Given the threat of terrorism and incidental spillover of radio-active materials, it is important to develop in vivo, non-invasive imaging techniques to assess the effect of total body irradiation¹. Diffusion MRI, especially with high b-values, has been shown to be sensitive to tissue microstructural and pathologic changes associated with a number of diseases² and cancer radiation therapy³. The purpose of this IACUC-approved study is to characterize hematopoietic Acute Radiation Syndrome (hARS) due to excessive total body irradiation in a Göttingen minipig model using quantitative diffusion MRI. The outcome of this study may provide helpful information for the follow-up evaluations of radiation exposure using medical countermeasures.

Animals: Thirteen male Göttingen minipigs (5 to 7 months old, 9-17 kg) were obtained from Marshall Bioresources (North Rose, NY). The minipigs were fed once daily with Harlan No. 8753 (Harlan, Madison, WI). Water was provided ad libitum. The minipigs were fasted at least 12 hours prior to irradiation and imaging.

Irradiation: Total body irradiation of the minipigs was performed using a 6 MeV LINAC photon source (Varian model #EX-21) at 60 to 80 cGy/min for a total dose of either 1.65 Gy (LD30/45 radiation dose at which 30% of the animals succumb at 45 days) or 1.90 Gy (LD70/45)⁴.

MR Imaging: All animals underwent diffusion MRI scans prior to irradiation to establish a baseline and imaged again at 8, 15, or 22 days post irradiation. Diffusion weighted abdominal images of the anesthetized minipig’s were acquired using a 3T MRI scanner (MR750, GE Healthcare, Milwaukee, WI) with a 32-element phased-array coil and a customized single-shot spin-echo EPI sequence (TR/TE = 8000/77 ms, field of view = 240x240 mm², matrix = 256x256, slice thickness = 5 mm, number of slices = 64, diffusion b-value = 0, 250, 500, 750, 1000, 1500, 2000, 2500 s/mm²).

Image Analysis: Using a noise resilient Theil-Sen linear regression algorithm⁵, the multi-b-value diffusion images were fit to an intravoxel incoherent motion (IVIM) model⁶ on a voxel basis. Histograms of the diffusion coefficient (D) were obtained from the regions of interest (ROI)of the spleen on the b=0 images and confirmed with the corresponding high-resolution T₁- and T₂-weighted images. Statistical difference of the mean D values between pre- and post- irradiation was determined using a one-tailed unequal-variance Student t-test. Significance was prescribed at p<0.05.

Figure 1 shows a set of representative quantitative diffusion images of the abdomen before and after irradiation. The pre-irradiated spleen diffusion coefficient D was consistent among all animals (0.53±0.03 10^-3mm²/s). In all but two of the irradiated minipigs, the mean D for the spleen significantly decreased after irradiation (e.g., ΔD between 0.04 and 0.18 x10^-3 mm²/s, p<0.05), as shown in Fig. 2. The decrease in D was not statistically different between the LD30 and LD70 groups. In eight of the pre-irradiation diffusion maps, the histograms of the spleen D values were unimodal (Fig. 3a), whereas five were trimodal (Fig 3b) which fitted well (R²>0.99) to a tri Gaussian model. After irradiation, the histograms for all animals became unimodal (green in Fig. 3).

The spleen, as a multifaceted organ, plays a key role in the maintenance of the body’s blood supply. The spleen pools both red and white blood cells, and, filters and breaks down senescent erythrocytes. Both functions may be affected by a hematopoietic insult^4,7. The release of pooled red and white blood cells by the spleen’s red pulp and white pulp, respectively, is consistent with the reduced production of blood cells due to damaged hematopoietic cells^4,7. The reduction in the spleen’s D may be due to the reduction in blood cells and hence the associated water content. The spleen’s red pulp’s activity increases and expands in response to digestion⁸ and may explain the pre-irradiation multi-model histogram of D. The post-irradiated animals lack this variability which may be due to the compromised digestion. The lack of observed difference between the two radiation doses may be due to the small sample size.

Diffusion weighted imaging in the abdomen is challenging due to perfusion and noise due to motion artifacts. The use of the IVIM model helped filter out the perfusion effect and the Theil-Sen linear regression provided robustness against noise.

The consistent diffusion value prior to irradiation and the significant changes post-irradiation, both observed in this study, suggest that quantitative diffusion MR may be a viable marker for studying the effect of total body irradiation.