function of mean dose (28.4–53.9 Gy). R²-correlation coefficient. Numbers indicate study id.

Figure 10

Apparent diffusion coefficient (ADC) change from pre-RT to post-RT plotted as function of post treatment time in days (7–270 days). R²-correlation coefficient. Numbers indicate study id.