Dual energy CT accurately identifies ACL tears in emergency department

MedicalResearch.com:  Katrina N. Glazebrook, MB, ChB
Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905

MedicalResearch.com:  Why did you do the study?

Dr. Glazebrook: We felt CT was being underutilized for evaluation of knee injuries. The utility of CT has been well documented in the assessment of fractures, but little attention has been made on soft tissue evaluation.

CT now has high spatial resolution with very thin reconstructions in any desirable plane, and we have previously noted that this allowed injured soft tissue structures such as cruciate ligaments to be well visualized [presented at Society of Skeletal Radiology meeting March 2013]. We had determined in that prior study that the best reconstruction plane to evaluate both normal and torn anterior cruciate ligaments was the oblique sagittal plane parallel to the lateral femoral condyle as routinely used in MRI imaging of the knee The soft tissue window, single energy bone removal and Dual energy bone removal were the best reconstructions to determine the presence or absence of ACL disruption The bone removal techniques removed the distracting bone so the soft tissue structures were more apparent.

In this study, we wanted to determine if this technique would be successful with radiologists having a differing level of experience and MSK training. To do this, we compared the diagnostic performance of a 4th year resident and that of an experienced MSK fellowship-trained radiologist in the evaluation of acute ACL injury, with MRI as the gold standard.

MedicalResearch.com:  Do you have updates to the numbers that are in the abstract?

Dr. Glazebrook: No, we have not updated the numbers.

MedicalResearch.com:      What is the standard way for imaging acute knee injuries in the ER?

Dr. Glazebrook: The standard imaging for knee injuries in the ER is radiography. If a displaced or suspected fracture is noted, then CT is performed. The radiologist is routinely provided with images in the axial, straight sagittal, and coronal planes, all reconstructed with a bone (high resolution) algorithm to look at fracture alignment. S soft tissue images reconstructed with a smoother (lower resolution) algorithm are routinely provided only in the axial plane to evaluate the soft tissues. MRI of the knee is not routinely used in the acute setting due to cost, exam time, patient discomfort and availability.

MedicalResearch.com:  How does DECT differ from standard CT?

Dr. Glazebrook: Standard CT uses a single x-ray tube to scan the patient, usually employing 120-140 kVp for the knee. Dual energy CT scans the patient with 2 different energies (80kVp and 140 kVp). This can be achieved with a dual source scanner with 2 x-ray tubes scanning simultaneously. Alternatively, a single source scanner can be used with rapid switching between the two  kVp energies. Analysis of the differences in attenuation observed in images at the two different energies allows the composition of materials to be determined.

MedicalResearch.com:  In your conclusion you talk about DECT in the emergency room.  Why CT instead of MRI?

Dr. Glazebrook: While MRI is considered the gold standard for evaluation in internal derangement of the knee, MRI is rarely used acutely in the emergency room for an injured knee due to the long scan time requiring that the patient hold the injured knee still in a sometimes uncomfortable position, lack of immediate availability and cost. CT is very fast and readily available in the ER. The patient can place the knee in a comfortable position, and only has to hold it still for just a few seconds. CT is frequently used already to evaluate fracture alignment in patient with knee injuries. Our work points to additional helpful information that can be obtained simply by replacing the traditional CT exam with a dual-energy CT exam. No additional radiation or scan time is required. The scanners capable of these exams are now routinely available, although not at every medical facility.

MedicalResearch.com:  How common are acute knee injuries?

Dr. Glazebrook: Anterior cruciate ligament (ACL) tears are one of the most frequent ligamentous injuries of the knee, with skiing, soccer, basketball being common sports responsible for these injuries. There are approximately 175,000 ACL reconstructions performed in the United States annually.

MedicalResearch.com:  How does the combined sensitivity and specificity you found with DECT compare to the current standard way of imaging knee injuries in the ER?

Dr. Glazebrook: Radiographs of the knee are taken to determine if there is a displaced fracture that may require early intervention. There are indirect signs such as Segond fracture or deepened lateral sulcus sign  which have a high association with ACL injury, however these are not often seen radiographically. Knee effusions can also be identified radiographically, however these are non-specific for significant internal derangement of the knee. If there are no obvious fractures, the patients are sent home with a brace and crutches. If a fracture is suspected, the patient is sent to CT.

MedicalResearch.com:  Have you changed your practice based on the results of the study?

Dr. Glazebrook: Not yet. We are currently working on implementing DECT protocols and including the oblique sagittal soft tissue reconstruction plane in our routine trauma knee CT to evaluate for ACL integrity.

MedicalResearch.com:   Were you surprised by the results of the study? If yes, what surprised you?

Dr. Glazebrook: Our study shows that the resident had a very high sensitivity for detecting ACL injury and still had high specificity and accuracy, though slightly lower than that of the experienced radiologist. This did surprise us, however the resident was familiar with the MRI appearance of ACL injury and was able to use this knowledge to interpret the CT images.  Even though this is a new use for this modality, the CT images were of sufficient spatial resolution and diagnostic quality for radiologists with a  range of MSK training to visualize significant ligamentous injury. In the ER setting, acute findings such as fractures, joint effusion, and soft tissue edema would also be available to the interpreting resident to increase confidence in their interpretation of ACL integrity.

MedicalResearch.com:  Is there anything you’d like to add?

Dr. Glazebrook: The benefit of DECT in evaluation of knee injuries is not confined to assessment of the ACL and fractures. DECT can also be used to remove the calcium (virtual non-calcium technique) for the assessment of bone bruise or contusion within the marrow. Studies have shown that bone contusions are found in patients with more severe internal derangement of the knee. Also, there are characteristic patterns of bone contusions  which are often seen with ACL pivot shift injury that can be seen on DECT virtual non-calcium imaging. A study performed at our institution using the same cohort of patients as our current dual reader study, again using MRI as the gold standard, showed that DECT could provide reliable assessment for the presence or absence of bone bruising. The study also showed that approximately half of the MRI proven bone bruises resolved by 4 weeks and >90% were gone by 8 weeks following injury(unpublished data).

A CT scan can be performed in only a few seconds, compared to 30 to 40 minutes for an MR study. Modern CT systems have the ability to reconstruct very thin images in virtually any plane from the original scan data. This allows other soft tissue structures, such as menisci, collateral ligaments, and bone bruises to be evaluated in addition to fractures. MRI is still the preferred imaging method for evaluation of ACL knee injuries, however our current study shows that DECT can detect ACL disruption, making it a more versatile exam. This is clinically important because early identification of significant internal derangement of the knee could facilitate more rapid treatment for patients with knee trauma.


Glazebrook KN. E294. Presented at: American Roentgen Ray Society Annual Meeting; April 14-19, 2013; Washington, D.C.