3-D renderings find role in musculoskeletal pathology studies

November 1, 2001

CT has always played a prominent role in the evaluation of musculoskeletal pathology. With the advent of spiral CT and, most recently, multidetector row CT (MDCT), the data sets available for image analysis and for postprocessing and display have unprecedented image resolution and detail. Concurrent with this advance is the development of postprocessing techniques, especially three-dimensional volume rendering.

CT has always played a prominent role in the evaluation of musculoskeletal pathology. With the advent of spiral CT and, most recently, multidetector row CT (MDCT), the data sets available for image analysis and for postprocessing and display have unprecedented image resolution and detail. Concurrent with this advance is the development of postprocessing techniques, especially three-dimensional volume rendering.

The newest scanners provide increased options for protocol design and optimization of both data reconstruction techniques and postprocessing tools. The quality of a CT data set depends on many factors, from mAs to kVp to slice thickness. Routine CT imaging of bone has classically required thin-section imaging (3 mm or less) and close interscan reconstruction intervals (3 mm or less), especially when multiplanar reconstruction or 3-D imaging is needed.

With MDCT, the user has increased flexibility; instead of selecting the slice thickness prior to the study, the user chooses the scan collimation. On our Siemens Somatom Volume Zoom scanner, we use four detectors for data acquisition from each 500-msec scan rotation. The detectors typically chosen are either 1 mm or 2.5 mm, with the 1 mm now clearly the selection of choice in small-part imaging (e.g., wrist or hand). With the 1-mm detector a range of slice thickness can be obtained. For example, we can reconstruct data at slice thicknesses ranging from 1 to 8 mm. For the best detail and for the best 3-D reconstruction we will select 1-mm to 1.25-mm slice thickness and reconstruct the data at 1-mm intervals. The scan protocol typically includes 120 kVp, 150 mAs and a pitch of six to eight. On our scanner, pitch is defined as distance traveled per scan rotation/nominal slice thickness (e.g., 15 mm/sec travel with 2.5 mm collimators is a pitch of six). For smaller volumes, we can obtain a 0.5-mm slice thickness, although pitch is limited to two, which results in longer scan times for the volume. However, the advantage of 0.5-mm slice thickness is that the volume becomes an isotropic data set with the x, y, and z axes equal in size, as opposed to routine CT data sets, which are anisotropic. The advantages of an isotropic data set is that regardless of the plane in which the patient is scanned, reconstructed data in any plane are of equal resolution.

A past concern with spiral CT and skeletal imaging was that the image quality suffered on both the axial and 3-D display as higher pitches were used. This was due in part to the less than optimal scan reconstruction algorithms available and also to how the single spiral scans were obtained. With higher pitches, the slice profile might increase up to 27% (that is, a scan with a 5-mm collimation might actually be 6.5 mm) and the mAs would actually decrease on the higher pitches, resulting in poorer image quality. With MDCT, the slice thickness selected is the slice thickness achieved, without any blooming artifact. Also, the mAs are constant across the scan volume, regardless of the pitch selected.

Other routine choices in musculoskeletal CT imaging are key, and they depend on the following decisions:

  • Is IV contrast needed for the study? In cases of pelvic trauma, a CT angiogram may be done concurrently with the musculoskeletal imaging exam; numerous studies have shown that CTA is highly accurate in this situation. In addition, the presence of muscle pathology is best seen following infusion of IV contrast, regardless of whether one is looking at abscess, tumor, or myositis.

  • Will 3-D imaging or MPR (multiplanar reconstruction) be needed? These require use of thin collimation and close interscan spacing.

  • What anatomic area will need to be scanned? Scan protocols depend on the volume to be scanned (e.g., tibial plateau versus the entire tibia).

  • What reconstruction algorithm will need to be selected? The answer depends on the clinical application: In cases where soft tissue or muscle imaging is the primary process, a standard soft-tissue algorithm is used, but if bone detail is needed then a high-resolution algorithm is necessary. Note also that the ideal reconstruction algorithm will depend on whether 3-D rendering needs to be done. Some high-resolution algorithms will create images with too much noise and yield 3-D images of suboptimal quality.

Trauma Applications

One of the classic applications for CT and for 3-D imaging is the evaluation of musculoskeletal trauma. Even in the earliest days of CT, many articles pointed out the advantages of CT and 3-D imaging as both a diagnostic and clinical management tool. Patients commonly referred to CT were sent either because plain films were indeterminate as to fracture, or in cases of extensive fracture, where the true extent of injury needed to be determined for surgical or nonsurgical planning (Figure 1). The 3-D images in these cases changed the patient's management in up to 30% of cases.

In the last 20 years, while little has changed in referral patterns for many trauma applications, other changes have occurred. CT scanners have become more available, speeds have increased, CT scanners have been placed in or near emergency rooms, and staffs have inaugurated systems for rapid triage of patients, with CT expediting diagnoses in crowded emergency rooms. In the ER, new imaging algorithms for spinal trauma as the initial study are becoming common. (Figure 2).

The use of MDCT is ideal in all trauma applications. CT can limit the need for sedation and provide a single comprehensive exam for imaging the solid organs and the skeletal structures. This is especially true in pediatric patients, for whom MDCT virtually has eliminated sedation.

The high-quality data sets generated by MDCT are important in small-part imaging, as in the wrist (Figures 3 and 4).

The use of 0.5-mm slice thickness and isotropic data sets are ideal for this application. Supplemented by 3-D renderings, they can produce all of the necessary views for surgical decision-making. Isotropic data sets eliminate the need for two views and speed up the examination without compromising quality.

In the evaluation of pelvic trauma, MDCT is ideal for combining skeletal imaging with more complicated studies including CT angiography, where it looks for vascular injury and where CT cystograms can help to exclude bladder injury in patients with pelvic trauma. CT cystograms are as accurate as classic cystograms for detecting bladder injury.

Oncologic Applications

The role of CT scanning has continued strong even with the continuing evolution of MRI. Following are some clinical scenarios in which CT is used:

  • Inconsistencies arise between interpretation of plain radiographs and patient symptoms, or between bone scans and plain films.

  • The extent of tumor needs determination before choosing between surgery and radiation therapy.

  • A requirement arises to define the potential biopsy of a lesion, where biopsy can be done with CT guidance and, if necessary, CT fluoroscopy. (CT fluoroscopy allows biopsy of smaller lesions in more difficult locations and can also guide radio-frequency ablation of osteoid osteoma.)

  • One needs to define the presence and extent of disease that may be detected with bone scanning or PET scanning, with CT able to help determine whether increased uptake in a bone scan is Paget's disease or metastases in indeterminate cases.

The use of MDCT with optimal scan protocols is routinely supplemented by 3-D reconstructions in these patients (Figure 5). We routinely combine volumetric 3-D reconstruction with multiplanar coronal and sagittal reconstruction. The use of MDCT data as a volume display rather than as a slice-based display is critical, especially when looking at joint surfaces and articular zones. With ever-larger datasets being generated, the 3-D display will become the dominant display in the future.

Congenital Deformities

CT has always evaluated congenital hip disease. Murray and Crim, in a recent review on the radiologic imaging and treatment of developmental dysplasias of the hip, recommended using multidetector CT with thin-section imaging.

In addition to the virtual elimination of sedation, MDCT provides ideal data sets for 3-D reconstruction. Numerous authors have documented that CT and 3-D imaging are invaluable in both preoperative planning and postoperative follow-up in patients with developmental dysplasias of the hip. The more severe the deformity, the more valuable the reformation of the data set becomes. In younger patients, it is important to select the lowest scan parameters possible to minimize the radiation dose to the patient.

Similarly, in areas such as the chest wall, imaging can be valuable in detecting benign anatomic variations that can simulate disease. We have also found CT to be helpful in the evaluation of a patient's pre-pectus excavatum repair as well as in patients with failed pectus repairs (Figure 6). In the latter group of patients, Pretorius et al previously showed how management is affected by the use of 3-D CT.

The use of isotropic data sets with 0.5-mm collimation is ideal for evaluation of the painful foot where a tarsal coalition is one of the more common diagnoses. The ability to reconstruct data in any plane obviates a second set of scans and provides a volume display that can help detect and define bony or fibrous unions. It is not surprising that in cases where tarsal coalition is suspected that other diagnoses are made, based on the CT scan, to explain the patient's symptoms.

Postoperative Imaging

One of the most challenging clinical scenarios occurs in the patient who has had prior orthopedic surgery with the insertion of pins, plates, screws, or joint replacement. Classic axial CT may be limited due to artifact generated on axial CT scans caused by the metal implants. Image postprocessing can solve this problem. Some CT scanners provide postprocessing software that can reduce artifact or extend the CT scale to minimize the artifacts generated as a result of beam hardening.

A technique that we find especially valuable is the generation of multiplanar reconstruction and/or 3-D volume renderings, using thin collimation and narrow interscan spacing. Since artifact typically is random on each individual CT slice, the reconstruction of overlapping CT scans will limit the artifacts and result in diagnostic-quality CT images. Specific applications where we have found this valuable include assessment of fracture healing or suspected postoperative infection, evaluation of suspected failure of orthopedic hardware, and evaluation of patients with persistent pain with hardware in place. When using 3-D we have found the ideal: using color for metal (blue works best) against the white opacification usually selected for bone. Our experience with MDCT involves slice thicknesses ranging ideally from 0.5 to 1.25 mm, with a 0.5 to 1-mm reconstruction interval (Figure 7).

Growing Role for CT

The role of CT in the evaluation of musculoskeletal pathology will continue to grow and evolve with the diffusion of MDCT and its evolution. New patient protocols will be developed, especially in the emergency room setting, where CT in many cases will be the initial and only study performed in patients with trauma. The use of CT in a volume display mode will also be ideal for rapid screening of the multisite trauma patient and will potentially become a cost-effective, time-effective study technique. The next generation of MDCT scanners will also allow routine use of isotropic data sets of 0.5-mm thickness. This is ideal for high-resolution imaging, which will be especially valuable in musculoskeletal imaging, especially for small-part CT scanning. Whether for trauma, infection, or oncologic applications, the role of CT is just beginning to be understood.

Dr. Fishman is a professor of radiology and oncology at Johns Hopkins University.

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