Chest: Angiography: Real Time Visualization of Volume Data Applications in CT Angiography

Elliot K. Fishman, M.D.

CT Angiography has developed over the past few years from a study that seemed to have potential to a study that can replace up to 80% of catheter based classic diagnostic CT angiography. The advantages of CT Angiography are numerous including:

Accuracy equal to angiography across a wide range of clinical applications.

Cost almost 70% less than classic angiography.

Better patient compliance with less potential complications.

Faster exam study times even in the most difficult patients.

The ability to combine routine CT studies with a CT angiographic exam to decrease overall patient cost to the system.



The usefulness of CT Angiography and its acceptance by the general radiologic community is very dependent on the performance of the actual image processing. Resistance to the use of CT Angiography often focuses on several factors including:

Lack of availability of needed scanners or 3D workstation.

Lack of understanding of the role of CT angiography as compared to classic angiography.

Workflow and manpower issues result in lack of use or introduction of new technologies.

Lack of understanding as to how to perform a CT Angiogram or what to do with the post processed data.



A summary of all possible applications of CT Angiography is impossible because of both time requirements of this talk as well as to the fact that applications are in a state of rapid development. For example, CT Angiographic coronary artery stenosis is not yet a routine study but by next year might be. However, to provide a feel of the depth of applications some published results are presented.



Renal Applications

"Imaging with 3D CT provides a comprehensive evaluation and the perspective necessary for evaluating the patient with a renal mass." Planning Nephron-Sparing Renal Surgery Using 3D Helical CT Angiography
Smith PA, et al.
J Comput Assist Tomogr 1999; 23(5): 649-654

"The 3D helical CT uniquely assists the urologist by providing preoperative information in a flexible display that aids in determining whether nephron-sparing surgery is possible and planning the surgical procedure."

Planning Nephron-Sparing Renal Surgery Using 3D Helical CT Angiography
Smith PA, et al.
J Comput Assist Tomogr 1999; 23(5): 649-654

"The 3-D volume rendering CT accurately depicts the renal parenchymal and vascular anatomy in a format familiar to most surgeons. The data integrate essential information from angiography, venography, excretory urography and conventional 2-D CT into a single imaging modality, and can obviate the need for more invasive imaging."

3-Dimensional Volume Rendered Computerized Tomography for Preoperative Evaluation and Intraoperative Treatment of Patients undergoing Nephron-Sparing Surgery
Coll DM, et al.
J Urology 1999; 161:1097-1102

"The enhanced 3-D computerized tomography reformation and reconstruction process appears to be as accurate as renal angiography for arterial anatomy, and more sensitive than angiography and IVP in evaluating venous and parenchymal anatomy."

Interactive three-dimensional computerized tomography reconstruction in evaluation of living renal donors
Lerner LB, et al.
J Urology 1999; 161:403-407

"In the evaluation of renal artery stenosis, CT angiography with VR is faster and more accurate than CT angiography with MIP. Accessory arteries not depicted with conventional angiography were depicted with both CT angiographic algorithms."

Renal Artery Stenosis: CT Angiography- Comparison of Real-time Volume-rendering and Maximum Intensity Projection Algorithms
Johnson PT, et al.
Radiology 1999; 211:337-343

"The 3-D volume rendering CT accurately depicts the renal parenchymal and vascular anatomy in a format familiar to most surgeons. The data integrate essential information from angiography, venography, excretory urography and conventional 2-D CT into a single imaging modality, and can obviate the need for more invasive imaging. Additionally the use of videotape in an operative setting provides concise, accurate and immediate 3-D information to the surgeon, and it has become the preferred means of data display for these procedures at our center."

3-Dimensional Volume Rendered Computerized Tomography for Preoperative Evaluation and Intraoperative Treatment of Patients undergoing Nephron-Sparing Surgery
Coll DM, et al.
J Urology 1999; 161:1097-1102

"Of 77 renal arteries identified at surgery 74 were detected by 3-D volume rendered CT (96%). 3 small accessory arteries were missed including 1 in a cross-fused ectopic kidney. All major venous branches and anomalies were identified including 3 circumaortic left renal veins. Of 69 renal veins identified at surgery 64 were detected by 3-D volume rendered CT (93%). All 5 renal veins missed by CT were small, short duplicated right branches of the main renal vein."

3 Dimensional Volume Rendered Computerized Tomography for Preoperative Evaluation and Intraoperative Treatment of Patients undergoing Nephron- Sparing Surgery

Coll DM, et al.
J Urology 1999; 161:1097-1102

"When compared with US or nonenhanced MR angiography, CT is the preferred method for evaluation of accessory renal arteries and proximal branches of the renal artery."

US, CT, and MR Evaluation of Accessory Renal Arteries and Proximal Renal Arterial Branches
Halpern EJ, et al.
Acad Radiol 1999; 6:299-304

"Three-dimensional helical CT angiography of renal transplant recipients presenting with hypertension, graft dysfunction, or both after transplantation yields valuable information that can be used to guide further therapy."

Three-Dimensional Helical CT Angiography in Renal Transplant Recipients:A New Problem-Solving Tool
Hoffman LV, et al
AJR 1999; 173:1085-1089

Liver

"Volume rendered 3D-CT images during intravenous injection produced 3D images of high quality with excellent visualization of tumors and their relationship to vital structures." Three Dimensional Imaging of Liver Tumors Using Helical CT During Intravenous Injection of Contrast Medium
Uchida M, et al.
J Comput Assist Tomogr 1999; 23(3):435-440

"Helical CT angiography holds promise as a screening modality for the detection of TIPS stenosis or occlusions."

Transjugular Intrahepatic Portosystemic Shunt: Accuracy of Helical CT Angiography in the Detection of Shunt Abnormalities
Chopra S, et al.
Radiology 2000; 215:115-122

"Helical CT angiograms correctly demonstrated 30 (97%) of the 31 morphologic abnormalities and allowed correct diagnosis of 22 (92%) of the 24 hemodynamically significant abnormalities."

Transjugular Intrahepatic Portosystemic Shunt: Accuracy of Helical CT Angiography in the Detection of Shunt Abnormalities
Chopra S, et al.
Radiology 2000; 215:115-122

Vascular System

"Helical CT angiography with dual-slice scanning is a useful and minimally invasive technique that can provide with high accuracy all the necessary information for treatment of abdominal aortic aneurysms." Abdominal Aortic Aneurysm: Pretherapy Assessment with Dual-Slice Helical CT Angiography
Qanadli SD, et al.
AJR 2000; 174:181-187

"CT angiography is the preferred method for establishing the presence of perigraft leakage following treatment of aortoiliac aneurysms with stent-grafts."

Aortic Aneurysmal Disease: Assessment of Stent-Graft Treatment- CT versus Conventional Angiography.
Armerding MD, et al.
Radiology 2000; 215:138-146

"Perigraft leakage was the most commonly identified complication. Twenty perigraft leaks were detected in the results of 42 examinations. Sensitivities and specificities for detecting perigraft leakage was 63% and 77% for conventional angiography and 92% and 90% for CT angiography, respectively."

Aortic Aneurysmal Disease: Assessment of Stent-Graft Treatment- CT versus Conventional Angiography
Armerding MD, et al.
Radiology 2000; 215:138-146

"Assuming equal diagnostic utility and procedure related morbidity, institutions may have substantial cost savings whenever CT angiography can replace intraarterial DSA for imaging AAA."

Cost Identification of Abdominal Aortic Aneurysm Imaging by Using Time and Motion Analyses
Rubin GD, et al.
Radiology 2000; 215:63-70

"The mean total direct cost of intraarterial DSA was $1,052 +/- 71, and that of CT angiography was $300+/- 30, which are significantly different. With 95% confidence, intraarterial DSA cost 3.2-3.7 times more than CT angiography for the assessment of AAA."

Cost Identification of Abdominal Aortic Aneurysm Imaging by Using Time and Motion Analyses
Rubin GD, et al.
Radiology 2000; 215:63-70



Nearly all of these articles however have used 3D imaging in a non- interactive mode; that is, the images were preselected based on planes around the body or preselected planes or projections like every 45 degrees around the x or y axis. These preset techniques do have an advantage in providing a constant display for the user from study to study as well as the ability to use radiologic technologists to supervise the 3D operation. However, the use of preset images ignores the reality that the ideal projection is nearly impossible to choose from case to case. We have previously shown that even in a process as simple as an acetabular fracture that the ideal imaging plane cannot be selected in advance.

With vascular imaging one of the limitations of classic catheter angiography has always been that a preset projection is not always the ideal projection because of anatomic variations. This can be easily overcome when CT angiography is performed using a real time display.

Discussion of real time display first must require a definition of the meaning of real time imaging. Our definition is the ability to interactively look at a CT volume without any lag in movement through the volume as if it was in your hand and could be moved through 360 degrees of freedom. The ability to use cut planes to slice into and/or through the volume is also required for real time 3D imaging. The ability to do real time imaging requires enough computer processing speed to provide a frame rate of 15-20 frames per second. This is important to recognize as the need for faster computer processing will be required if we are to be able to do real time rendering with MDCT datasets.

The advantages of real time rendering can be measured in both abstract as well as absolute forms. They include:

A radiologist is more likely to use a system where their action provide an immediate result.

Referring physicians are more apt to spend the time to review images on a system that is interactive.

The ability to obtain the correct diagnosis/information needed is enhanced in an interactive mode where any plane or perspective can be selected.

An interactive display is more likely to become integrated into the general workflow.

Enhancements to the 3D interactive display can make it the primary mode for viewing/reading/analyzing CT datasets.

Advanced display techniques like stereo display are only possible when images are viewed interactively on a workstation.

Interactive real time rendering allows optimizing of rendering parameters and the integration of the 3D volume with graphic tools.

The use of cut-plane editing in real time is usually adequate in most cases. In select cases prestudy editing of the volume may be more useful.

Interactive display allows easy to use tools to change between volume rendering and MIPS technique. Both techniques are helpful in select cases especially in vascular imaging and hepatic imaging.

Real time display will become the standard review mode for all CT datasets in the near term. Volume imaging and display will become core functionality and represent the next challenge that CT must meet in this new millennium.