Noninvasive 3d angiography-CT or MRI
INTRODUCTION-
Angiography is imaging of blood vessels. Traditionally this has been accomplished by injecting contrast material [usually containing iodine] into the blood vessels so that an xray can be taken, showing the outline of the lumen of the vessel. This is a two dimensional image, and an accurate diagnosis often necessitated more than one view. Conventional angiography involves placement of catheters within various blood vessels in the body, a procedure not without complications. Digital angiography improved some aspects of vascular imaging, but generally still requires a catheter.
Ultrasound with color doppler is a combined method of imaging and physiologic measurement that can evaluate for the presence of stenoses and compromised flow. Some arteries are not easily seen with ultrasound, and therefore cannot be evaluated. Complex surgical anatomy is also difficult to evaluate.
Even as improvements are made in conventional and digital angiography as well as doppler
ultrasound, within the past few years two new vascular imaging modalities have emerged. These
are the subject of the following discussion.
MR ANGIOGRAPHY
Although CT has been available for a longer tim than MR, it was MR that first developed techniques for vascular imaging. It was noticed early in MR that vessels within which there was flow generally were black on images [no signal]. This allowed for an inherent contrast modality. Various pulse sequences were developed that resulted in signal being emitted only from certain blood vessels. Because MR is a volume imaging modality, 3 dimensional images could then be generated from this information.
The more technical aspects of MR angio are discussed in "An Introduction to MR Angiography"
by David Soloner, PhD [Radiographics 1995;15:453-465].
"This article provides an overview of the basic principles of magnetic
resonance (MR) angiography. The parameters in MR imaging manipulated to generate high contrast between flowing nuclei and stationary
tissue are discussed. Two primary strategies are used: time-of-flight (TOF) MR angiography, which creates differences in magnetization
magnitude between flowing and stationary nuclei, and phase-contrast MR
angiography, which induces changes in the spatial orientation, or phase, of flowing nuclei relative to stationary nuclei. The end result of an MR
angiographic study is typically a three-dimensional data set composed of either sequential two dirnensional
sections or true three dimensional data. Two-dimensional TOF methods are sensitive
to slow flow and are valuable for differentiating between slow flow and occlusion.
Three-dimensional TOF methods have better resolution and are more useful in imaging tortuous
vessels. Phase contrast MR angiography can be effectively used to avoid problems of magnetization saturation that occur in three dimensional
TOF studies and to eliminate signal from high-intensity stationary material such as blood products, which
may appear bright and mimic flow signal in TOF studies. Careful use of postprocessing
tools aids in the assessment of vascular abnormalities once the MR angiographic data
have been acquired."
CT ANGIOGRAPHY
With the recent introduction of extremely fast CT scanners which use a continuously spiralling
xray tube [i.e., spiral CT], high resolution imaging of a vascular bolus is possible with computer
analysis of the data resulting in a 3 dimensional model. Disadvantages inherent to CT and not
present in MRA include the interference of bone and calcific plaque in generating these images.
Various corrections can be used. An inherent advantage is the relative speed of CT and
potentially higher resolutions [<1mm]. This is definitely an advantage in areas of motion such as
the chest and abdomen. CT is often more readily available in emergent situations, and magnetic
hardware that accompanies ill patients is not a problem with CT.
CAROTID IMAGING
MRA and CT angio are both used in the evaluation of extracranial cerebral circulation.It has been conclusively established by the NASCET study that stenoses of the internal carotids greater than 70% benefit from surgical correction. Therefore accurate diagnosis and evaluation of this condition of of great interest to treating physicians. In some institutions, ultrasound followed by MR angiography is adequate for pre-operative surgical evaluation. Of note is recent Medicare decision to deny reimbursement to patients and physicians for MR angiographic studies unless there is a contraindication to invasive angiography. This decision of course was based on the wisdom of government bureaucrats who reviewed ancient literature on the subject.
Many comparisons between the different modalities is anecdotal or more importantly based on
the level of imaging equipment available in a given place. One study in 1993 showed fairly good
results for MR angiography, and I suspect the conclusion in 1996 might be different. In this
article entitled "Carotid Artery: Prospective Blinded Comparison of Two-dimensional Time-of
Flight MR Angiography with Conventional Angiography and Duplex US,"[Radiology
1993;186;339-344] J. Huston III MD et al report that:
A prospective blinded comparison of two-dimensional (2D) time-of-flight
(TF) magnetic resonance (MR) angiography and color duplex flow ultrasound (US) with
conventional angiography as a standard of reference was performed in 50 patients with
hemispheric ischemic symptoms. The guidelines of the North American Symptomatic Carotid
Endarterectomy Trial for measuring stenosis of the internal carotid artery were
utilized for maximum-intensity-projection (MIP) images and conventional
angiograms. While the 2D TF MIP technique overrepresented the degree of stenosis in the internal
carotid arteries, it matched the performance of color duplex flow US as measured
by means of receiver operating characteristic curves. A signal void on the MIP images
corresponded to a 70% or greater internal carotid artery stenosis in 17 of 20 arteries.
At its current state of development, the accuracy of 2D TF MR angiography equals
that of US in characterizing the degree of carotid stenosis but cannot be considered a
replacement for conventional angiography.
INTRACRANIAL CIRCULATION
The major intracerebral vessels originate at the base of the skull and form an anastomotic ring called th Circle of Willis. Unfortunately for CT, bone at the base of the skull has always created imaging difficulty. With improved algorithms, however, CT angio has been shown to be at least equal to MRA, in the very challenging task of identifying intracranial aneurysms as reported by D. Katz et al in "Circle of Willis:Evaluation with Spiral CT Angiography, MR Angiography, and Conventional Angiography" [Radiology 1995;195:445-449]
PURPOSE: To evaluate the use of spiral computed tornographic (CT) angiography in the analysis of the arteries of the circle of Willis and compare these results with magnetic resonance (MR) angiography and conventional angiography.
MATERIALS AND METHODS: The results in 17 patients who underwent examination were prospectively studied in a blinded fashion. The presence or absence of the arteries of the circle of Willis was determined by using maximum intensity projection reconstructions from CT angiography and MR angiography. These results were compared with results from conventional angiography.
RESULTS. Similar sensitivities were determined for CT angiography (88.5%) and MR angiography (85.5%); however, MR angiography was found to differ significantly (P = .005) from conventional angiography. No significant differences (P> .05) were found between the two modalities and conventional angiography in the detection of the anterior, middle, or posterior cerebral arteries or the anterior communicating artery.
CONCLUSION: Spiral CT angiography is highly sensitive in the
detection of arterial anatomy in the circle of Willis and is a reliable alternative
to MR angiography.
ABDOMEN
Consensus opinion now is that the preoperative evaluation of abdominal aneurysms can usually
be accomplished nonivasively with CT or MR angiography in order to demonstrate the clinically
important anatomy. This was studied by K. Ecklund MD and reported in "MR Angiography as
the sole Method in Evaluating Abdominal Aortic Aneurysms: Correlation with Conventional
Techniques and Surgery" [Radiology 1994;192:345-350]
PURPOSE: To compare magnetic resonance (MR) angiography with conventional preoperative imaging
techniques and surgical findings in the evaluation of abdominal aortic aneurysms (AAAs).
MATERIALS AND METHODS: MR angiography was performed in 40 patients with an AAA. Two-dimensional time-of~flight MR angiography with maximum intensity projections was compared with conventional angiography, ultrasound, computed tomography, and surgery.
RESULTS: In 18 of 20 patients, MR angiography demonstrated more extensive disease than did angiography. MR angiography depicted 41 of 43 renal arteries and seven of eight renal artery stenoses (one false-negative finding of mild stenosis) identified at angiography. when iliac arteries were imaged (30 patients), good correlation with angiography was seen in all but one patient (resulting from surgical clip artifact). Ten iliac stenoses were seen at both studies. Angiography caused underestimation of the extent of seven iliac aneurysms.
CONCLUSION: MR angiography can provide all of the necessary preoperative information for evaluation of AAA and can replace conventional angiography in many cases.
PULMONARY
Some recent literature has indicated that spiral CT angio of the chest can increase the specificity
for the diagnosis of pulmonary embolus. Well seen on this modality are the larger and more
centrally positioned thrombi.
CONCLUSION
CT and MR angiography are powerful noninvasive techniques for the diagnosis of a variety of
vascular disorders, including carotid artery stenosis, intracranial aneurysms, abdominal
aneurysms, aortic dissections, renal artery stenosis. Rapid progress is being made in both CT and
MR technology improving image quality and resulting in improved diagnostic accuracy.
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*Of note in the socioeconomic arena is that Medicare has ceased reimbursing for MR
angiography other than head and neck and those only for extremely limited situations. CT
angiography is a minimal charge [3d reconstruction] in additional to the anatomy studied, and
therefore may be "cost-effective" if ordered in conjunction with an imaging study.
