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    Time-attenuation curves for undiluted iohexol solutions (A) and iohexol solutions diluted 1:1 (B), 1:2 (C), and 1:3 (D) with saline (0.9% NaCl) solution administered IV to 4 clinically normal Beagles for cardiac CT angiography. Three doses of iodine were evaluated for each set of dilutions (300 mg/kg [black circles], 400 mg/kg [white circles], and 800 mg/kg [black inverted triangles]), yielding 12 formulations of contrast solutions, each of which was evaluated in each dog by use of a crossover study design. A peak arc–type TAC (rapidly increasing, brief peak, then rapidly decreasing curve) was detected primarily for undiluted contrast formulations. A plateau-type TAC (more uniform and prolonged plateau peak) was attained with contrast formulations of greater dilutions. The magnitude of aortic enhancement increased proportionally with increasing iodine dose for each dilution ratio.

  • 1. Boyd DP, Lipton MJ. Cardiac computed tomography. Proc IEEE 1983; 3: 298307.

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  • 8. Makara M, Dennler M, Kühn K, et al. Effect of contrast medium injection duration on peak enhancement and time to peak enhancement of canine pulmonary arteries. Vet Radiol Ultrasound 2011; 52: 605610.

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  • 9. Bae KT. Intravenous contrast medium administration and scan timing at CT: considerations and approaches. Radiology 2010; 256: 3261.

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  • 11. Bae KT, Tran HQ, Heiken JP. Multiphasic injection method for uniform prolonged vascular enhancement at CT angiography: pharmacokinetic analysis and experimental porcine model. Radiology 2000; 216: 872880.

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  • 12. National Research Council Committee for the Update of the Guide for the Care and Use of Laboratory Animals. Guide for the care and use of laboratory animals. 8th ed. Washington, DC: The National Academies Press, 2010; 1220.

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  • 13. Awai K, Hiraishi K, Hori S. Effect of contrast material injection duration and rate on aortic peak time and peak enhancement at dynamic CT involving injection protocol with dose tailored to patient weight. Radiology 2004; 230: 142150.

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  • 16. Silverman PM, Brown B, Wray H, et al. Optimal contrast enhancement of the liver using helical (spiral) CT: value of SmartPrep. AJR Am J Roentgenol 1995; 164: 11691171.

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Dynamic computed tomographic determination of scan delay for use in performing cardiac angiography in clinically normal dogs

Jisun Kim DVM1, Yeonho Bae DVM2, Gahyun Lee DVM3, Sunghoon Jeon DVM4, and Jihye Choi DVM, PhD5
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  • 1 College of Veterinary Medicine, Chonnam National University, Gwangju 500-757, Republic of Korea.
  • | 2 College of Veterinary Medicine, Chonnam National University, Gwangju 500-757, Republic of Korea.
  • | 3 College of Veterinary Medicine, Chonnam National University, Gwangju 500-757, Republic of Korea.
  • | 4 College of Veterinary Medicine, Chonnam National University, Gwangju 500-757, Republic of Korea.
  • | 5 College of Veterinary Medicine, Chonnam National University, Gwangju 500-757, Republic of Korea.

Abstract

OBJECTIVE To determine the scan delay for use in performing cardiac CT angiography in dogs.

ANIMALS 4 clinically normal adult Beagles.

PROCEDURES In a crossover study, 12 formulations of iohexol solutions differing in iodine dose (300, 400, and 800 mg/kg) and concentration (undiluted and diluted 1:1, 1:2, and 1:3 with saline [0.9% NaCl] solution) were administered IV to each dog. Dynamic CT angiography was performed to evaluate enhancement characteristics of each formulation, with the region of interest set over the aorta. Time-attenuation curves (TACs) were obtained and analyzed.

RESULTS Peak arc–type TACs were obtained after administration of all undiluted formulations. Curve shape changed from peak arc type to plateau type as the total volume of the contrast solution (ie, dilution) increased. Prolonged peaks characteristic of plateau-type TACs suggested that a sufficient period of homogeneous attenuation could be achieved for CT scanning with administration of higher iohexol dilutions (1:2 or 1:3) containing higher iodine doses (400 or 800 mg/kg). In particular, attenuation values for plateau-type TACs remained between 200 and 300 Hounsfield units for > 16 seconds after the plateau endpoint was reached for 1:2 and 1:3 dilutions containing an iodine dose of 800 mg/kg. Scan delays of 13 to 17 seconds were computed for those 2 formulations.

CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that for clinically normal dogs, a scan delay of 13 to 17 seconds could be used to perform cardiac CT angiography with iohexol solutions containing an iodine dose of 800 mg/kg at dilutions of 1:2 or 1:3.

Abstract

OBJECTIVE To determine the scan delay for use in performing cardiac CT angiography in dogs.

ANIMALS 4 clinically normal adult Beagles.

PROCEDURES In a crossover study, 12 formulations of iohexol solutions differing in iodine dose (300, 400, and 800 mg/kg) and concentration (undiluted and diluted 1:1, 1:2, and 1:3 with saline [0.9% NaCl] solution) were administered IV to each dog. Dynamic CT angiography was performed to evaluate enhancement characteristics of each formulation, with the region of interest set over the aorta. Time-attenuation curves (TACs) were obtained and analyzed.

RESULTS Peak arc–type TACs were obtained after administration of all undiluted formulations. Curve shape changed from peak arc type to plateau type as the total volume of the contrast solution (ie, dilution) increased. Prolonged peaks characteristic of plateau-type TACs suggested that a sufficient period of homogeneous attenuation could be achieved for CT scanning with administration of higher iohexol dilutions (1:2 or 1:3) containing higher iodine doses (400 or 800 mg/kg). In particular, attenuation values for plateau-type TACs remained between 200 and 300 Hounsfield units for > 16 seconds after the plateau endpoint was reached for 1:2 and 1:3 dilutions containing an iodine dose of 800 mg/kg. Scan delays of 13 to 17 seconds were computed for those 2 formulations.

CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that for clinically normal dogs, a scan delay of 13 to 17 seconds could be used to perform cardiac CT angiography with iohexol solutions containing an iodine dose of 800 mg/kg at dilutions of 1:2 or 1:3.

Computed tomographic angiography facilitates the identification of complex cardiovascular malformations and provides, even for small animals, accurate information on the location, size, and number of intra- and extracardiac masses; density and characteristics of contrast enhancement; and delineation of adjacent anatomic structures.1–5

The main goal of CT angiography is to facilitate detection of pathological changes in the cardiovascular system by providing adequate and prolonged contrast enhancement within an anatomic ROI.6,7 The quality of contrast enhancement achieved is influenced by contrast medium–related factors (eg, concentration, volume, injection rate, injection bolus protocol, and whether a flush solution is used), patient-related factors (eg, cardiac output, cardiovascular circulation, age, and body weight), and CT scanning factors (eg, scan duration, equipment used [multi-detector- vs monodetector-row CT], scan delay [interval from initiation of contrast medium injection to image acquisition], and method used to control the timing of contrast medium arrival at the ROI [test bolus vs bolus-tracking method]).8–11

In human medicine, protocols for contrast medium injection have been evaluated for use in cardiac CT angiography. With a traditional uniphasic injection protocol, the resulting cardiac enhancement profile consists of a single peak in the degree of enhancement within the ROI followed by a rapid decrease once injection is complete. This profile reflects the rapid distribution of contrast medium away from the cardiovascular system.10 Consequently, nonuniform cardiovascular enhancement occurs during image acquisition when a uniphasic injection protocol is used.10 A modified version of the uniphasic injection protocol, the multiphasic approach, includes flushing with saline (0.9% NaCl) solution or diluted contrast medium after injection of the original contrast medium to prolong the duration of contrast enhancement.11 In veterinary medicine, a few CT protocols have been evaluated for contrast enhancement of the caudal vena cava, abdominal aorta, and pulmonary artery.8,10 However, to the authors’ knowledge, a protocol for performing cardiac CT angiography has not been reported for veterinary species.

A method is needed to help discriminate intra- and extracardiac structures in dogs better than is achieved with echocardiography. The purpose of the study reported here was to establish a protocol for CT angiography to facilitate cardiac examination, including dose and concentration of iodine to be used and total volume of contrast solution to be administered. We also sought to use information from TAC analyses to determine a scan delay that would allow adequate and consistent cardiovascular enhancement.

Materials and Methods

Animals

Four clinically normal adult (approx 1- to 2-year-old) male research Beagles weighing 10 to 12 kg were used in the study. Lack of cardiovascular abnormalities or other clinical problems affecting blood circulation was confirmed on the basis of results of physical examinations, CBCs, serum biochemical analyses, blood pressure measurements, radiography, and echocardiography. Dogs were cared for in accordance with the Laboratory Animal Research Center Guide for Care and Use,12 and the experimental protocol was approved by the Institutional Animal Care and Use Committee at Chonnam National University.

Study design

A crossover study design was used, with treatment order randomly assigned. Each of the 4 dogs was used to evaluate each of 12 formulations of contrast solutions, with a 3-day interval separating each administration session to allow for sufficient washout of contrast medium. Contrast protocols were classified by dose of iodine (300, 400, and 800 mg/kg) and concentration of contrast medium (iohexola) used. For each iodine dose evaluated, 4 concentrations of contrast solutions were prepared (undiluted and dilutions of 1:1, 1:2, and 1:3) by mixing iohexol with saline solution (Appendix).

Dynamic CT scanning

In preparation for CT scanning, dogs were anesthetized with a combination of medetomidine hydrochlorideb (0.03 mg/kg, IV) and zolazepam hydrochloride–tiletamine hydrochloride (0.75 mg/kg, IV)c and positioned in sternal recumbency. A 22-gauge, 1-inch catheterd (internal diameter, 0.9 mm; water flow rate, 35 mL/min) was then placed in a cephalic vein.

Computed tomographic scanning of each dog was performed with a 16-row multidetector CT scanner.e Heart rate was recorded during the scanning process by use of a multiparameter patient monitor.f First, precontrast CT scanning was performed in the craniocaudal direction at a setting of 16 rows × 0.5mm collimation, helical pitch of 0.5, rotation duration of 600 milliseconds, tube voltage of 130 kV, and effective tube current of 200 mA to position the ROI at the aorta. Then, dynamic scanning was performed by setting the ROI cursor over the aorta at the level of the pulmonary trunk. Contrast solution was administered through the cephalic catheter at a rate of 3 mL/s by use of a single-headed injector system,g and dynamic scanning was performed at 110 kVp, tube current of 40 mA, slice thickness of 10 mm, and rotation speed of 1.0 s/rotation. Dynamic CT images were acquired during a 54-second period at 0.9-second intervals. A TAC was generated for each contrast formulation administered to each dog by positioning a circular ROI cursor so that it was as large as possible within the aortic lumen.

The TACs were analyzed to determine attenuation values at the enhancement peak (peak attenuation values), interval from initiation of contrast medium injection to the enhancement peak, interval from initiation of injection to attainment of 200 HU before the enhancement peak, and interval from initiation to attainment of 300 HU after the enhancement peak. With these results, calculations were made to determine the duration of optimal attenuation maintained between 200 and 300 HU before (early optimal attenuation) and after (late optimal attenuation) the enhancement peak was attained. Durations of optimal attenuation were evaluated on the basis of findings from other studies6,13 in which optimal vascular and ventricular attenuation was identified in ROIs at 200 to 300 HU. Data from the TACs were also used to calculate the scan delay (ie, the starting point for cardiac CT scanning after completion of contrast medium injection).

Statistical analysis

Results are reported as mean ± SD. Differences among contrast formulations for peak attenuation values, interval to peak attenuation values, and interval to reach an attenuation value of 200 HU were assessed with a 1-way ANOVA. Values of P ≤ 0.05 were considered significant. When a significant difference was detected among the contrast formulations, post hoc analysis (Tukey honestly significant difference; α = 0.05) was performed to identify the specific nature of the differences. All statistical analyses were performed with statistical software,h and graphs were created by use of graphing software.i

Results

Animals

All 4 Beagles received all 12 formulations of iohexol solution for cardiac CT angiography. No significant differences were identified in overall mean ± SD heart rate before (66.69 ± 10.07 beats/min), during (64.44 ± 14.31 beats/min), and after (64 ± 15.21 beats/min) IV injection of contrast solution.

TACs

Two types of TACs were identified during assessment of the 12 formulations of contrast solutions: peak arc type and plateau type (Figure 1). In peak arc–type TACs, attenuation values within the ROI (over the aorta at the level of the pulmonary trunk) rapidly increased until a single enhancement peak was reached and then quickly decreased. This type of TAC was obtained with IV injection of undiluted iohexol solution at each iodine dose assessed (300, 400, and 800 mg/kg) and for injections containing a small total volume of contrast solution (ie, 300 mg of iodine/kg [undiluted and 1:1, 1:2, and 1:3 dilutions], 400 mg of iodine/kg [undiluted and 1:1 dilution], and 800 mg of iodine/kg [undiluted and 1:1 dilution]). Peak arc–type TACs were associated with a lack of uniform contrast enhancement, which appeared as a precipitous change within the curves.

Figure 1—
Figure 1—

Time-attenuation curves for undiluted iohexol solutions (A) and iohexol solutions diluted 1:1 (B), 1:2 (C), and 1:3 (D) with saline (0.9% NaCl) solution administered IV to 4 clinically normal Beagles for cardiac CT angiography. Three doses of iodine were evaluated for each set of dilutions (300 mg/kg [black circles], 400 mg/kg [white circles], and 800 mg/kg [black inverted triangles]), yielding 12 formulations of contrast solutions, each of which was evaluated in each dog by use of a crossover study design. A peak arc–type TAC (rapidly increasing, brief peak, then rapidly decreasing curve) was detected primarily for undiluted contrast formulations. A plateau-type TAC (more uniform and prolonged plateau peak) was attained with contrast formulations of greater dilutions. The magnitude of aortic enhancement increased proportionally with increasing iodine dose for each dilution ratio.

Citation: American Journal of Veterinary Research 76, 8; 10.2460/ajvr.76.8.694

As the total volume of contrast solution increased, TAC shape changed from peak arc type to plateau type. Plateau-type TACs were characterized by a prolonged period of enhancement reflecting maintenance of attenuation values throughout the plateau or a prolonged increase in attenuation values followed by a more rapid decrease. For example, injection of the 1:2 dilution of contrast medium containing an iodine dose of 800 mg/kg resulted in a definite plateau shape in which a uniform degree of attenuation was maintained, whereas injection of the 1:3 dilution of the same iodine concentration resulted in a peak in attenuation values that increased consistently during the plateau period. Injection of the 1:2 and 1:3 dilutions of contrast medium at a dose of 400 mg of iodine/kg also yielded a TAC that represented a continuous, slowly increasing enhancement peak over a prolonged period.

Peak attenuation values for ROIs were determined for each formulation of contrast medium (Table 1). No significant differences in values were identified among the 4 dilutions (undiluted and dilutions of 1:1, 1:2, and 1:3) for contrast medium containing an iodine dose of 300 mg/kg. For the 400 mg/kg dose, each dilution yielded a significant difference in peak attenuation values within the ROI. In general, values for the 400 mg/kg dose decreased with increasing dilution, except that the 1:1 dilution yielded a significantly higher attenuation value than did the undiluted formulation. Results of post hoc comparisons indicated that peak attenuation values for the 1:1 and 1:3 dilutions were significantly different. Attenuation values for the dose containing 800 mg of iodine/kg decreased significantly with increasing dilutions. Post hoc comparisons revealed that peak attenuation values differed significantly among all 4 dilutions.

Table 1—

Mean ± SD peak attenuation values (HU) attained within the aorta after IV administration of various formulations of iohexol solutions evaluated for use in performing cardiac CT angiography in 4 clinically normal Beagles.

 Dilution*
Iodine dose (mg/kg)Undiluted1:11:21:3
300416.35 ± 45.84498.50 ± 61.34436.70 ± 48.21390.63 ± 46.09
400535.45 ± 42.29a601.25 ± 48.89b552.38 ± 46.40a,b,c444.18 ± 54.77a,c
800960.65 ± 21.32a719.78 ± 83.39b559.20 ± 50.66c521.00 ± 86.32d

Dilutions were made with saline (0.9% NaCl) solution.

Within a row, values with different superscript letters differ significantly (P ≤ 0.05).

No significant differences in intervals to the enhancement peak were identified among the 4 dilutions for contrast medium containing an iodine dose of 300 mg/kg (Table 2). For formulations containing an iodine dose of 400 mg/kg, interval from initiation of injection to attainment of the enhancement peak increased with increasing dilution ratio. Post hoc comparisons among dilutions of the 400 mg/kg dose revealed that the interval for the 1:3 dilution differed significantly from those for the undiluted formulation and 1:1 dilution. Intervals to the enhancement peak for formulations containing an iodine dose of 800 mg/kg significantly increased with increasing dilutions, with significant differences identified among all dilutions.

Table 2—

Mean ± SD values from TAC analysis of various formulations of iohexol solutions administered IV to the dogs in Table 1.

Iodine dose (mg/kg)DilutionInterval to attain 200 HU before peak (s)Duration of early optimal enhancement* (s)Interval to attain peak enhancement (s)Interval to attain 300 HU after peak (s)Duration of late optimal enhancement* (s)
300Undiluted12.50 ± 2.031.78 ± 0.5116.45 ± 1.6921.28 ± 2.443.20 ± 0.80
 1:112.25 ± 1.481.46 ± 0.5517.96 ± 1.6422.84 ± 2.783.13 ± 1.44
 1:214.00 ± 2.321.46 ± 0.9718.88 ± 1.64223.63 ± 3.472.13 ± 1.58
 1:314.13 ± 1.753.37 ± 0.8320.20 ± 3.8426.95 ± 3.893.09 ± 0.19
400Undiluted11.25 ± 0.431.47 ± 0.7115.13 ± 1.29a22.58 ± 0.574.32 ± 1.67
 1:112.38 ± 2.381.75 ± 0.4518.65 ± 1.69a26.13 ± 2.338.12 ± 1.17
 1:214.13 ± 2.381.87 ± 0.4921.98 ± 1.59a29.48 ± 2.342.51 ± 0.84
 1:314.50 ± 2.152.39 ± 0.3526.13 ± 3.32b32.85 ± 3.584.83 ± 0.75
800Undiluted11.63 ± 1.981.66 ± 0.4818.65 ± 2.19a26.38 ± 5.3713.10 ± 0.47
 1:112.13 ± 0.541.66 ± 1.2524.60 ± 1.58b37.68 ± 6.4712.42 ± 1.22
 1:213.25 ± 1.642.54 ± 0.6333.85 ± 4.18c47.13 ± 1.99> 16
 1:314.88 ± 4.226.35 ± 0.4045.75 ± 2.59d52.21 ± 1.80> 16

Starting point for all intervals was initiation of contrast medium injection.

Optimal enhancement was defined as an attenuation value of 200 to 300 HU. Early refers to the period before peak attenuation values were attained, and late refers to the period after this peak.

Within a column within an iodine dose, values with different superscript letters differ significantly (P ≤ 0.05).

None of the 12 formulations differed with respect to interval from initiation of injection to attainment of an attenuation value of 200 HU within the ROI (Table 2). Duration of the optimal enhancement (attenuation, 200 to 300 HU) maintained in the ROI was identified for 2 periods before and after the enhancement peak. Duration of early optimal enhancement was less than the duration of late optimal enhancement by < 7 seconds for all contrast formulations, regardless of iodine dose. Formulations that yielded plateau-type TACs provided a sufficient period with uniform enhancement to allow performance of cardiac CT angiography, considering that cardiac CT scanning could be performed within 15 seconds by use of the 16-channel multi-detector CT. In particular, among the contrast formulations that yielded plateau-type TACs, the 1:2 dilution containing an iodine dose of 800 mg/kg yielded a TAC with 2 peaks in attenuation values while maintaining a uniform degree of contrast enhancement throughout the peaks for approximately 18 seconds. When the 1:3 dilution of this iodine dose was administered, peak attenuation values slowly increased to yield a plateau. For both of those formulations, attenuation values at the beginning of the plateau remained > 400 HU and those after the end of the plateau remained between 200 to 300 HU for > 16 seconds (ie, the duration of late optimal enhancement was achieved). For the other formulations that yielded a plateau-type TAC, duration of late optimal enhancement was briefer by < 13 seconds and would be insufficient to allow cardiac CT scanning. Scan delay for the 1:2 and 1:3 dilutions containing an iodine dose of 800 mg/kg was calculated as 13 to 17 seconds after completion of contrast medium injection.

Discussion

In the present study, TACs and scan delay for cardiac CT angiography in clinically normal dogs were determined by use of 12 formulations of iohexol contrast solutions. The TACs were evaluated to determine whether an optimal degree of contrast enhancement (attenuation values, 200 to 300 HU) was attained within the ROI (over the aorta at the level of the pulmonary trunk) and whether that enhancement was uniform in consistency.

During cardiac CT angiography, it is important to attain attenuation values > 200 HU to allow visual evaluation of small vessels and to maintain enhancement at < 350 HU because enhancement at higher values may obscure calcifications and thrombi, leading to false-negative results.6,14 Consequently, a target range of 200 to 300 HU was used in the present study because it reportedly provides optimal vascular and ventricular attenuation.1,13 This degree of contrast enhancement should be maintained uniformly throughout data acquisition to ensure quality results. Accurate timing of contrast medium injection and use of an appropriate scan delay are required to allow synchronization with the duration of CT scanning when multidetector CT scanners are used. Meeting those requirements is challenging, particularly in animals with small body weights, because the volume of injected contrast medium is small and the duration of cardiovascular enhancement is brief. The point at which injected contrast medium arrives at an ROI is affected by patient characteristics such as heart rate and cardiovascular function. Use of the fixed scan-delay technique with a test bolus or use of the bolus tracking technique can help to control the timing of contrast medium arrival within an ROI. In the fixed scan-delay technique, scan delay is estimated from the TAC attained after injection of a small test bolus of contrast medium, and then diagnostic CT scanning is performed with additional contrast medium. With the bolus tracking technique, CT scanning begins when the degree of contrast enhancement within an ROI exceeds a predetermined threshold that is determined through measurement of temporal changes in contrast enhancement at the ROI after bolus injection of contrast medium. The bolus tracking technique is more efficient than the fixed scan-delay technique because the scan-delay method requires 2 injections of contrast medium and additional examination time. On the other hand, the bolus tracking technique requires use of specific computer software.15–18

In the present study, the fixed scan-delay technique was used for cardiac CT angiography and Beagles with similar body size were used to control for the influence of dog characteristics on the results. Heart rate was considered a constant variable. Contrast medium was diluted with saline solution to increase the total amount of contrast solution administered without a proportional increase in iodine dose. In human medicine, a saline solution flush and dual injector are used for uniformity and prolongation of contrast enhancement. Flushing improves the efficiency of contrast medium injection by pushing contrast medium in the injection tubing and peripheral veins into the central blood volume, thereby increasing the magnitude of the enhancement peak and prolonging the interval to the enhancement peak. By this method, the degree of contrast enhancement after the enhancement peak decreases more rapidly because there is no slow, delayed flow of contrast medium from peripheral venous spaces as occurs with the single-injector method used in the present study. Scanning duration may be insufficient because of this rapid decrease.19 We presumed that injection of an increased (diluted) volume of contrast solution through a single rather than dual injector would prolong the period of uniform enhancement within cardiac and vascular lumens and provide enough time to perform cardiac scanning while avoiding a rapid decrease in enhancement after the enhancement peak was attained. For example, the undiluted iohexol formulation containing 800 mg of iodine/kg and the 1:1, 1:2, and 1:3 dilutions of the same formulation contained the same iodine dose, and the period of contrast enhancement increased as the total injected volume increased.

In the peak arc–type patterns of contrast enhancement obtained in the present study, attenuation values within the ROI rapidly decreased shortly after injection was completed because a small amount of contrast medium quickly diffused away from the cardiovascular system. This situation can lead to nonuniform cardiovascular enhancement during image acquisition.11 For undiluted contrast medium or contrast solutions with small volumes, peak arc–type TACs in the present study were characterized by sharp increases, brief peaks, and rapid decreases. Duration of optimal attenuation (200 to 300 HU) was < 8 seconds for those formulations. Considering that ≥ 15 seconds would be needed for cardiac CT scanning even with use of a 16-channel multidetector, the observed period was too brief to be useful. On the contrary, the plateau-type TACs obtained with large volumes of diluted contrast solutions (ie, 400 mg of iodine/kg at dilutions of 1:2 and 1:3 or 800 mg of iodine/kg at dilutions of 1:2 and 1:3) consisted of plateaus > 16 seconds in duration. However, attenuation values attained during the plateaus were not indicative of an optimal degree of enhancement because they were > 400 HU during the plateau period. Rather, attenuation values attained after the endpoint of the plateau were maintained at 200 to 300 HU for > 16 seconds.

Scan delay should be determined on the basis of 3 factors: duration of contrast medium injection, timing of arrival of contrast medium at the ROI, and duration of CT scanning. In general, scan delay for the abdominal aorta or liver can be determined by use of the following equation from the human medical literature9:

article image

However, in the present study, scan delay was determined by TAC data because had this equation been used instead, a superfluous degree of contrast enhancement would have been achieved. For example, for the 1:2 and 1:3 dilutions containing an iodine dose of 800 mg/kg, scan delay calculated by use of this equation would be 31 to 36 seconds, and that period would result in an excessive degree of contrast enhancement (> 550 HU). It should be considered, however, that the equation for scan delay was derived for CT angiography of abdominal organs or noncardiac structures and not for CT angiography of cardiac structures. Interval from initiation of contrast medium injection to the enhancement peak is calculated as the sum of the duration of contrast medium injection and the interval from completion of contrast medium injection to its arrival within a target organ (ie, transit time).20 Transit time for contrast medium is influenced by duration of the injection, timing of contrast medium arrival at the ROI, and circulation path to the target organ.1,5 However, the interval for the arrival of injected contrast medium at cardiac structures in the present study was less than that which might be required for medium to arrive at abdominal organs; therefore, use of calculations for abdominal organs would necessarily lead to an excessively high amount of contrast medium within cardiac and vascular lumens for evaluation of the intra- and extracardiac structures.

For the 1:2 and 1:3 dilutions containing an iodine dose of 800 mg/kg, attenuation values were maintained at > 400 HU from the beginning of the TAC plateau and were too high to provide optimal image quality. As contrast medium circulates in the body, it becomes diluted by the blood, disperses through the circulatory system, and returns to the heart. The blood recirculation visibly enhanced with contrast medium may contribute to the overall pattern of contrast enhancement obtained during CT image acquisition.8 When contrast medium is injected at a constant rate for a prolonged versus brief period, freshly injected contrast medium and recirculating contrast medium already dispersed through the body mix and accumulate, resulting in a more gradual increase in aortic enhancement with time. Without this recirculation, the TAC for aortic contrast enhancement would consist of a rapid increase followed by a uniform, steady-state plateau with a flat, broad peak as the rate of contrast medium clearance from the central blood compartment equilibrates with the injection rate of contrast medium.21 The interval from initiation of contrast medium injection to the end of the enhancement plateau may correspond to the duration of the injection plus the period needed for contrast medium to arrive at an ROI; however, because of recirculation, a steady-state plateau in contrast enhancement cannot be sustained and instead consistently increases. Therefore, in the study reported here, contrast formulations with large volumes such as those for the 1:2 and 1:3 dilutions containing iodine doses of 400 and 800 mg/kg were associated with a steady plateau. Attenuation values attained after the end of the plateau were consistently maintained within a range of 200 to 300 HU for > 16 seconds for the 800 mg/kg formulations at the 1:2 and 1:3 dilutions. Approximately 13 to 17 seconds elapsed between completion of the total injection of contrast solution and attainment of an attenuation value of 300 HU after the enhancement plateau. Therefore, we recommend that a scan delay of 13 to 17 seconds be used for performing cardiac CT angiography in dogs when either of these 2 formulations of contrast medium is administered.

The present study had some limitations. First, the ROI used for creation of TACs was set over the aorta, but the contrast protocols evaluated were intended to pertain to cardiac structures as well as large vessels such as the aorta and pulmonary arteries. We presumed that, in general, the degree of contrast enhancement in the aorta and pulmonary artery would reflect that in the left ventricle and right ventricle.20 Second, the total volume of the 1:2 and 1:3 dilutions of contrast medium containing an iodine dose of 800 mg/kg (8.0 and 10.7 mL/kg, respectively) would be too large to administer and might cause adverse effects such as congestive heart failure and pulmonary edema, particularly in dogs with cardiovascular disease, even if that total volume was injected at a fixed flow rate. The total volume of the 1:2 and 1:3 dilutions containing an iodine dose of 400 mg/kg (4.0 and 5.3 mL/kg, respectively) was smaller than that for the 800 mg/kg dose, but optimal attenuation was maintained for only 12 to 13 seconds (vs > 16 seconds). These findings suggested that a need remains to identify a contrast formulation that would provide an appropriate iodine dose at a large enough volume to maintain the plateau while also posing a low risk to dogs with cardiovascular disease. Third, the flow rate of contrast medium injection, which is an important factor in contrast enhancement, was not considered. Injection rates of 2 to 5 mL/s via the cephalic vein are commonly used for clinical CT imaging in dogs.9 The rate of 3 mL/s was chosen for the dogs in the present study because we believed a rapid injection rate would allow a fast scan, but additional research is needed to identify an optimal flow rate for cardiac CT scanning in dogs.

Dynamic CT scanning was performed in the study reported here to determine an optimal scan delay and appropriate contrast formulations for use in cardiac CT angiography in dogs. Diluted contrast solutions of large volumes provided a plateau in contrast enhancement with homogeneous attenuation. Scan delay was 13 to 17 seconds for 1:2 and 1:3 dilutions of contrast medium containing an iodine dose of 800 mg/kg in clinically normal Beagles.

Acknowledgments

Supported in part by the Animal Medical Institute of Chonnam National University.

Presented in part at the American College of Veterinary Radiology Annual Scientific Conference, Savannah, Ga, October 2013.

ABBREVIATIONS

HU

Hounsfield unit

ROI

Region of interest

TAC

Time-attenuation curve

Footnotes

a.

Omnihexol 300, Korea United Pharm Co, Seoul, Republic of Korea.

b.

Domitor, Orion Corp, Espoo, Finland.

c.

Zoletil, Virbac, Carros, France.

d.

BD Angiocath Plus, Becton Dickinson Infusion Therapy Systems Inc, Singapore, Republic of Singapore.

e.

Somatom Emotion, Siemens Medical Systems, Erlangen, Germany.

f.

VP-1200/1000, Votem, Chuncheon, Republic of Korea.

g.

Medrad Vistron C-T Injector System, Medrad Inc, Indianola, Pa.

h.

SPSS, version 20, IBM Corp, Armonk, NY.

i.

SigmaPlot, version 10.0, Systat Software Inc, Calif.

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Appendix

Characteristics of various formulations of iohexol solutions evaluated for use in performing cardiac CT angiography in 4 clinically normal Beagles.

Iodine dose (mg/kg)DilutionTotal volume (mL/kg)Iodine concentration (mg/mL)Total volume per dog (mL)*Injection duration (s)*
300Undiluted1.030011.50 ± 0.504.00 ± 0.00
 1:12.015023.00 ± 1.008.00 ± 0.00
 1:23.010034.50 ± 1.5011.50 ± 0.50
 1:34.07546.00 ± 2.0015.50 ± 0.50
400Undiluted1.330014.75 ± 0.835.25 ± 0.43
 1:12.715029.50 ± 1.6610.25 ± 0.43
 1:24.010044.25 ± 2.4914.75 ± 0.83
 1:35.37559.00 ± 3.3219.75 ± 0.83
800Undiluted2.730029.75 ± 1.7910.50 ± 0.50
 1:15.315059.50 ± 3.5720.25 ± 1.30
 1:28.010089.25 ± 5.3629.75 ± 1.79
 1:310.775119.00 ± 7.1439.90 ± 2.62

For all contrast imaging procedures, iohexol (300 mg/mL) was administered via a cephalic vein at a rate of 3 mL/s.

Values are mean ± SD.

Contributor Notes

Address correspondence to Dr. Choi (imsono@chonnam.ac.kr).