Feasibility and clinical usefulness of deep learning-accelerated MRI for acute painful fracture patients wearing a splint: A prospective comparative study

Objective To evaluate the feasibility and clinical usefulness of deep learning (DL)-accelerated turbo spin echo (TSEDL) sequences relative to standard TSE sequences (TSES) for acute radius fracture patients wearing a splint. Methods This prospective consecutive study investigated 50 patients’ preoperative wrist MRI scans acquired between July 2021 and January 2022. Examinations were performed at 3 Tesla MRI with body array coils due to the wrist splint. Besides TSES obtained according to the routine protocol, TSEDL sequences for axial T2-, coronal T1-, and coronal PD-weighted TSE sequences were scanned for comparison. For quantitative assessment, the relative signal-to-noise ratio (rSNR), the relative contrast-to-noise ratio (rCNR), and the relative contrast ratio (rCR) were measured. For qualitative assessment, all images were assessed by two independent musculoskeletal radiologists in terms of perceived SNR, image contrast, image sharpness, artifacts disturbing evaluation, overall image quality and diagnostic confidence for injuries using a four- or five-point Likert scale. Results The scan time was shortened approximately by a factor of two for TSEDL compared to TSES. TSEDL images showed significantly better rSNR, rCNR, and rCR values for all sequences, and scored significantly better in terms of both image quality and diagnostic confidence for both readers than TSES images (all p < .05). Interrater reliabilities were in almost perfect agreement. Conclusion The DL-accelerated technique proved to be very helpful not only to reduce scan time but also to improve image quality for acute painful fracture patients wearing a splint despite using body array coils instead of a wrist-specific coil. Based on our study, the DL-accelerated technique can be very useful for MRI of any part of the extremities in trauma settings just with body array coils.


Introduction
The distal radius is one of the most common sites of fracture, and the incidence of distal radius fracture is increasing among people in all age groups. It accounts for approximately 18% of fractures in patients 65 years and older, and it hinders one's ability to perform daily activities, such as preparing meals and performing housekeeping duties [1][2][3].
Furthermore, associated soft tissue injuries (e.g., intrinsic scapholunate and lunotriquetral ligament, triangular fibrocartilage with or without concomitant distal radioulnar joint instability) are reported with high incidence [3]; MRI evaluation is crucial for the detection of these soft tissue injuries [4]. Additionally, MRI is used to identify potential radiographically occult fractures, such as associated occult scaphoid fractures [5]. More than 60% of patients diagnosed with distal radius fracture undergo surgical correction [6]. In the case of acute wrist fracture patients in need of surgical correction, preoperative MRI scans might have an important role in the evaluation of associated soft tissue injury and surgical planning.
When patients with acute distal radius fracture need to undergo preoperative MRI, they are prone to suboptimal MRI quality for the following reasons. These patients tend to undergo manual reduction and splinting as soon as possible after diagnosis to prevent further injury. However, the splints are not easily detachable, and with the wrist splint on, wrist-specific coils are not suitable for MRI scans, causing impaired image quality and subsequent inaccurate diagnosis for soft tissue injuries. Additionally, patients are in such pain that it is difficult for them to remain still for the long time required for an MRI scan, which makes them susceptible to motion artifacts. Over the years, various attempts have been made to reduce the scan time, such as parallel imaging [7][8][9] and compressed sensing [10][11][12]. More recently, deep learning (DL)-based reconstruction and acceleration techniques have been developed. With concurrent denoising, DL-based reconstruction and acceleration techniques have achieved some promising results in reducing scan time and improving image quality simultaneously for various body parts, including the prostate [13], pituitary gland [14], liver [15], pelvis [16], shoulder and hip joints [17]. A preemptive study about the feasibility of implementing DL reconstruction in musculoskeletal turbo spin echo (TSE) imaging has been conducted with positive results [18]. To the best of our knowledge, no previous studies have applied deep learningaccelerated MRI in a painful trauma clinical setting. It was hypothesized in this study that with this DL approach, high image quality wrist MRI scans can be acquired and a reduction in scan time can be achieved simultaneously, when utilizing body array coils instead of a wrist-specific coil.
The purpose of this study was to evaluate the feasibility and clinical usefulness of deep learning-accelerated turbo spin echo (TSE DL ) sequences relative to standard TSE (TSE S ) sequences for acute distal radius fracture patients wearing a splint on the wrist.

Materials and methods
This prospective study was approved by the institutional review board of Seoul National University Hospital, and written informed consent was obtained from all subjects before inclusion in the study (IRB No. H-2105-079-1218).

Study population
Fifty-three consecutive consenting patients with acute distal radius fracture diagnosed based on wrist CT scan findings and who were scheduled for preoperative wrist MRI were prospectively included in this study between July 2021 and January 2022. Three patients were excluded for the following reasons: One had a scaphoid fracture with screw fixation in the affected wrist. Another two were in so much pain that they had to end the scanning process as fast as possible. Finally, 50 patients (41 female and 9 male patients; mean age ± SD, 64.68 ± 10.83 years; age range 31-86 years) were included in this study.

Image acquisition
All MRI examinations were performed using a 3 Tesla MRI scanner (MAGNETOM Skyra fit , Siemens Healthcare) with a pair of 30-channel body array coils (Body 30, Siemens Healthcare) instead of a wrist-specific coil. All study participating patients who underwent sugar-tong splint immobilization for distal radius fracture (anteroposterior compression after partial closure reduction via traction and release to prevent joint dislocation; the distal end of the splint was not to cross the distal ends of the metacarpal bones) were scanned in supine position with the hand above the head (Fig 1). First, TSE S sequences were obtained according to the routine MRI protocol for distal radius fractures in our institute, which was as follows: axial T2-weighted TSE S sequence with fat suppression (FS), axial T1-weighted TSE S sequence, coronal T1-weighted TSE S sequences, coronal T2-weighted and PD-weighted TSE S sequences with FS, and sagittal T2-weighted TSE S sequences. In addition, axial T2-weighted TSE DL sequence with FS, coronal T1-weighted TSE DL sequence, and coronal PD-weighted TSE DL sequence with FS were acquired for comparison. The acquisition parameters for each sequence are summarized in Table 1. The prototypical TSE DL sequence employs a deep-learning reconstruction which is designed for improving the signal-to-noise ratio of acquisitions with higher accelerations. It comprises an unrolled variational network [19] and we refer to references [13,18] for more details on the technical implementation used in this study.

Quantitative image analysis
For objective comparison of image quality, the relative signal-to-noise ratio (rSNR) for bone and muscle, relative contrast-to-noise ratio (rCNR), and relative contrast ratio (rCR) between bone and muscle were measured on TSE DL and TSE S images by a 2 nd year radiology resident (S.H.R). For the corresponding TSE DL and TSE S images of each sequence, the same sized circular ROIs with a diameter of 5 mm were placed in the same location. For axial images, the thenar muscle and 2 nd metacarpal base of the same plane were selected for muscle and bone analysis measurements (Fig 2A). For six patients whose 2 nd metacarpal base was not covered in the axial scan, the flexor digitorum muscle and distal radius or ulna of the same plane were selected instead. For coronal images, the 1 st interosseous muscle and 2 nd metacarpal base of the same image plane were selected ( Fig 2B). Circular ROIs were placed in homogeneous areas away from other structures, such as vessels, edema or cysts. Regions affected with partial volume averaging were avoided as well. We measured signal intensity (SI) values and standard deviation (SD) within the ROI for comparison analysis. With those values, rSNR for bone and muscle, rCNR and rCR between bone and muscle were calculated using the following expressions [14,17]:

Qualitative image analysis
All images were independently evaluated by two independent board-certified musculoskeletal radiologists (H.J.Y. with 17 years of experience; J.Y.C., with 23 years of experience). For each case, two image sets of TSE DL and TSE S sequences were anonymized and distributed in random order. The readers were blinded to the clinical information, radiological report, and scan parameters. For each image set, image quality parameters, including the perceived signal-tonoise ratio, image contrast, image sharpness, artifacts (motion, grid) disturbing evaluation, and overall image quality, were evaluated using a 4-point Likert scale (Table 2). Grid artifact was defined as a pattern of coarse lines that cross each other to form squares on the images (Fig 3). In addition, diagnostic confidence levels were assessed in terms of the presence of a distal radius fracture, ulnar styloid fracture/bone contusion, and triangular fibrocartilaginous complex (TFCC) injury. Diagnostic confidence for each abnormality was measured based on a 5-point Likert scale. Detailed information on the scale used for qualitative evaluation of image quality and diagnostic confidence is provided in Table 2.

Statistical analysis
For comparison of the rSNR, rCNR, and rCR between TSE DL and TSE S sequences, the paired t test was used. For qualitative image analysis and diagnostic confidence, the Wilcoxon signed rank test was utilized. A p value of less than .05 was considered to indicate a significant difference. All statistical analyses were performed by using commercially dedicated software (IBM SPSS Statistics 27 software for Windows; IBM). The levels of interrater agreement were evaluated using Gwet's agreement coefficient (AC) [20] due to the skewed marginal distribution of qualitative scores [21]. Gwet's ACs were calculated using STATA/MP 17 for Windows; Stata-Corp LLC [22]. Interrater reliability was categorized as poor (<0), slight (0-0.

Results
The total scan time for axial T2-, coronal T1-, and coronal PD-weighted TSE sequences was 11 minutes and 55 seconds for TSE S, and 6 minutes and 6 seconds for TSE DL (Table 1).

PLOS ONE
Usefulness of deep learning-accelerated MRI for acute painful fracture patients wearing a splint Quantitative image quality assessment TSE DL images showed significantly better (p < .05) bone rSNR and muscle rSNR for all sequences than TSE S images; axial T2-, coronal T1-, and coronal PD-weighted TSE (Table 3). TSE DL images showed significantly better (p < .05) rCNR and rCR between bone and muscle for all sequences as well (Table 3). See Figs 4-6 for example images.

Qualitative evaluation of image quality and diagnostic confidence
TSE DL sequences demonstrated significantly better image quality than TSE S sequences for both readers (p < .05) in terms of perceived SNR, image contrast, image sharpness, motion  Fig 7). Regarding diagnostic confidence, TSE DL sequences showed significantly higher confidence levels for all distal radius fractures, ulnar styloid fractures (Fig 8), and TFCC injuries (Fig 9) (p < .001 in all) for both readers except one reader's TFCC injury evaluation, which was due to both TSE DL sequences and TSE S sequences scoring the highest point for all patients. Interrater reliabilities showed almost perfect agreement for qualitative evaluation of image quality and diagnostic confidence except in the following cases: distal radius fracture and ulnar styloid fracture/bone contusion diagnostic confidence for TSE S sequences (moderate) and image sharpness for TSE S sequence (fair) ( Table 4).

Discussion
In the current study, TSE DL images showed significantly better rSNR, rCNR, and rCR than TSE S images for all sequences for patients with acute painful distal radius fracture wearing a splint. For qualitative analysis, TSE DL sequences were significantly better than TSE S sequences in terms of both image qualities and diagnostic confidence. It is inspiring that the TSE DL sequences allow increased spatial resolution without image quality deterioration, while scan time is reduced by approximately half of that required for TSE S sequences. It is difficult for patients with acute fracture to tolerate the long scan time due to pain even without motion. Therefore, we think that the longer it takes to acquire images, the more susceptible images are to motion artifacts (Fig 7). These artifacts may degrade image quality and subsequently lead to decreased diagnostic accuracy, thus incurring expensive costs for the patient and the institution due to potential exam failure or repeat exam. Our results showed that motion artifacts were less frequent in TSE DL images than in TSE S images. It goes without saying that reduced scan time itself is a considerable advantage for the patients and the institution in terms of convenience and cost-effectiveness. Furthermore, in the present study, despite two unfavorable conditions for wrist MR scans, 1) using body array coils instead of a wrist-specific coil and 2) keeping the splint on at scan time (the culprit for off-center scanning or foreign body artifacts), TSE DL images demonstrated excellent image quality. Based on our study, the TSE DL technique can be very useful for MRI of any part of the extremities in trauma settings, such as fractures with splints or cast immobilization just with body array coils.
A disadvantage of TSE DL images compared with TSE S images was the minor 'grid' artifact that was sometimes observed (Fig 3). This minor byproduct hardly altered the diagnostic performance of the images, but it would be a candidate for future improvement.
There were several limitations in this study. First, although both quantitative and qualitative analysis generated congruent results, in favor of TSE DL images over TSE S images, there was no direct comparison between TSE DL images and the hypothetical images with splint removal and wrist-specific coil application. However, it was difficult to remove the splint for a while during the MRI scan for comparison in daily practice, which might cause patient discomfort and failure of mechanical reduction. Second, this study investigated a vendor provided preset denoising level for DL-based reconstruction. However, appropriate levels of denoising could be further studied [14,17], as too much denoising could impair the edge margins of distinct structures and impair image quality [13].
In conclusion, the deep learning-accelerated technique proved to be very helpful not only to reduce scan time but also to improve image quality simultaneously for patients with acute painful distal radius fracture wearing a splint on the wrist despite using body array coils instead of a wrist-specific coil. Based on our study, the deep-learning accelerated technique has potential for being applicable to MRI for any part of the extremities in trauma settings just with body array coils.