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DOI:10.2214/AJR.07.3254
AJR 2008; 190:W247-W254
© American Roentgen Ray Society

Technical Innovation

Contrast Volume Reduction with Superior Vena Cava Catheter-Directed Coronary CT Angiography: Comparison with Peripheral IV Contrast Enhancement in a Swine Model

Anil N. Shetty1, Kostaki G. Bis1, Amit R. Vyas1, Arun Kumar1, Andrew Anderson2 and Mamtha Balasubramaniam3

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.

Received February 26, 2007; accepted after revision October 24, 2007.

 
Support provided by Medrad, Inc.

Address correspondence to A. N. Shetty (ashetty{at}beaumont.edu).

WEB

This is a Web exclusive article.


Abstract
Top
 Abstract
Introduction
 Experimental Methods
 Results
 Discussion
 References
 
OBJECTIVE. Conventional MDCT angiography uses a traditional peripheral IV approach for contrast injection; however, we describe our experience with a superior vena cava (SVC) catheter approach for coronary artery MDCT angiography as a potential means of decreasing iodinated contrast volume.

CONCLUSION. Central SVC contrast injection can decrease the contrast volume by 50% while maintaining coronary attenuation similar to that of peripheral IV injection. This approach has potential in reducing the contrast volume on coronary MDCT angiography studies and therefore the risk of contrast-induced nephropathy in certain high-risk patients. Further studies with higher injection rates and faster scan acquisition are needed for defining a lower contrast volume threshold.

Keywords: catheter angiography • coronary artery angiography • CT angiography • reduced contrast burden • superior vena cava approach


Introduction
Top
 Abstract
 Introduction
Experimental Methods
 Results
 Discussion
 References
 
Although conventional cardiac angiography remains the imaging reference standard for evaluating coronary artery anatomy and pathology, recent technologic advances are establishing peripheral IV contrast-enhanced MDCT angiography as a popular technique for coronary imaging [1–4]. The advantages of peripheral IV contrast-enhanced MDCT angiography over car diac angiography include noninvasiveness, 3D and 4D anatomic display, and plaque char acterization. In current practice, optimum peripheral IV contrast-enhanced MDCT angiography achieves mean coronary attenuation values of approximately 250–300 H using 80–100 mL of iodinated contrast agent [5].

Several limitations of MDCT angiography have been noted in the literature [6–8]. They include lower spatial resolution and higher radiation exposure than cardiac angiography, motion artifacts, and incomplete evaluation of all coronary arterial segments. The risk of contrast-induced nephropathy in high-risk patients and its relation to the volume of radiocontrast material administered is also an important consideration. The use of iodinated contrast material is not without risks especially among patients with renal insufficiency (serum creatinine level of > 1.5 mg/dL) [9, 10]. Additional risk factors for contrast-induced nephropathy include diabetes, age, congestive heart failure, dehydration, and malignancy [11–14].

Our primary objective in this study was to evaluate an alternative minimally invasive approach to peripheral IV contrast-enhanced coronary MDCT angiography that could potentially decrease the volume of iodinated contrast material in high-risk patients. Previous studies at our institution have shown the feasibility of coronary MDCT angiography using catheter-directed contrast delivery to the coronary arteries with nonselective aortic root pigtail and selective catheters [15, 16]. Recent data [17] indicate that aortic root injections of 20 mL of contrast material yield coronary artery attenuation values that are statistically significant (p < 0.05, ≥ 250 H) and similar to attenuation values obtained with peripheral IV contrast-enhanced MDCT angiography using 100 mL of iodinated contrast material. This approach, however, is more invasive and exposes the patient to risks associated with arterial vascular access such as hemorrhage and stroke.

In an ongoing effort to minimize the volume of iodinated contrast material and to decrease the invasiveness of the procedure, we present a technique that uses superior vena cava (SVC) catheter-directed enhancement and coronary MDCT angiography for imaging.


Experimental Methods
Top
 Abstract
 Introduction
 Experimental Methods
Results
 Discussion
 References
 
Animal Preparation
Our hypothesis was tested in four female farm swine (40–60 kg). Before the study, institutional animal review board approval was obtained. These swine were delivered to our on-site animal facility 7–10 days before the study to allow acclimatization. The standard institutional animal protocol was used for preoperative and intraoperative study of swine [16, 18]. They were pretreated with atenolol (Tenormin, AstraZeneca) (100 mg orally twice a day) for 2 days as the initial means for heart rate reduction. Even after acclimatization, the heart rate is usually in the range of 100–120 beats per minute (bpm), which needs to be brought down to an acceptable range of 65 bpm. Analgesics including carprofen (Rimadyl, Pfizer) 2 mg subcutaneously, buprenorphine (Buprenex, Reckitt Benckis) 0.01 mg/kg sub cutaneously, and acetylsali cylic acid 25 mg/kg orally were administered for 24 hours before cathe terization. On the day of the exami nation, the initial sedation was obtained with intramuscular ketamine (Ketaset, Wyeth) 33 mg/kg. Induction was then achieved with oxygen 1–2 L/min and a 2–5% isofluorane (Forane, Arkema) face mask. The animals were then intubated and an IV lidocaine drip (0.3 mg/kg/h) was started for arrhythmia prevention. The swine were transported to the CT scanner from the animal facility. Sodium thiopental (Pentothal, Abbott) 15–30 mg/kg was used as needed during transportation to the hospital CT scanner. During transportation, the animals were mechanically ventilated by hand. Once on the CT scanner table, the animals were placed on mechanical ventilation. Sedation was maintained with oxygen 1 L/min and 1–2% isoflurane (Forane) and a sufentanil (Sufenta, Janssen-Ortho) 0.015–0.030 mg/kg/h drip. IV access with a 20- or 22-gauge catheter was obtained in an ear and an upper extremity vein (for peripheral IV contrast-enhanced studies). IV fluids (2 mL/kg/h) with normal saline were provided for hydration. Typical heart rates under this sedation protocol ranged from 100 to 120 bpm. The heart rate was further reduced to the desired 65 bpm with IV diltiazem (Cardizem, Abbott) in a 20-mg bolus followed by an IV diltiazem drip (15 mg/h).


Figure 1
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  		Fig. 1 —Photograph shows 5-French Vanguard diffusion catheter (Medrad) with 640 laser-drilled side holes (arrows) and distal tip constrictor (white end).

 
SVC Vascular Access
Unilateral venous access through the right internal jugular vein was obtained on the CT table under sterile conditions with a 5-French micro puncture kit (5 FR MPIS-501-SST, Cook) using a hand-held probe and sonographic guidance (Site Rite unit, Bard Access Systems). A 5-French sheath was inserted. A 0.035-inch guidewire (5 FR MPIS-501-SST, Cook) was placed into the right atrium with portable C-arm fluoroscopy guidance (Series 9800, OEC Medical Systems).


Figure 2
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  		Fig. 2A —Bar graphs of proximal (blue), mid (red), and distal (yellow) coronary attenuation values. Bar graphs show coronary attenuation values for right coronary artery (RCA) (A), left anterior descending coronary artery (LAD) (B), and left circumflex coronary artery (LCX) (C) on superior vena cava (SVC) (25-mL and 50-mL studies) and IV (100-mL) studies. Significance of grouped segments (proximal, mid, and distal) for each injection is compared with threshold value of 250 H.

 


Figure 3
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  		Fig. 2B —Bar graphs of proximal (blue), mid (red), and distal (yellow) coronary attenuation values. Bar graphs show coronary attenuation values for right coronary artery (RCA) (A), left anterior descending coronary artery (LAD) (B), and left circumflex coronary artery (LCX) (C) on superior vena cava (SVC) (25-mL and 50-mL studies) and IV (100-mL) studies. Significance of grouped segments (proximal, mid, and distal) for each injection is compared with threshold value of 250 H.

 


Figure 4
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  		Fig. 2C —Bar graphs of proximal (blue), mid (red), and distal (yellow) coronary attenuation values. Bar graphs show coronary attenuation values for right coronary artery (RCA) (A), left anterior descending coronary artery (LAD) (B), and left circumflex coronary artery (LCX) (C) on superior vena cava (SVC) (25-mL and 50-mL studies) and IV (100-mL) studies. Significance of grouped segments (proximal, mid, and distal) for each injection is compared with threshold value of 250 H.

 
SVC Catheter Placement
SVC injection studies were performed with a 5-French Vanguard diffusion catheter (Medrad) [17]. The catheter has an end-hole constrictor excepting a guidewire and a series of laser-drilled micro side holes (n = 640) within the pigtail end (Fig. 1). This catheter provides a uniform cloudlike 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 right atrium. The catheter was placed over the guidewire and placed within the SVC using C-arm fluoroscopy guidance. Its position was confirmed by injecting 5 mL of Visipaque (iodixanol, GE Healthcare).

SVC Catheter-Directed Studies
Contrast-enhanced MDCT angiography with catheter-directed SVC injection was performed with 25 mL (n = 4) and 50 mL (n = 4) of undiluted contrast material (Visipaque) volume at 10 mL/s followed by a 50-mL saline flush at 10 mL/s. A dual-head venous power injector (325 psi pressure limit) was used (Stellant Injector, Medrad). The 25-mL volume injection was followed by the 50-mL volume injection. A time period of 15 minutes was allowed for contrast material to clear between the 25-mL and 50-mL volume injections.

Peripheral IV Studies
For comparison, peripheral IV-enhanced MDCT angiography studies (n = 4) were performed in the same pigs with a standard undiluted (full-concentration of 320 mg I/mL) 100-mL contrast (Visipaque) volume at 5 mL/s with a 75-mL saline flush at 5 mL/s. This was per formed 15 minutes after the 50-mL SVC injection study to allow the contrast to clear from the blood volume. The same dual-head venous power injector was used. The upper extremity venous access site was used.

Bolus Tracking
Bolus tracking was used for initiating data acquisitions for the peripheral IV contrast-enhanced and SVC contrast-enhanced studies. A series of low-dose dynamic axial scans were obtained every 2 seconds at the level of the mid ascending aorta to monitor contrast enhancement within the ascending aorta. Coronary MDCT angiography was initiated with a trigger threshold value of 160 H above baseline [19].

MDCT Angiography Protocol and Parameters
CT angiography (CTA) studies (SVC and peripheral IV) were performed on a 64-MDCT scanner (Sensation 64, Siemens Medical Solutions). MDCT angiography commenced with respiratory termination using a cranial-to-caudal acquisition to cover the heart with retrospective ECG gating and the following para meters: collimation and slice thick ness, 0.6 mm; recon struction increment, 0.3 mm; tube rota tion time, 0.33 second; tube voltage, 120 kVp; effec tive current, 850 mAs; pitch, 0.2; recon struction field of view sized to the region of interest. ECG pulse gating for tube current modu lation was not implemented.

This allowed for multiple phase reconstructions at regular intervals for a 4D presentation. Single- and dual-sector reconstructions were performed. The default data reconstruction was set at 65% of the R-R interval for all the studies; however, in addition, reconstruction at every 5% interval was also performed.

Radiation Exposure
Although ECG-controlled dose modulation was available on our 64-MDCT scanner, we ac quired data using an unmodulated exposure to gen e rate reconstructed images at every 5% of the R-R interval. The acquisition of the entire data set for coronary CTA consists of three steps: topo gram, monitoring for bolus arrival time, and coronary MDCT angiography scan. Topograms were obtained with a low-energy scan with the least amount of radiation exposure (~ 0.085 mSv). The radiation exposure for IV-based coronary angiography using 64-MDCT is estimated to be approximately 11–22 mSv [3]. In our study, the cumulative exposure for the topogram, monitoring, and MDCT angiography was 14.688 mSv. The radiation exposure levels used in the IV study and SVC study were nearly equal (~ 14.5 mSv). As a comparison, conventional catheter-directed selec tive coronary angiography using fluoroscopy guidance has a cumulative radiation exposure of 3–5 mSv, and the annual exposure from background radiation is about 2.5 mSv [3, 20, 21].

After Imaging
At the completion of the imaging study, the sedated animals were sacrificed using standard hospital animal protocol procedure with a lethal IV injection of 1 mL/10 lb (4.5 kg) of Euthasol (pentobarbital sodium [390 mg/mL] and phenytoin sodium [50 mg/mL], Virbac AH) [18].

Data Processing
Using a Wizard workstation (Siemens Medical Solutions), 4D analysis was performed by reconstructing multiple 3D data sets at 5% intervals of the R-R length. Finally, reconstructed images were transferred to a stand-alone workstation (Aquarius workstation with software version 3.5.2.1, TeraRecon) for generation of maximum-intensity-projection (MIP) images using the full volume and variable slice thickness including 5 mm, 30 mm, and 60 mm. The mean and peak coronary artery attenuation values were measured by one of the authors on the TeraRecon workstation using per pendicular measurements for the proximal, middle, and distal segments of the right coronary artery (RCA), left anterior descending coronary artery (LAD), and left circumflex coronary artery (LCX). Coronary measurements were performed using the American Heart Association (AHA) [22] anatomy definitions at the following levels: proximal, mid, and distal seg ments of the RCA and LAD. The AHA anatomy definition, however, does not segment the LCX into proximal, middle, and distal components. To get similar measurements to that of the RCA and LAD, the LCX was segmented into proximal (beyond left main bifurcation or trifurcation), mid (defined halfway down the posterior atrioventricular groove), and distal (proximal aspect of the last obtuse marginal branch). A full-width at half-maximum (FWHM) technique was used [15]. The two measurements were made by displaying the coronary segments perpendicular to their long axes. Perpendicular diameters traversing the cross sections 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. For each segment, the mean signal FWHM, SD, and median values were noted both in SVC- and peripheral IV-generated images.

Statistical Analysis
A value of 250 H was used as adequate for coronary artery attenuation [15, 16, 23, 24]. The average FWHM values for each artery from each injection were first analyzed to see if they met the distributional assumptions of the statistical tests that were being used to analyze them. On the basis of this preliminary assessment, either the parametric Student's t test or the nonparametric Wilcoxon's signed rank test was used to determine if the mean or median coronary attenuation value was significantly different from 250 H at each of these injections. Pooled data were analyzed with a two-factor interaction linear mixed model using PROC MIXED (SAS) because FWHM was normally distributed with independent errors and no indication of nonlinearity. We fit the following linear mixed model to the data: FWHM = intercept + concentration + segment + concen tration x segment. Here FWHM is the response, concentration and segment are the two individual (i.e., main) effects, and concentration x segment is the two-factor interactive (i.e., combined) effect. The corresponding pairwise comparisons were performed using least-square means based on the Tukey-Kramer adjustment. We considered p values less than an alpha of 0.05 (probability of type 1 error) statistically significant. Statistical analysis was performed using the SAS System for Windows version 9.1.3, Service Pak 2.


Figure 5
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  		Fig. 3 —Bar graph shows pooled mean coronary attenuation values compared for each vessel on superior vena cava (SVC) (25-mL and 50-mL studies) and IV (100-mL) studies. In comparison of 25 mL SVC and 100-mL IV studies, for left anterior descending coronary artery (LAD), p = 0.0055; for left circumflex coronary artery (LCX), p = 0.0113; and for right coronary artery (RCA), p = 0.0017. Also, in comparison between SVC 50-mL and IV 100-mL studies, for LAD, p = 0.7664; for LCX, p = 0.5724; and for RCA, p = 0.9797. Blue = RCA, Red = LAD, Yellow = LCX. Note: Although some of pairwise comparisons are statistically significant, there is no significant difference in overall full-width at half-maximum (FWHM) because of interactive (i.e., combined) effect of segment x concentration (p value of type 3 test of fixed effects = 0.9102).

 


Figure 6
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  		Fig. 4A —Coronary artery CT angiography with superior vena cava (SVC) (50 mL) injection. RA = right atrium, RVOT = right ventricular outflow tract. 10-mm maximum-intensity-projection images of left main bifurcation (A), left anterior descending coronary artery (LAD) (B), proximal and middle right coronary artery (RCA) (C), and distal RCA (D) are shown using central SVC injection of 50 mL of contrast material at 10 mL/s. Coronary arteries are displayed with similar attenuation as in Figures 5A, 5B, 5C, and 5D (peripheral IV injection) but with much lower enhancement of right heart structures.

 

Figure 10
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  		Fig. 5A —Coronary artery CT angiography with peripheral IV (100 mL) injection. RA = right atrium, RVOT = right ventricular outflow tract. 10-mm maximum-intensity-projection images of left main bifurcation (A), left anterior descending coronary artery (LAD) (B), proximal and middle right coronary artery (RCA) (C), and distal RCA (D) are shown using peripheral IV injection of 100 mL of contrast material at 5 mL/s.

 

Figure 11
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  		Fig. 5B —Coronary artery CT angiography with peripheral IV (100 mL) injection. RA = right atrium, RVOT = right ventricular outflow tract. 10-mm maximum-intensity-projection images of left main bifurcation (A), left anterior descending coronary artery (LAD) (B), proximal and middle right coronary artery (RCA) (C), and distal RCA (D) are shown using peripheral IV injection of 100 mL of contrast material at 5 mL/s.

 

Figure 12
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  		Fig. 5C —Coronary artery CT angiography with peripheral IV (100 mL) injection. RA = right atrium, RVOT = right ventricular outflow tract. 10-mm maximum-intensity-projection images of left main bifurcation (A), left anterior descending coronary artery (LAD) (B), proximal and middle right coronary artery (RCA) (C), and distal RCA (D) are shown using peripheral IV injection of 100 mL of contrast material at 5 mL/s.

 

Figure 13
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  		Fig. 5D —Coronary artery CT angiography with peripheral IV (100 mL) injection. RA = right atrium, RVOT = right ventricular outflow tract. 10-mm maximum-intensity-projection images of left main bifurcation (A), left anterior descending coronary artery (LAD) (B), proximal and middle right coronary artery (RCA) (C), and distal RCA (D) are shown using peripheral IV injection of 100 mL of contrast material at 5 mL/s.

 


Figure 7
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  		Fig. 4B —Coronary artery CT angiography with superior vena cava (SVC) (50 mL) injection. RA = right atrium, RVOT = right ventricular outflow tract. 10-mm maximum-intensity-projection images of left main bifurcation (A), left anterior descending coronary artery (LAD) (B), proximal and middle right coronary artery (RCA) (C), and distal RCA (D) are shown using central SVC injection of 50 mL of contrast material at 10 mL/s. Coronary arteries are displayed with similar attenuation as in Figures 5A, 5B, 5C, and 5D (peripheral IV injection) but with much lower enhancement of right heart structures.

 


Figure 8
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  		Fig. 4C —Coronary artery CT angiography with superior vena cava (SVC) (50 mL) injection. RA = right atrium, RVOT = right ventricular outflow tract. 10-mm maximum-intensity-projection images of left main bifurcation (A), left anterior descending coronary artery (LAD) (B), proximal and middle right coronary artery (RCA) (C), and distal RCA (D) are shown using central SVC injection of 50 mL of contrast material at 10 mL/s. Coronary arteries are displayed with similar attenuation as in Figures 5A, 5B, 5C, and 5D (peripheral IV injection) but with much lower enhancement of right heart structures.

 


Figure 9
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  		Fig. 4D —Coronary artery CT angiography with superior vena cava (SVC) (50 mL) injection. RA = right atrium, RVOT = right ventricular outflow tract. 10-mm maximum-intensity-projection images of left main bifurcation (A), left anterior descending coronary artery (LAD) (B), proximal and middle right coronary artery (RCA) (C), and distal RCA (D) are shown using central SVC injection of 50 mL of contrast material at 10 mL/s. Coronary arteries are displayed with similar attenuation as in Figures 5A, 5B, 5C, and 5D (peripheral IV injection) but with much lower enhancement of right heart structures.

 

Results
Top
 Abstract
 Introduction
 Experimental Methods
 Results
Discussion
 References
 
The attenuation values of the proximal, middle, and distal segments of the coronary vasculature for the SVC (25 mL and 50 mL) and peripheral IV (100 mL) MDCT angiography studies are provided in Tables 1, 2, 3 and in Figures 2A, 2B, and 2C. Grouping of vessel segments for each injection was compared with a cutoff of 250 H using either a Student's t test or Wilcoxon's signed rank test as described in the statistical analysis section. As can be seen from measured p values, many measurements failed to be statistically significant (H > 250) for all studies.


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  		TABLE 1: Proximal Coronary Attenuation Values

 

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  		TABLE 2: Middle Coronary Attenuation Values

 

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  		TABLE 3: Distal Coronary Attenuation Values

 

Table 4 and Figure 3 show the pooled mean FWHM attenuation values from proximal, mid, and distal coronary segments for each study. The pooled attenuation values were significantly higher (> 250 H, p < 0.05) for the SVC 50-mL catheter-directed and for the IV 100-mL studies and significantly lower (< 250 H, p < 0.05) for the 25-mL (SVC) catheter-directed studies. Also, pooled mean FWHM attenuation values for each segment for SVC 50-mL and SVC 25-mL were compared with IV 100-mL studies. There was no statistically significant difference of pooled values between SVC 50-mL and IV 100-mL studies for LAD (p = 0.7664), LCX (p = 0.5724), and RCA (p = 0.9797). However, there was a statistically significant difference of pooled mean values between SVC 25-mL and IV 100-L studies for LAD (p = 0.0055), LCX (p = 0.0113), and RCA (p = 0.0017).


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  		TABLE 4: Pooled Mean Coronary Attenuation Values

 

The right atrium, right ventricle, and pulmonary arteries were of significantly lower attenuation on the SVC studies compared with peripheral IV studies. Figures 4A, 4B, 4C, and 4D shows this observation of a true CT levogram on SVC injections. The right heart structures (right atrium, right ventricle, and pulmonary arteries) were always of higher attenuation on the peripheral IV (Figs. 5A, 5B, 5C, and 5D) studies but less than the left heart anatomy (pulmonary veins, left atrium, left ventricle, aorta, and coronary arteries). The coronary arteries are shown in Figures 4A, 4B, 4C, 4D, 5A, 5B, 5C, and 5D using MIP images, and for comparative purposes, the RCA is shown in Figures 6A, 6B, 6C, and 6D using MIP and multiplanar reformation (MPR) images.


Figure 14
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  		Fig. 6A —Comparison between maximum-intensity-projection (MIP) and multiplanar reformatted (MPR) images for IV contrast-enhanced (100-mL) and superior vena cava (SVC) (50-mL) studies. MIP image (2 mm) for 100-mL IV contrast-enhanced study (A) and corresponding image from MPR (0.6 mm) (B) of same data set.

 

Figure 15
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  		Fig. 6B —Comparison between maximum-intensity-projection (MIP) and multiplanar reformatted (MPR) images for IV contrast-enhanced (100-mL) and superior vena cava (SVC) (50-mL) studies. MIP image (2 mm) for 100-mL IV contrast-enhanced study (A) and corresponding image from MPR (0.6 mm) (B) of same data set.

 

Figure 16
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  		Fig. 6C —Comparison between maximum-intensity-projection (MIP) and multiplanar reformatted (MPR) images for IV contrast-enhanced (100-mL) and superior vena cava (SVC) (50-mL) studies. MIP (2 mm) for 50-mL SVC study (C) and corresponding image from MPR (0.6 mm) (D) of same data set.

 

Figure 17
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  		Fig. 6D —Comparison between maximum-intensity-projection (MIP) and multiplanar reformatted (MPR) images for IV contrast-enhanced (100-mL) and superior vena cava (SVC) (50-mL) studies. MIP (2 mm) for 50-mL SVC study (C) and corresponding image from MPR (0.6 mm) (D) of same data set.

 


Discussion
Top
 Abstract
 Introduction
 Experimental Methods
 Results
 Discussion
References
 
Although there are animal and human coronary MDCT angiography studies using arterial catheter-directed contrast enhancement, this is the first study to our knowledge in an animal model showing the feasibility of coronary MDCT angiography after catheterdirected contrast material delivery using a pigtail catheter placed into the SVC. We used a jugular approach for placement of the catheter into the SVC; however, in humans, a peripheral upper extremity venous approach would be more appealing and desirable. Central venous line placements are routinely used in clinical practice [25, 26]. The SVC catheter-directed MDCT angiography studies depicted the coronary anatomy with adequate (H > 250) attenuation using 50 mL of contrast material at an injection rate of 10 mL/s. In fact, the pooled attenuation was similar to peripheral IV studies performed with 100 mL of contrast material injected at 5 mL/s. Unfortunately, SVC injection of 25 mL at 10 mL/s did not result in adequate attenuation. Adequate coronary attenuation with a lower contrast volume (25 mL), however, may be feasible if the injection rate is increased to 20 mL/s. This would require a faster scan acquisition because the bolus profile would be very narrow. The current scan acquisition time of 10 seconds would likely have to be reduced to less than 5 seconds for imaging the coronary arteries at these high injection rates. Faster scan acquisitions, however, are becoming feasible with newer MDCT scanners such as the dual-source MDCT recently introduced.

There are several limitations to this study. First, this investigation was limited with respect to the number of animals and the results are based on a small number of examinations. As such, due to a small number, several of the attenuation values for the proximal, middle, and distal segments did not show statistical significance with respect to the cutoff of 250 H. A wide SD on several measurements could potentially be due to suboptimal cardiac phase selection for attenuation measurements. However, with pooled data, the mean attenuation values for the 50-mL SVC and 100-mL peripheral IV studies were significantly greater than the 250-H threshold. Furthermore, there was no statistically significant difference of the pooled coronary attenuation values between the 50-mL SVC and 100-mL peripheral IV studies for LAD, LCX, and RCA segments.

Although we concluded that there was no significant effect on FWHM because of the interactive effect of concentration and segment (p value of type 3 test of fixed effects = 0.9102), we performed post hoc comparisons (which are typically done only when the interactive effect is statistically significant). These results, as shown in Figure 3, indicate that for each segment there is no significant difference between the SVC 50-mL and IV 100-mL studies for RCA (p = 0.9797), LCX (p = 0.5724), and LAD (p = 0.7664). However, because the interactive term in the model was not statistically significant, we cannot conclude with certainty that there is no significant difference between these two concentrations for each of these segments. Second, the additional limiting factor was the scanner speed. The reason for limiting the SVC contrast injection rate to 10 mL/s was to synchronize it with the injection time (5 seconds for contrast injection and 5 seconds for saline flush) of 10 seconds. However, with higher-speed scanners, a further reduction of contrast material may be possible with higher injection rates. Lowering the iodinated contrast volume is important clinically when there is a need for defining the coronary anatomy and pathoanatomy in renal insufficiency patients to avoid contrast-induced nephropathy. Its less invasive nature compared with arterial catheter-directed MDCT angiography renders the SVC catheter-directed approach an appealing alternative. Although there would be a cost increase because of the added physician's time for catheter placement, the cost would be less than cardiac angiography.

Adequate contrast enhancement of all coronary segments and tributaries is an important factor in delineating atherosclerotic disease with MDCT angiography. There are a variety of factors influencing the degree of enhancement on IV studies, and they include the iodine content, contrast osmolality, contrast injection rate, timing of data acquisition, systemic dilutional effects, individual differences in cardiac output, and size of vascular segments being interrogated. In this study, central venous injection of contrast material via the SVC was shown to be feasible with a reduced volume. This could have clinical applications for patients who are at risk for contrast-induced nephropathy. Further animal studies using smaller contrast volumes (≤ 25 mL) at higher injection rates of (≥ 20 mL/s) are needed using shorter acquisition times of 5 seconds or less. Furthermore, any improved diagnostic value of SVC-enhanced coronary MDCT angiography would need to be confirmed in animal or clinical trials.


Acknowledgments
 
The authors acknowledge the scanner support provided by Victoria Hollingsworth of the Department of Cardiology and the veterinary support by Karen M. Rossman and the Animal Care Committee of the Research Institute of William Beaumont Hospital.


References
Top
 Abstract
 Introduction
 Experimental Methods
 Results
 Discussion
 References
 

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