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High Frequency Spectral Ultrasound Imaging Detects Early Post Traumatic Muscle Fibrosis In Rodents
Johanna H. Nunez, MD1, Nicole J. Edwards, PhD2, Geoffrey Hespe, MD1, Cassie Rowe, PhD3,4, Amanda Huber, PhD1, Chase Pagani, B.A.1, Ashish Chowdary, B.S.1, Jan Stegemann, PhD5, Cheri Deng, PhD5, Thomas Davis, PhD3,4, Benjamin Levi, MD1.
1University of Texas Southwestern, Dallas, TX, USA, 2University of Michigan, Ann Arbor, MI, USA, 3Uniformed Services University, Bethesda, MD, USA, 4The Henry M. Jackson Foundation for the Advancement of Military Medicine, Bethesda, MD, USA, 5University of Michigan, Ann Arbor, MD, USA.

PURPOSE: Muscle fibrosis is a devastating condition that can occur following traumatic injury leading to loss of function and poor outcomes. Limited noninvasive modalities exist to diagnose and characterize the extent of muscle damage. Here we investigate spectral ultrasound imaging (SUSI), a quantitative imaging technique which detects tissue parameters based on intrinsic structure and composition, to identify early muscle fibrosis based on fibrotic changes established in a rat model. We hypothesize that SUSI will detect early traumatic muscle fibrosis in a clinically translational rat muscle injury model.
METHODS: Adult male Sprague Dawley rats were sedated, prepped, and underwent a soft tissue crush injury that generated 20lbs/inē of pressure using a digital force gauge. A superficial 1.5 cm incisional wound was then made over the quadriceps and closed. Injured and contralateral uninjured hind limbs were collected and fixed for SUSI imaging. Imaging was performed with a VisualSonics VEVO770 high resolution small animal ultrasound imaging system, using a 30 MHz center frequency single element transducer. Injured muscles were imaged at the crush injury and the corresponding place on the contralateral limb. Analysis of radiofrequency data was done as previously described in literature and SUSI parameters were calculated using the average acoustic concentration (AAC) of the fibrotic region and the corresponding region in the contralateral muscle (Fig. 1A). AAC values were grouped by time point and tissue type for statistical comparison using unpaired t tests.
RESULTS: In our rodent model, SUSI identified muscle fibrosis as early as 7 days post injury. The average AAC of injured fibrotic versus uninjured muscle was significantly increased at post injury day 7 (36.33 vs 18.36 db/mm3, injured versus uninjured, p = 0.003) and day 28 (54.64 vs 28.29 db/mm3, injured versus uninjured, p<0.0001) (Fig.1B). Though not statistically significant at day 14 (38.69 vs 27.61 db/mm3, injured vs uninjured, p=0.14) the fibrotic muscle did trend toward a higher AAC value. These results indicate that at day 7 and 28, the SUSI measurement AAC can be used to distinguish injured fibrotic muscle from uninjured muscle.
CONCLUSIONS: SUSI offers a portable, non-invasive, and safe diagnostic modality to detect muscle fibrosis, making it an ideal modality for post traumatic injuries. Here, we show SUSI distinguishes muscle fibrosis from healthy tissue at day 7 and 28 post injury in our model allowing for early treatment. Unlike qualitative grayscale imaging, SUSI determines unique parameters of the tissue based on a calibrated spectrum related to the inherent properties of the tissue, making it less user-dependent and more reproducible.


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