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DOI:10.2214/AJR.06.0945
AJR 2007; 188:W415-W422
© American Roentgen Ray Society

Original Research

Aortic Root Catheter-Directed Coronary CT Angiography in Swine: Coronary Enhancement with Minimum Volume of Iodinated Contrast Material

Arun Kumar1, Kostaki G. Bis1, Anil Shetty1, Amit Vyas1, Andrew Anderson2, Mamtha Balasubramaniam3, William O'Neill2,4 and Wendy Stein1

1 Department of Diagnostic Radiology, William Beaumont Hospital, 3601 W 13 Mile Rd., Royal Oak, MI 48073.
2 Division of Cardiovascular Disease, William Beaumont Hospital, Royal Oak, MI.
3 Research Institute-Biostatistics, William Beaumont Hospital, Royal Oak, MI.
4 Present address: Leonard Miller School of Medicine, Miami, FL.

Received July 21, 2006; accepted after revision October 31, 2006.

 
Address correspondence to K. G. Bis (kbis{at}beaumont.edu).

Supported by a research grant from Medrad, Inc.

WEB This is a Web exclusive article.


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of this study was to evaluate the minimum amount of contrast material for coronary imaging with aortic root catheter-directed enhancement and 64-MDCT angiography (MDCTA).

MATERIALS AND METHODS. A 64-MDCT scanner was used after animal institutional review board approval to study four swine (40-60 kg). Heart rate reduction to 65 beats per minute was achieved with atenolol by mouth and IV Cardizem. Common femoral artery access was obtained with a 5-French micropuncture kit and sonographic guidance. A diffusiontip (640 side holes), 5-French pigtail catheter was positioned in the aortic root on the CT table with a retrofitted C-arm fluoroscopy unit and connected to an arterial power injector. Aortic root MDCTA (retrospective ECG gating; collimation, 0.6 mm; tube rotation time, 0.33 second; scanning time, 10-12 seconds; tube voltage, 120 kVp; effective mAs, 850 mAs; pitch, 0.2; field of view, 109-123 mm; slice thickness and increment, 0.6 and 0.3 mm) was begun 1 second after the injection of 100 mL of various Visipaque (iodixanol) concentrations (10%, 20%, 30%, 40%) at 10 mL/s. Coronary mean and peak densities, 3D maximum intensity projections, and 4D projections were obtained.

RESULTS. The mean pooled coronary attenuation values (H ± SD) for the right (RCA), left anterior descending (LAD), and left circumflex (LCx) coronary arteries at various concentrations (10%, 20%, 30%, 40%) were as follows: 10% (RCA [232.6 ± 64.0], LAD [180.4 ± 45.1], and LCx [176.6 ± 56.2]); 20% (RCA [383.0 ± 98.7], LAD [324.3 ± 60.1], and LCx [331.8 ± 105.5]); 30% (RCA [441.8 ± 137.6], LAD [401.3 ± 125.8], and LCx [418.5 ± 173.0]); and 40% (RCA [717.3 ± 377.7], LAD [573.3 ± 233.3], and LCx [584.8 ± 189.0]). Coronary imaging with aortic root MDCTA was feasible at all concentrations, and the attenuation values were statistically significantly greater than 250 H at 20%, 30%, and 40% (p < 0.05). The attenuation values with aortic root MDCTA using one fifth of the volume of contrast material are comparable to those currently achieved both clinically and experimentally with peripheral IV MDCTA.

CONCLUSION. Aortic root MDCTA can depict the coronary arteries with as little as 20 mL of contrast material. This may provide an alternative means of coronary evaluation in patients with renal insufficiency.

Keywords: angiography • cardiovascular disease • cardiovascular imaging • catheters • CT coronary arteriography


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
In the past few years, several technologic advances for the evaluation of coronary artery anatomy have occurred. Conventional X-ray angiography has been the gold standard but has several technical and diagnostic limitations [1-3]. Recently, noninvasive peripheral IV contrast-enhanced 3D coronary imaging techniques with MDCT angiography (MDCTA) have become popular in the evaluation of the coronary arteries [4-6]. Although MDCTA is proving to be a promising technology, it also has several limitations [7-9].

A common feature of both conventional angiography and peripheral IV contrast-enhanced MDCTA (IV MDCTA) is their need for relatively larger doses of contrast material. At our institution, 100 mL of nonionic contrast material is administered via a peripheral IV injection for MDCTA. It is common to use as much or more with conventional angiography. Unfortunately, a large group of patients cannot tolerate this contrast load. The patients at risk include those with preexisting renal dysfunction, diabetes mellitus, dehydration, congestive heart failure, malignancy, and the elderly [10]. It has been shown that most patients develop a mild increase in creatinine levels after a contrast-enhanced imaging study. However, the increase is transient and clinically insignificant [11]. In patients with risk factors, however, the chance of developing clinically significant contrast-induced nephropathy is much higher [11]. For example, a 21fold increased risk of contrast-induced nephropathy occurs if a patient's creatinine level is > 1.5 mg/dL [12].


Figure 1
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  		Fig. 1 —5-French Medrad Vanguard diffusion catheter with 640 laser-drilled side holes (along gray pigtail end), distal tip constrictor (white end), and manually placed distal catheter bend along orange component.

 


Figure 2
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  		Fig. 2 —Pooled mean coronary attenuation with increasing concentration of Visipaque (iodixanol, GE Healthcare) using aortic root MDCTA (first four groups) and peripheral IV contrast-enhanced MDCTA (last group) with 100 mL of full-strength Visipaque. Light gray bars represent left anterior descending artery; dark gray bars, right coronary artery; and white bars, left circumflex artery.

 
Although the increased risk of developing contrast-induced nephropathy in patients with diabetes mellitus and normal renal function is low (0.6%), those patients with diabetes mellitus and renal insufficiency are three times more likely to suffer contrast-induced nephropathy than those with renal insufficiency alone, 19.7% versus 6% [12].

Contrast-induced nephropathy is defined as an absolute rise of 0.5 mg/dL in serum creatinine level or a > 25% increase from baseline [13]. Contrast-induced nephropathy is the third most common cause of hospital-acquired renal failure and a well-recognized risk of coronary angiography. It occurs in 1-20% of hospitalized patients, with rates as high as 50% among those with the strongest risk factors, including preexisting renal disease and diabetes [11, 13]. Furthermore, up to 30% of patients who develop contrast-induced nephropathy might develop a permanent decline in renal function [13, 14]. Dialysis may be required in some of these patients, with mortality rates ranging from 29% to 36% [15, 16].

Studies have shown that the volume of radiocontrast material administered directly correlates to risk of contrast-induced nephropathy [17]. On the basis of these findings, researchers have suggested that efforts be made to minimize the volume of iodinated contrast material given, particularly in high-risk patients [7, 18-20].

As a result of this, we studied this hybrid technique that uses aortic root catheter-directed enhancement and MDCTA (aortic root MDCTA) for imaging. Aortic root MDCTA has the benefits of IV MDCTA but with a fraction of the contrast load. This may allow physicians to safely evaluate the large group of patients who were previously contraindicated from undergoing IV MDCTA or even conventional angiography.


Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Animal Preparation
Animal institutional review board approval to conduct a series of studies in four female swine (40-60 kg) was obtained from our institution. Farm swine were acclimated 7-10 days in our research animal facility before any procedure. The animals were pretreated with atenolol (Tenormin, AstraZeneca) (100 mg by mouth twice a day) for 2 days as the initial means of heart rate reduction. Analgesics were administered for 24 hours before catheterization (carprofen [Rimadyl, Pfizer Animal Health]), 2 mg/kg subcutaneously; buprenorphine (Buprenex, Reckitt Benckiser), 0.01 mg/kg subcutaneously; acetylsalicylic acid (aspirin), 25 mg/kg by mouth. The animals were initially sedated with intramuscular ketamine (Ketaset, Fort Dodge Laboratories) (33 mg/kg). Induction of anesthesia was then achieved with oxygen, 1-2 L/min, and a 2-5% isoflurane (Forane, Anaquest) face mask. The animals were then intubated and an IV lidocaine drip (0.3 mg/kg/hr) was started for prevention of arrhythmia. Sodium thiopental (Pentothal, Hospira) (15-30 mg/kg) was used as needed during transportation to the hospital CT scanner. During the transportation, the animals were mechanically ventilated by hand.

Once on the CT scanner table, the animals were switched to mechanical ventilation. Sedation was maintained with oxygen, 1 L/min, and 1-2% isoflurane (Forane) and a sufentanil (Sufenta, Akorn) drip (0.015-0.030 mg/kg/hr). IV access with 20- or 22-gauge catheters was obtained in an ear and a lower extremity vein. IV fluids (2 mL/kg/hr) with normal saline were provided for hydration. Typical heart rates under this sedation protocol ranged from 100 to 120 beats per minute (bpm). The heart rate was further reduced to the desired 65 bpm with an IV bolus (20 mg) of diltiazem (Cardizem, Biovail Pharmaceuticals) followed by an IV diltiazem (Cardizem) drip (15 mg/hr).

Vascular Access
Common femoral artery access was obtained on the CT table under sterile conditions with a 5-French micro puncture kit (501 set, Cook) using a handheld probe and sonographic guidance (Site Rite unit, Bard Access Systems) for directing the access needle into the femoral artery. A 5-French sheath was inserted, and arterial pressure via the sheath was monitored with a portable unit. A guidewire (0.035 inches, Long Tapered, Cook) was placed into the ascending aorta using portable C-arm fluoroscopy guidance (series 9800 unit, OEC Medical Systems).

Arterial Catheter
Aortic root injection studies were performed with a 5-French Vanguard diffusion catheter (Medrad) (Fig. 1). The catheter has an end-hole constrictor accepting a guidewire and a series of laser-drilled micro side holes (n = 640) in the pigtail end. This catheter provides a uniform cloud-like distribution of contrast material. It also has significantly less associated catheter whipping when compared with a standard pigtail catheter. These effects are desired for more uniform enhancement of the aortic root. A bend was placed manually at the distal end of the catheter to direct the pigtail end centrally within the aortic root.

Nonselective Aortic Root Catheter Study
The location of the pigtail catheter end was confirmed in the aortic root using a retrofitted C-arm fluoroscopy unit. Total fluoroscopic time was less than 30 seconds. A hand injection of 5 mL of Visipaque (iodixanol, GE Healthcare) was made to confirm the position of the pigtail end. Aortic root contrast-enhanced MDCTA studies were initiated 1 second after the onset of arterial power injection (Provis Injector, Medrad; 1,000 psi pressure limit). One hundred-milliliter volumes of various concentrations (10%, 20%, 30%, 40%) of Visipaque (320 mg/mL) were mixed with normal saline and injected at 10 mL/s (n =4). Each pig was initially injected with the 10% solution followed by the 20% solution, the 30% solution, and finally, the 40% solution. We waited 10 minutes between injections to ensure the pig was stable clinically and to allow contrast clearance from the blood pool.

IV Study
We performed IV MDCTA in our swine experiments as a control. IV MDCTA studies (n = 4) were performed with a standard undiluted volume of 100 mL of contrast material at 5 mL/s with a 50-mL saline flush at 5 mL/s. A dual-head venous power injector was used (Stellant Injector, Medrad). The power injection was performed at 5 mL/s at a pressure limit of 325 psi. This was performed approximately 15 minutes after the completion of the arterial study. The lower extremity venous access site was used. Bolus tracking was performed with the region of interest being the mid ascending aorta. A trigger threshold value of 150 H above baseline was used.

MDCTA Protocol and Parameters
A 64-MDCT scanner (32 detectors) (Sensation 64, Siemens Medical Solutions) was used. MDCTA began with respiratory termination using a cranial-to-caudal acquisition to cover the heart with retrospective ECG gating and the following parameters: collimation and slice thickness, 0.6 mm; reconstruction increment, 0.3 mm; tube rotation time, 0.33 second; tube voltage, 120 kVp; effective current, 850 mAs; pitch, 0.2; and reconstruction field of view, 109-123 mm. ECG pulse gating for tube current modulation was not implemented to achieve homogeneous attenuation of the coronary vasculature throughout the cardiac cycle on 4D reconstructions. The radiation exposure using these parameters is similar for both the aortic root and the IV studies and was calculated at 13.9 mSv per MDCTA acquisition. Radiation exposures for the preprocedural topogram and bolus tracking (IV studies) were 0.085 and 0.95 mSv, respectively.

Single-sector and dual-sector reconstructions were performed. The data were reconstructed at 65% of the R-R length for all studies and then modified to a different phase start if motion artifacts were seen. This was directed with a preview series whereby one axial slice was reconstructed at every 10% of the R-R interval at the level of the motion artifact. Finally, 4D analysis was performed by reconstructing multiple 3D data sets at 5% intervals of the R-R length. These reconstructions were performed on the Siemens Wizard workstation and then transferred to a conventional angiography workstation for maximum intensity projections (MIPs) using the full volume and variable slab thicknesses, including 5, 30, and 60 mm.

After Imaging
At the completion of the imaging study, the sedated animals were sacrificed according to our standard hospital animal protocol procedure using a lethal IV injection of Euthasol (1 mL/10 lb [4.5 kg]), which is composed of pentobarbital sodium (390 mg/mL) and phenytoin sodium (50 mg/mL).

Postprocessing
The mean and peak coronary artery attenuations were measured on the Tera Recon workstation using two perpendicular measurements for the proximal, middle, and distal segments of the right coronary artery (RCA), the left anterior descending coronary artery (LAD), and the left circumflex coronary artery (LCx). A full-width-half-maximum technique was used [1]. The two measurements were made by displaying the coronary segments perpendicular to their long axes. Perpendicular diameters traversing the cross section of the coronary arteries were made, and the corresponding densities were recorded and then averaged for that particular segment. A total of three measurements (spaced 0.6 mm apart) were made and then averaged for the particular coronary segment. Results were then pooled over the entire vessel.

Statistics
A value of 250 H was used as adequate for coronary artery attenuation [1, 3, 21, 22]. The mean values for each artery at each concentration were first analyzed to see whether they met the distributional assumptions of the statistical tests that were being used to analyze them. Based on this preliminary assessment, either the parametric Student's t test or the nonparametric Wilcoxon's signed rank test was used to test whether the mean or median coronary attenuation value was significantly higher or lower than 250 H at each of these concentrations. Values for p of less than an alpha of 0.05 (probability of type 1 error) were considered statistically significant. Statistical analysis was performed using the SAS System for Windows (Microsoft), version 9.1.3, Service Pak 2.


Figure 3
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  		Fig. 3A —60-mm maximum intensity projections of right coronary artery obtained with aortic root MDCTA and various concentrations of Visipaque (iodixanol, GE Healthcare). Septal perforator branches of posterior descending artery (PDA) seen distally are better delineated with increasing contrast material concentration. Note faint enhancement of middle cardiac vein subjacent to PDA. A = anterior, L = left. Images obtained at concentrations of 10% (A), 20% (B), 30% (C), and 40% (D).

 


Figure 4
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  		Fig. 3B —60-mm maximum intensity projections of right coronary artery obtained with aortic root MDCTA and various concentrations of Visipaque (iodixanol, GE Healthcare). Septal perforator branches of posterior descending artery (PDA) seen distally are better delineated with increasing contrast material concentration. Note faint enhancement of middle cardiac vein subjacent to PDA. A = anterior, L = left. Images obtained at concentrations of 10% (A), 20% (B), 30% (C), and 40% (D).

 


Figure 5
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  		Fig. 3C —60-mm maximum intensity projections of right coronary artery obtained with aortic root MDCTA and various concentrations of Visipaque (iodixanol, GE Healthcare). Septal perforator branches of posterior descending artery (PDA) seen distally are better delineated with increasing contrast material concentration. Note faint enhancement of middle cardiac vein subjacent to PDA. A = anterior, L = left. Images obtained at concentrations of 10% (A), 20% (B), 30% (C), and 40% (D).

 


Figure 6
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  		Fig. 3D —60-mm maximum intensity projections of right coronary artery obtained with aortic root MDCTA and various concentrations of Visipaque (iodixanol, GE Healthcare). Septal perforator branches of posterior descending artery (PDA) seen distally are better delineated with increasing contrast material concentration. Note faint enhancement of middle cardiac vein subjacent to PDA. A = anterior, L = left. Images obtained at concentrations of 10% (A), 20% (B), 30% (C), and 40% (D).

 

Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
In current practice, optimum peripheral IV MDCTA achieves a mean coronary attenuation of approximately 250-300 H. We therefore used 250 H as our minimum desired value for mean coronary attenuation. Peripheral IV MDCTA produced mean coronary attenuation values of approximately 300 H for all three coronary arteries (Tables 1, 2 and 3). In the experimental studies, the injection of 100 mL of Visipaque at the 10% concentration was not able to produce mean coronary attenuation greater than 250 H in the proximal, middle, or distal coronary segments (Tables 1, 2 and 3). The injection of 100 mL of Visipaque at 20%, however, was able to produce mean coronary attenuation greater than 250 H in the proximal, middle, and distal segments; however, these results were not statistically significant (Tables 1, 2 and 3). As would be expected, increasing the concentration of Visipaque to 30% and 40% led to proportional increases in the mean coronary attenuation values (Fig. 2, Table 4). Increasing the concentration of Visipaque also produced better clarity of the coronary tributaries such as the RCA distal perforator and diagonal and obtuse marginal branches (Figs. 3A, 3B, 3C, 3D and 4A, 4B, 4C, 4D).


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  		TABLE 1: Coronary Attenuation Values for Aortic Root MDCT Angiography (MDCTA) and IV MDCTA in Proximal Segments of the Coronary Arteries in Four Swine at Various Concentrations of Visipaque (iodixanol, GE Healthcare)

 

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  		TABLE 2: Coronary Attenuation Values for Aortic Root MDCT Angiography (MDCTA) and IV MDCTA in Middle Segments of the Coronary Arteries in Four Swine at Various Concentrations of Visipaque (iodixanol, GE Healthcare)

 

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  		TABLE 3: Coronary Attenuation Values for Aortic Root MDCT Angiography (MDCTA) and IV MDCTA in Distal Segments of the Coronary Arteries in Four Swine at Various Concentrations of Visipaque (iodixanol, GE Healthcare)

 

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  		TABLE 4: Three Pooled Coronary Artery Attenuation Values in Four Swine at Various Concentrations of Visipaque (iodixanol, GE Healthcare) with Aortic Root Injection

 

Figure 7
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  		Fig. 4A —60-mm maximum intensity projections of left anterior descending and left circumflex artery obtained with aortic root MDCTA and various concentrations of Visipaque (iodixanol, GE Healthcare). Diagonal branches of left anterior descending artery (LAD) and obtuse marginal branches of left circumflex artery are better delineated with increasing contrast material concentration. Note faint enhancement of great cardiac vein subjacent to LAD. A = anterior, L = left, F = foot. Images obtained at concentrations of 10% (A), 20% (B), 30% (C), and 40% (D).

 

Figure 8
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  		Fig. 4B —60-mm maximum intensity projections of left anterior descending and left circumflex artery obtained with aortic root MDCTA and various concentrations of Visipaque (iodixanol, GE Healthcare). Diagonal branches of left anterior descending artery (LAD) and obtuse marginal branches of left circumflex artery are better delineated with increasing contrast material concentration. Note faint enhancement of great cardiac vein subjacent to LAD. A = anterior, L = left, F = foot. Images obtained at concentrations of 10% (A), 20% (B), 30% (C), and 40% (D).

 

Figure 9
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  		Fig. 4C —60-mm maximum intensity projections of left anterior descending and left circumflex artery obtained with aortic root MDCTA and various concentrations of Visipaque (iodixanol, GE Healthcare). Diagonal branches of left anterior descending artery (LAD) and obtuse marginal branches of left circumflex artery are better delineated with increasing contrast material concentration. Note faint enhancement of great cardiac vein subjacent to LAD. A = anterior, L = left, F = foot. Images obtained at concentrations of 10% (A), 20% (B), 30% (C), and 40% (D).

 

Figure 10
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  		Fig. 4D —60-mm maximum intensity projections of left anterior descending and left circumflex artery obtained with aortic root MDCTA and various concentrations of Visipaque (iodixanol, GE Healthcare). Diagonal branches of left anterior descending artery (LAD) and obtuse marginal branches of left circumflex artery are better delineated with increasing contrast material concentration. Note faint enhancement of great cardiac vein subjacent to LAD. A = anterior, L = left, F = foot. Images obtained at concentrations of 10% (A), 20% (B), 30% (C), and 40% (D).

 

Our results show that using 100 mL of Visipaque at a concentration of 20% produces mean coronary attenuation equivalent to that of peripheral IV MDCTA, which uses 100 mL of full-strength Visipaque (Fig. 5A, 5B, 5C, 5D). Our statistical analysis shows that when the results obtained in the proximal, middle, and distal segments of each of the coronary arteries are averaged over the entire vessel, we obtain statistically significant (p < 0.05) results, including mean attenuation values greater than 250 H using 20%, 30% and 40% contrast concentrations (Table 4, Fig. 2). We obtained slightly higher attenuation values in the RCA compared with the LCx artery and LAD artery. This may be due to the close approximation of the distal catheter end and the right coronary sinus. However, the differences in attenuation are not statistically significant.


Figure 11
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  		Fig. 5A —5-mm maximum intensity projections obtained with peripheral IV MDCTA after injection of 100 mL of full-strength Visipaque (iodixanol, GE Healthcare). A = anterior, L = left, F = foot. Images obtained in right coronary artery (A), left anterior descending artery (B), left circumflex artery (C), and diagonal branches (limited visualization) off left anterior descending artery (D).

 

Figure 12
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  		Fig. 5B —5-mm maximum intensity projections obtained with peripheral IV MDCTA after injection of 100 mL of full-strength Visipaque (iodixanol, GE Healthcare). A = anterior, L = left, F = foot. Images obtained in right coronary artery (A), left anterior descending artery (B), left circumflex artery (C), and diagonal branches (limited visualization) off left anterior descending artery (D).

 

Figure 13
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  		Fig. 5C —5-mm maximum intensity projections obtained with peripheral IV MDCTA after injection of 100 mL of full-strength Visipaque (iodixanol, GE Healthcare). A = anterior, L = left, F = foot. Images obtained in right coronary artery (A), left anterior descending artery (B), left circumflex artery (C), and diagonal branches (limited visualization) off left anterior descending artery (D).

 

Figure 14
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  		Fig. 5D —5-mm maximum intensity projections obtained with peripheral IV MDCTA after injection of 100 mL of full-strength Visipaque (iodixanol, GE Healthcare). A = anterior, L = left, F = foot. Images obtained in right coronary artery (A), left anterior descending artery (B), left circumflex artery (C), and diagonal branches (limited visualization) off left anterior descending artery (D).

 
The blood pool remains unopacified with all catheter-directed studies, unlike with peripheral IV MDCTA. This lack of cardiac chamber enhancement results in the ability to render immediate MIPs of the full volume for a robust 3D display of the entire coronary anatomy (Fig. 6). Peripheral IV MDCTA requires thin (5-mm) MIPs with continuous navigation using a variety of planes for each of the coronary segments, which results in a longer time for postprocessing and interrogation of the entire coronary bed. Likewise, 4D analysis of the entire coronary bed can be obtained as a result of the ability to work with multiple thick-slab 3D data over the entire cardiac cycle without obscuring the coronary anatomy from underlying enhanced cardiac chambers. Four-dimensional analysis of catheter MDCTA studies also expedites ideal cardiac phase selection for display of different coronary segments and provides a conventional angiography-like temporal display of anatomy but in 3D.


Figure 15
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  		Fig. 6 —Three-dimensional whole-volume maximum intensity projection of coronary arteries obtained with aortic root contrast-enhanced MDCTA using 40% Visipaque (iodixanol, GE Healthcare) injection. No blood pool contrast interference is seen.

 
There were no significant complications related to catheter placement or catheter injections.


Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
This is the second feasibility study from our institution using MDCTA after catheter-directed contrast material delivery to the coronary arteries with a nonselective aortic root pigtail catheter [1]. Whereas previously, nonselective delivery of contrast material with a diffusion tip (side holes, n = 360) pigtail catheter (Vanguard, Medrad) yielded nonuniform visualization of the coronary anatomy, our current results with a diffusion-tip, 5-French Vanguard pigtail catheter (Medrad) using 640 side holes and a distal bend (Fig. 1) with a dedicated arterial power injector yields more promising results.

Although there was slightly increased peak attenuation in the RCA compared with the LCx and LAD, the differences were not statistically significant. Our data also show that aortic root MDCTA can produce results similar to those of IV MDCTA at a fraction of the contrast load. Aortic root MDCTA produces mean coronary attenuation values of approximately 250-300 H in all three coronary arteries with a 20% concentration of Visipaque. Thus, only 20 mL of contrast material is needed to produce the same results as a peripheral IV MDCTA study, which requires 100 mL of contrast material. In addition, there is no need for a test bolus, which may be used with peripheral IV MDCTA studies, thus saving an additional 10-20 mL of contrast material.

IV MDCTA requires 80-100 mL of full-strength contrast material to produce its results. This is sufficient in producing significant mean coronary attenuation values to evaluate for coronary stenosis of the major epicardial anatomy such as the left main coronary artery, RCA, LAD, and LCx. Coronary attenuation, however, may be limited in evaluating the smaller coronary tributaries, such as the diagonal and obtuse marginal branches. However, our results with nonselective aortic root MDCTA show that relative increases in concentration of contrast material result in a trend for higher coronary attenuation that may result in increased specificity of coronary artery disease grading.

Our study has several limitations. First, this investigation was limited with respect to the number of examinations performed. As a result, the mean attenuation of proximal, middle, and distal segments did not show statistically significant results with respect to our 250-H threshold. However, when the results of the studies for the various segments were averaged for the entire vessel, results were statistically significantly greater than 250 H when using the 20%, 30%, and 40% contrast concentrations. Further testing using animal models or the application of our method to human patients would be helpful.

In addition, our hybrid technique has certain disadvantages. Aortic root MDCTA requires more physician time and cost than IV MDCTA. With IV MDCTA, the physician is not required to perform the imaging study. In addition, the cost of doing an aortic root MDCTA study is primarily due to the added physician reimbursement and equipment use, such as the C-arm fluoroscopy unit and angiography supplies. However, less time and expertise are required with the hybrid technique than with traditional conventional angiography because the physician must be present initially only during the placement of the catheter in the aortic root. With conventional angiography, the physician must be present during the entire study to inject the various coronary arteries, which requires increased time and skill.

Another disadvantage of the hybrid MDCTA study is the increased radiation exposure to the patient compared with conventional angiography. The radiation exposure of an aortic root MDCTA acquisition is similar to the exposure of an IV MDCTA acquisition. This is because the imaging parameters themselves are similar. Although aortic root MDCTA does not require bolus arrival assessment or bolus tracking, it does require the use of fluoroscopy for guidewire and catheter placement. In the literature, the radiation exposure of MDCTA (11-14 mSv) is higher than that of traditional coronary angiography (6 mSv) [23]. However, the exposure from MDCTA can be decreased to levels similar to those obtained with conventional angiography by using dose pulsing and reduced tube-voltage techniques [24].

Finally, the patient undergoing aortic root MDCTA is at risk of major complications that are also seen with conventional angiography and include pseudoaneurysm, hemorrhage, dissection, stroke, and even death. Studies show that the risk of these major complications, excluding contrast reactions and radiation risk, is approximately 1.3% [25].

Although conventional angiography remains the reference standard, MDCTA is rapidly improving and has gained tremendous interest. Previous studies have shown that MDCTA is preferred to traditional conventional angiography in many respects, including less professional time used, a lower radiation exposure to staff, 3D and 4D presentations of disease, and cross-sectional display of not only the lumen but also the coronary arterial wall and the ability to characterize underlying atherosclerotic disease. However, peripheral IV contrast studies have some limitations.

First, they require higher contrast loads, similar to those of traditional coronary angiography. This excludes certain patient groups who may be at risk for coronary artery disease, such as those with renal insufficiency or diabetes. Using aortic root MDCTA, these patients may obtain the same evaluation with a fraction of the contrast volume. Even in patients with significant renal insufficiency, 20 mL of isoosmolar contrast material, if used in the proper clinical setting with hydration and other measures, may be a safe procedure. With future technologic advances and faster scanners, we may be able to inject at higher rates using lower contrast volumes.

Another limitation of peripheral IV studies is that they are technically limited to evaluation of the main coronary arteries [7, 8]. Aortic root MDCTA, however, shows that with the increasing concentration of contrast material, the trend is toward better depiction of coronary anatomy, including branch vessels, which may be important for treatment planning of atherosclerotic disease involving bifurcation segments.

This hybrid technique for coronary artery evaluation shows promising results and may be useful in a variety of clinical scenarios. It may be useful in patients who are currently excluded from peripheral IV MDCTA or conventional angiography because of their risk of developing contrast-induced nephropathy, such as patients with renal insufficiency and those with diabetes. Further work may also show benefits over conventional angiography as a result of the depiction of the entire coronary arterial anatomy using 3D and 4D projections that are not available with conventional angiography.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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