|Marc J. Fenstermacher, MD|
Since the early reports of three-dimensional techniques for displaying data from CT examinations first began to appear in the literature, these methods and the technology that support them have undergone tremendous advances. 1-4 3D CT is now utilized routinely in most academic institutions for CT angiography (CTA) 5 and is being utilized with a growing number of imaging applications. Although comparable techniques using CT and MRI are rarely evaluated together in published studies, 3D and multiplanar display techniques in CT are competing with direct multiplanar and postprocessed 3D MR imaging such as MR cholangiopancreatography (MRCP) and MR angiography (MRA). 6-8 The advent and increased utilization of multidetector (multislice) helical CT units have allowed this development to supplant some applications of the more time-consuming and expensive MR techniques. Where specific soft-tissue contrast provided only with MRI is crucial to answering a clinical question, or iodinated contrast cannot be administered, CT may never supplant MRI. When CT provides the contrast resolution required for particular clinical situations, 3D CT with volume rendering and multiplanar reformatted images can provide higher resolution, artifact-free images in any plane, or curved “planes” along pertinent anatomic structures, in a shorter period of actual imaging time (acquisition of source images), with easier patient acceptance. 9
|Paul M. Silverman, MD|
A comprehensive analysis of the cost-effectiveness of 3D CT in general, or relative to MRI techniques, has not been performed to our knowledge. The relative values of these techniques are influenced by a number of subjective factors, including clinicians’ practices and requirements, and individual radiologists’ bias. Intangible or other nonreimbursable costs include radiologists’ time for image processing and interpretation of the additional images employed in 3D and multiplanar techniques, and the costs involved in acquiring, transferring, and archiving multiple image sets with multiple prospective and retrospective reconstructions. Although valid cost analysis is lacking, some of the hidden costs involved in these techniques may be reimbursed at the present time through judicious use of CPT codes assigned for 3D image processing and CT angiography, combining them when appropriate with routine CPT codes and charges. Some currently accepted CPT codes and their associated approximate technical and professional charges at one institution are provided in Table 1.
As technology has progressed from single-detector helical CT to multidetector row helical scanning, a number of techniques have been employed for reconstructing and displaying the helical data set in novel ways. 10 Multiplanar and curved reformations, maximum intensity projection (MIP), minimum intensity projection (minIP), volume rendered, and real-time interactive volume rendered (endoscopic) images have been applied to many organ systems and clinical problems. 11-20
|Table 1. Current Procedural Terminology codes for 3D imaging in use at MD Anderson.|
Many of these applications have been in the area of oncologic imaging. There are a variety of techniques under development, and at times there may be a disconnect between what has been scientifically proven to be advantageous vs what is clinically useful at a particular institution, and in the hands of a given team of imagers and clinicians. At our institution, the current use of multiplanar and 3D CT in clinical practice can be categorized by organ system.
CTA is currently supplanting MRA for the evaluation of the abdominal aorta, major branches (including renal arteries), iliac vessels, and specific branch vessels that abut potentially resectable disease 5,8,14,21,22 (Figure 1). The same techniques employed in the abdomen and pelvis can be applied to the chest. Multiplanar reformations and 3D displays of arterial anatomy are utilized particularly by urologists for revealing the relationship of nodal masses to the aorta, vena cava, and renal arteries in cases of testicular cancer prior to retroperitoneal lymph node dissection (Figure 2). We have found multiplanar volume reformation and curved reformation to be ideal for depicting the relationship of soft-tissue masses to vessels in cases of planned resections. All bolus-contrast-enhanced angiographic techniques depend on matching the timing of image acquisition to the transit of contrast through the vessels of interest. The advent of multidetector helical CT has allowed the acquisition of larger areas of coverage with thinner slice collimation and much faster imaging times per series compared with single-detector units. We routinely image the entire abdominal aorta and proximal common iliac arteries in less than 20 seconds with 2.5-mm-thick slice reconstruction, 1.25-mm interval. This is repeated during multiple phases of contrast enhancement when indicated such that arterial, portal venous, and delayed venous phases may be acquired (Figure 3). Extremely small accessory renal vessels may require 1.25-mm-thick slice reconstruction with 0.63-mm reconstruction interval. When transferred to a workstation for angiographic image creation, the result is very high-quality curved, multiple oblique, and volume-rendered images. Pertinent vessels are demonstrated, the adjacent soft tissues are well depicted, and their relationship to vessels visualized. In the oncologic setting, tailoring the examination to each clinical situation allows the radiologist to provide the angiographic images needed by the clinician as well as the appropriate phase(s) of contrast-enhanced axial images for evaluating other organs, such as the liver.
Data are transferred to a workstation for creating and filming curved and multiple oblique reformations. This step is time-consuming for the radiologist but rewarding for the clinician. While a technologist with special interest and training in image processing can perform this step in many cases, optimizing reformatted imaging planes often requires knowledge of anatomy and pathology that may be beyond the technologist’s reach. At the current stage of development of these techniques, interested cross-sectional radiologists should be encouraged to perform the reformations and reconstructions personally. The choice of optimum angiographic display (3D volume rendered, MIP reconstruction, curved reformation, and/or multiple oblique reformation) should be individualized for each case in order to avoid the generation of extremely large numbers of superfluous images that must be archived in addition to the routine axial images.
3D and volumetric techniques have been shown to be helpful for the preoperative evaluation of patients for partial liver resections, to measure the volume of the future liver remnant, and to choose patients requiring preoperative portal vein embolization, in order to grow a larger residual liver prior to resection. 20,23 For the evaluation of bile duct tumors and lesions involving or abutting the biliary tree, we have used multiplanar volume reformation in the oblique coronal plane, and occasionally other optimized planes, and curved reformation along bile ducts and pertinent vessels. This is an application for which standard coronal multiplanar volume reformations are provided by a specially trained technologist, obviating the need for time-consuming image processing by the radiologist except in selected cases. Routine acquisitions include 2.5-mm-slice thickness, 1.25-mm reconstruction interval, and separate acquisitions during arterial and portal venous phases of contrast enhancement.
Pancreatic cancer is evaluated in a manner similar to hilar liver tumors, with a single phase of contrast enhancement for parenchymal enhancement of the pancreas and optimal imaging of arterial anatomy. Multidetector helical CT scanning allows for imaging of the entire liver and pancreas in a single breath hold with 2.5-mm slice thickness and 1.25-mm reconstruction interval. While authors have reported using a variety of 3D techniques for angiographic evaluation in preoperative staging, 14,21 others have suggested that standard thin section transaxial helical CT images should be used in the evaluation of vascular involvement. 24,25 We utilize the transaxial thin section helical images for the primary identification of the pancreatic lesion and involvement of vascular structures, and supplement these with multiplanar volume reformations and curved reformations in optimal planes for depicting the mass and relationship to ducts and other structures when needed in selected cases.
Besides the depiction of the location and number of renal vessels using CTA techniques mentioned above, 3D and multiplanar imaging are employed in preparation for nephron-sparing surgery (partial nephrectomy), increasingly utilizing laparoscopic surgical techniques that require a thorough preoperative understanding of pertinent anatomy. 26 We primarily use multiplanar volume reformations in standard coronal and sagittal planes, and curved reformations, in arterial, nephrogram, and collecting system phases of contrast enhancement, to depict the relationship of a renal mass to vessels and renal pelvis, as well as provide a vascular road map. 3D volume rendering techniques are employed in selected cases. Reconstruction slice thickness is 2.5 mm with 1.25-mm reconstruction interval.
3D and multiplanar CT images have been shown to be useful supplements to the analysis of routine thin section helical CT images in the evaluation of pulmonary emboli (PE), and to outperform MRA in laboratory studies.27,28 The patient with possible PE is a very common clinical problem in the oncologic radiology department due to the patient’s condition, the underlying malignancy that may be associated with a hypercoagulable state, mechanical and hemodynamic effects of tumors, postoperative status, and other treatment-related factors. If a patient has an abnormal chest radiograph, multidetector helical CT of the chest with a PE protocol is performed instead of a radionuclide V/Q scan. Reconstruction slice thickness is 1.25 mm.
There are other 3D CT techniques currently under investigation at our institution and elsewhere that show promise for future routine clinical use. Real-time volume rendering techniques include virtual bronchoscopy,13 CT colonography (also known as virtual colonoscopy), 16,17 and other virtual endoscopic techniques such as virtual angioscopy. 29 3D CT for use in radiation therapy planning is under development.30 Plastic surgeons and orthopedists show great interest in using 3D techniques for designing plastic reconstructions and prostheses. There are many applications of 3D CT outside the clinical scope of oncologic imaging. As workstations become faster and more automated, and reimbursement standards catch up with imaging technology and clinical practice, 3D and multiplanar CT imaging will become routine components of a growing percentage of CT examinations across the spectrum of imaging facilities..
Marc J. Fenstermacher, MD, is associate professor of radiology in the Section of Body Imaging, and Paul M. Silverman, MD, is professor of radiology, Gerald D. Dodd Jr Distinguished Chair in Diagnostic Imaging, and chief, Section of Body Imaging, MD Anderson Cancer Center, Houston.
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- Smith PA, Fishman EK. Clinical integration of three-dimensional helical CT angiography into academic radiology: results of a focused survey. AJR. 1999;173: 445-447.
- Masui T, Takehara Y, Fujiwara T, et al. MR and CT cholangiography in evaluation of the biliary tract. Acta Radiol. 1998;39:557-563.
- Bendib K, Poirier C, Croisille P, Roux JP, Revel D, Amiel M. Characterization of arterial stenosis using 3D imaging: comparison of 3 imaging techniques (MRI, spiral CT and 3D DSA) and 4 display methods (MIP, SR, MPVR, VA) by using physical phantoms. J Radiol. 1999;80:1561-1567.
- Bourlet P, De Fraissinnette B, Garcier JM, et al. Comparative assessment of helical CT-angiography, 2D TOF MR-angiography and 3D gadolinium enhanced MRA in aorto-iliac occlusive disease [in French]. J Radiol. 2000;81:1619-1625.
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- Park SJ, Han JK, Kim TK, Choi BI. Three-dimensional spiral CT cholangiography with minimum intensity projection in patients with suspected obstructive biliary disease: comparison with percutaneous transhepatic cholangiography. Abdom Imaging. 2001;26:281-286.
- Wigmore SJ, Redhead DN, Yan XJ, et al. Virtual hepatic resection using three-dimensional reconstruction of helical computed tomography angioportograms. Ann Surg. 2001;233:221-226.
- Fishman EK, Horton KM, Urban BA. Multidetector CT angiography in the evaluation of pancreatic carcinoma: preliminary observations. J Comput Assist Tomogr. 2000;24:849-853.
- Rubin GD, Shiau MC, Leung AN, Kee ST, Logan LJ, Sofilos MC. Aorta and iliac arteries: single versus multiple detector-row helical CT angiography. Radiology. 2000;215:670-676.
- Vauthey JN, Chaoui A, Do FA, et al. Standardized measurement of the future liver remnant prior to extended liver resection. Methodology and clinical associations. Surgery. 2000;127:512-519.
- Loyer EM, David CL, Dubrow RA, Evans DB, Charnsangavej C. Vascular involvement in pancreatic adenocarcinoma: reassessment by thin-section CT. Abdom Imaging. 1996;21:202-206.
- Baek SY, Sheafor DH, Keogan MT, DeLong DM, Nelson RC. Two-dimensional multiplanar and three-dimensional volume-rendered vascular CT in pancreatic carcinoma: interobserver agreement and comparison with standard helical techniques. AJR. 2001;176:1467-1473.
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- Smith PA, Heath DG, Fishman EK. Virtual angioscopy using spiral CT and real-time interactive volume-rendering techniques. J Comput Assist Tomogr. 1998;22:212-214.
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