Surg Radiol Anat (2008) 30:443–447 DOI 10.1007/s00276-008-0349-y

O R I G I N A L A R T I CL E

An initial qualitative study of dual-energy CT in the knee ligaments Cong Sun · Fan Miao · Xi-ming Wang · Tao Wang · Rui Ma · Dao-ping Wang · Cheng Liu

Received: 2 November 2007 / Accepted: 7 April 2008 / Published online: 22 April 2008 © Springer-Verlag 2008

Abstract Objective To study the clinical application of dual-energy CT (DECT) in the knee ligaments. Methods Twelve cases (24 knees) were scanned using dual-energy CT for the knee. Two- and three-dimensional images were used for display in all cases by means of multi-planar reformation (MPR) and volume rendering technique (VRT). All images were ranked by two radiologists according to the grade of knee ligament displayed, the deWnition of edge and attachment points of the knee ligament. Results The partial ligaments of 24 knees, such as the patellar ligament, Wbular collateral ligament, anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) were clearly displayed; the tibial collateral ligament was not satisfactorily displayed. The transversal ligaments, such as lateral patellar retinaculum and medial patellar retinaculum, and the posterior ligament, such as oblique popliteal ligament could not be shown clearly. Conclusion The dual-energy CT is a new and valuable tool to qualitatively display the main ligaments of the knee.

Introduction It is known that visualization of the knee ligament is the weak link of computed tomography (CT) and it is the ligament that forms the major stable structure of knee. Consequently, CT is highly restricted in its ability to diagnose knee diseases. MRI is the golden standard for examining intra-articular structures of the knee and surrounding soft tissue [9]. MRI can make two-dimensional (2D) and threedimensional (3D) acquisitions such as isotropic 3D fast spin echo with extended echo train acquisition (3D FSE XETA), 2D FSE and 2D fast recovery FSE (2D FRFSE) [3, 4, 6]. But all the images that it provides (e.g., coronal, sagittal and axial images or reformation images) are in 2D display and MRI has not been found to be capable of displaying the tendon and ligament in 3D images. With the application of dual-energy CT (DECT), the soft tissues, in particular ligaments, have higher resolution and can be displayed with multi-angle views and three-dimensionally. Therefore, as a new approach for clinical application, DECT has good prospects. In our studies, we retrospectively analyzed and reported 12 cases of patients for DECT knee scan.

Keywords Dual-energy CT · Knee · Ligament · X-ray computed tomography Materials and methods Clinical data

C. Sun · X.-m. Wang · T. Wang · R. Ma · D.-p. Wang · C. Liu (&) Shandong Provincial Medical Imaging Institute, Road jing-wu, No. 324, 250021 Jinan, Shandong, China e-mail: [email protected] F. Miao Shandong Provincial hospital, Jinan, Shandong, China

There were a total of 12 patients (8 men, 4 women; mean age 46.5 years; age range 36–57 years), and 24 knees. All patients underwent DECT at our institute as a pre-treatment evaluation for knee bone related diseases and all patients had no ligament related diseases of knee in clinical. The study protocol was approved by the local ethics committee. All patients consented to participate in the study.

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Equipment and scanning technique Twelve patients were scanned on a SOMATOM DeWnition Dual Source CT (DSCT; Siemens, Forchheim, Germany) system in dual-energy mode at tube voltages of 140 and 80 kVp and a ratio of 1:3 between tube currents. The eVective mAs were 56 and 234 mAs for tube A and tube B; collimation 0.6 mm, pitch 0.7, reconstruction increment 0.75 mm, FOV 250 mm, rotation time 1.0 s, total coverage approximately 150–200 mm, total scan time approximately 38.04–49.79 s and CTDIvol 4.70–6.26 mGy. Image post-processing The data obtained from DECT were transferred to a workstation (Volume Wizard) and images of the knee were reconstructed by multiplanar reconstruction (MPR) and volume rendering technique (VRT). They were rotated to a diVerent position in order to observe 3D and multi-location images of the knee and two experienced radiologists diVerentiated and analyzed the knee ligaments. To grade the knee ligament displayed, the MPR and VRT images were evaluated. The criteria were as follows: (1) the attachment points of the ligament, clearly displayed (2 points); fuzzily displayed (1 point); could not be displayed (0 point); (2) The edge and proWle of the ligament, clearly displayed (2 points); displayed fuzzily (1 point); could not be displayed (0 point). The total scores of the MPR and VRT images shown were all four points, and the total is eight points.

Results Some ligaments (such as the patellar ligament, Wbular collateral ligament, ACL and PCL) from the 24 knees were displayed clearly, and the scores were all over 6 points (Figs. 1, 2). The tibial collateral ligament, the transversal ligaments (lateral patellar retinaculum and medial patella retinaculum), and the posterior ligament (oblique popliteal ligament) were poorly shown, and the scores were all below two points. The scores of knee ligaments displayed by DECT are shown in Table 1.

Discussion The background of the study The tendon and ligament form the main stable structure of the knee, in particular, injuries of the ligament will have a great impact on the function of the knee. How can the knee ligament be clearly displayed and in three-dimensions?

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These are the objectives that are pursued by radiologists in this specialty. Although CT arthrography can display the intra-articular structures, it is an invasive examination and its wider application in the clinical setting is thereby restricted. With the development of multi-slice computed tomography (MSCT), tissue resolution increased, but the weak link is still how to reveal the ligament of the knee [2, 8]. Therefore, CT in the diagnosis of disease has great restrictions. Dual-energy technologies and principle The diVerentiation of material in computed tomography (CT) is based on their X-ray attenuation as quantiWed in HounsWeld Units and displayed in shades of gray at diVerent window levels in normal CT scans. Attenuation is caused by absorption and scattering of radiation by the material [7]. In the process of X-ray through human tissue, its attenuation mainly includes the Compton scatter and the photo eVect. The contribution of these two processes to the attenuation of diVerent materials varies and also depends on the energy of the X-ray photons. Inside the energy range considered, the total cross section of the Compton eVect is almost independent of photon energy, whereas the total cross section of the photo eVect is strongly energy dependent. The information from the two attenuation eVects above is integrated in conventional CT images. CT numbers do not vary very much with beam energy for soft tissues, but do vary noticeably for high z materials. Thus, materials can be diVerentiated further by applying diVerent X-ray spectra and analyzing the diVerences in attenuation. Then, an image similar to that of histochemistry will be obtained [1]. The DECT system evaluated is equipped with two X-ray tubes and two corresponding detectors. The two acquisition systems are mounted onto the rotating gantry with an angular oVset of 90°. One detector (A) covers the entire scan Weld of view (50 cm in diameter) while the other detector (B) is restricted to a smaller, central Weld of view (26 cm in diameter) due to space limitations on the gantry. Each of the two rotating envelope X-ray tubes allows up to 80-kW peak power from the two on-board generators. Both tubes can be operated independently with regard to their kilovolt (kV) and milliampere (mA) settings. This allows the acquisition of dual-energy data, and can be helpful in distinguishing tissue characterization [5]. Early DECT technology in diVerent tube voltages can carry out two consecutive independent scans, but cannot connect seamlessly in a manner suitable for imaging the organizational structure. This prevented its development for use in routine clinical applications. Now, DECT could overcome this limitation. For dualenergy post-processing algorithms, the image noise in both

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Fig. 1 Images in 34-year-old man with Dual Energy CT in the knee ligaments. a The MPR image showing ACL, b the MPR image showing the PCL, c the transverse image showed ACL and PCL, d the VRT image showing ACL, e the VRT image showing PCL

image data sets (acquired at 80 kV and at 140 kV) has to be similar, which was not possible in the early days of DECT due to insuYcient power reserves for the low kV scan. The dual-energy data can be acquired nearly simultaneously with subsecond scan times. The ability to overcome data registration problems should provide a variety of information that makes it possible to identify the characteristics of imaging organizations [5]. The diVerentiation of collagen

makes it possible to depict ligaments. And ligaments could also be multi-angled and displayed three-dimensionally. Therefore, as a new method, it will broaden applications in the clinical setting. The value of the study: MRI is the golden standard for examining intra-articular structures of the knee and surrounding soft tissue [9], but no reports had been found that MRI could display the tendon and ligament in 3-dimensions,

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Fig. 2 Images in 41-year-old woman with Dual Energy CT in the knee ligaments. a The MPR image showing the Wbular collateral ligament, b the VRT image showing the Wbular collateral ligament (white arrow)

Table 1 The degree of DECT displayed ligaments of the knee

Ligament

VRT (n = 24)

MPR (n = 24)

Total average point

Attachment points

Edge and proWle

Attachment points

Edge and proWle

Patella ligament

2

2

2

2

ACL

2

1

2

1

6

PCL

2

2

2

2

8

8

Fibular collateral ligament

2

2

1

1

6

Tibial collateral ligament

1

1

0

1

3

Lateral patellar retinaculum

0

0

0

0

0

Medial patellar retinaculum

0

0

0

0

0

Oblique popliteal ligament

0

0

0

0

0

since the spatial resolution of MR scans is not achieved. While at least some tendons are already visible on single energy CT images, DECT can be especially helpful for ligaments. DECT could also give a multi-angled image of the tendon and ligament. Using VRT images, DECT could Wrst reconstruct a 3D model of the knee tendon and ligament in the clinical setting. The main ligaments that maintain knee stability include: patellar ligament, anterior curciate ligament and posterior curciate ligament, Wbular collateral ligament and the tibial collateral ligament. Except for the tibial collateral ligament, DECT could clearly display the other major ligaments stereoscopically. Meanwhile, DECT could be combined with MPR images to observe the details of ligaments multi-directionally. Meanwhile, DECT could provide the precision in deWning anatomic landmarks and achieving accurate location and orientation of the ligament and tendon. Some limitations existed in the study: Although DECT could provide the exciting images of the knee tendon and

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and patella ligament (yellow arrow), c the VRT image showing the tibial collateral ligament

ligament, there were still a number of problems that need further improvement at the same time. Firstly, the thinner and transverse ligaments, such as, the tibial collateral ligament, the lateral patellar retinaculum and medial patellar retinaculum were displayed not so satisfactorily; Secondly, because of the coverage of its rear muscles, the posterior ligament, such as oblique popliteal ligament could not be showed clearly; Thirdly, the scanning time that DECT used for the knee is about 2–3 times of MSCT. So, it needs to be braked. However, almost all the subjects received acceptable. Fourthly, since the largest FOV of DECT is 260 mm, the scanning bed must adjust at the center position and only a few of persons were constrained for both knees scanning. A limitation of this study was the sample was relatively small; another limitation was that we only had a preliminary discussion in the knee tendon and ligament shown and the injury and other diseases of the knee tendon and ligament had not yet involved. And we had made no comparison with images obtained by

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MRI. So, further studies are required to draw conclusions on the diagnostic value on it. With the improvement of DECT technology, we believe that it has good prospects in clinical.

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