Preliminary Application of High-Definition CT Gemstone Spectral Imaging in Hand and Foot Tendons

Tendon and ligament imaging is problematic for routine computed tomography (CT), with low specificity and sensitivity, limiting applications in this area (1). Magnetic resonance imaging (MRI) has fine soft-tissue contrast resolution and multi-planar capability, making it a useful tool in diagnosis (2), but the long duration and high cost of MRI and occasional contraindications limit usefulness in this application. Recently the emergence of gemstone spectral imaging (GSI) technique may allow broader application in musculoskeletal diseases by dint of its high resolution and spectral imaging, especially for tendons and ligaments (3, 4). The aim of this study was to investigate the primary value of GSI in hand and foot tendon anatomy and disease diagnosis.

Hand and foot tendon injuries are very common in routine work. Diagnosis is usually based on trauma history and clinical examination in the absence of direct imaging modality. MRI, because of good tissue contrast resolution, remains the gold standard for musculoskeletal imaging (6). However, there is a wide range of situations when MRI cannot be performed, such as patients with pacemakers or claustrophobia, or in emergency situations. Though CT techniques was earlier used to diagnose tendon injuries and proved to be a useful tool, tendon imaging is still weak with conventional CT for lack of density contrast (7, 8). Recently, a new technique with higher soft tissue resolution, GSI, may improve the application of tendon imaging for multi-detector computed tomography (MDCT).

Now GSI images can be obtained from HDCT (GE Discovery CT750 HD; GE Healthcare, Milwaukee, WI, USA). This spectral CT has undergone many technical innovations, such as momentary switching between high and low energy (140 kV and 80 kV) in a single X-ray tube, a high-performance gemstone detector, and powerful image post-processing. This makes material differentiation with monochromatic energy feasible (9, 10). The monochromatic imaging can help to eliminate the ray-hardening artifacts, optimizes imaging quality, and increases image contrast (11). With GSI imaging, we can get 101 monochromatic images from 40 keV to 140 keV. The use of optimal monochromatic energy allows selection of the best CNR image for tendon and surrounding tissues. In this study, the monochromatic images at about 65 keV provided the best CNR for hand and foot tendons, with quality superior to conventional polychromatic images. Most hand and foot tendons could be shown clearly, including the tendon shape and stop points. These improvements result in a great increase in the ability of GSI to display minor lesions.

The chief advantage of GSI over MRI and sonography is the ability to provide a three-dimensional (3D) display of tendons (12). Although MRI can make 3D acquisition now, it requires long scanning times and the resolution is poor. Many patients cannot tolerate staying in one position for long durations, especially for surgical trauma (13). MDCT in combination with 3D image reconstruction has shown to be useful for the evaluation of a variety of tendon abnormalities (5, 7). MDCT may serve as a valuable alternative imaging tool for the assessment of soft tissues when ultrasound or MRI cannot be used; for example, in the presence of large open wounds or when there is post surgical metallic hardware close to the anatomical area of interest (14). Ohashi et al. (15) showed that VR technique with MDCT could display various kinds of tendon injuries for fracture more directly. We also found that GSI images with 3D model could detect tendon diseases such as tendon thickening, absence, compression and rupture more directly than MRI. These direct reconstruction images help define the best surgical procedure and post-operative therapy for specific injury.

However, GSI also has limitations in visualizing hand and foot tendons, such as the inner lesions of tendons and paratendons. In this study, there were six tendon degeneration and twelve tendinous sheath disorders missed in GSI because no change of density could be found, although detected readily with MRI. This problem might be resolved by using spectral analysis. The change of spectral analysis curve might suggest presence of tendon injuries before density changes appear. The cases in this study were too few to examine this. We hope to examine this in further studies. Furthermore, not all tendons could be shown clearly with GSI, especially in the distal end of digit extensor tendon and musculus flexor digitorum sublimis tendon, limiting application in this area. Another shortfall of GSI is the radiation dose. The average radiation dose in GSI mode may be increased for switching between high and low energy. In this study, the average CTDIvol was 18.25 mGy, much higher than routine CT (5, 16), so it is prohibited for pregnant women and children.

In our study, the number of patient cases was so small that we could evaluate only the primary effectiveness of GSI and MRI. Further studies should properly classify the injury lesions detection of GSI.

In summary, we consider that GSI with monochromatic images at 65 keV improves the image quality. GSI can show tendons' whole shape, stop points and many conditions, such as tendon thickening, compression, absence and rupture more directly than MRI. So it is well-suited to patients with acute external injuries, extremity malformation, mental disorders and other patients who are contraindicated for MRI examination. But GSI has less sensitivity in displaying tendon adherence, degeneration and tendinous sheath diseases than MRI. Therefore, the examination method should be chosen in line with condition type and individual situation.