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DOI:10.2214/AJR.07.3576
AJR 2008; 191:1589-1594
© American Roentgen Ray Society

Original Research

Aortic and Hepatic Contrast Enhancement with Abdominal 64-MDCT in Pediatric Patients: Effect of Body Weight and Iodine Dose

Kyongtae T. Bae1, Amisha J. Shah1, Sherry S. Shang1,2, Jin Hong Wang1, Samuel Chang1, Masayuki Kanematsu3 and Charles F. Hildebolt4

1 Department of Radiology, University of Pittsburgh School of Medicine, 3362 Fifth Ave., Pittsburgh, PA 15213.
2 Department of Radiology, Hahnemann University Hospital, Drexel University College of Medicine, Philadelphia, PA.
3 Department of Radiology, Gifu University School of Medicine, Gifu, Japan.
4 Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO.

Received December 22, 2007; accepted after revision May 29, 2008.

 
Address correspondence to K. T. Bae (baek{at}upmc.edu).


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of our study was to retrospectively evaluate the effect of body weight and iodine dose on aortic and hepatic contrast enhancement in pediatric patients who underwent 64-MDCT of the abdomen and pelvis.

MATERIALS AND METHODS. Eighty-seven consecutive pediatric patients (50 boys and 37 girls; median age, 12.1 years; age range, 3.8–17.6 years) underwent standard abdominopelvic CT with a 64-MDCT scanner. Contrast medium (350 mg I/mL) was injected using a power injector at 2 mL/s followed by 15–20 mL of saline flush. According to our CT protocol, the volume of administered contrast medium was approximately 1.8 mL/kg of body weight, up to the maximum volume of 80 mL. CT scanning was initiated 60 seconds after the start of the contrast medium injection. CT attenuations of the aorta and liver were measured. For each patient, the injected contrast medium iodine mass per body weight index (g I/kg) (hereafter, iodine mass body index) was calculated. Linear regression analysis was performed between iodine mass body index and aortic and hepatic attenuations.

RESULTS. A wide range of patient weights (19–82 kg; mean, 48.6 kg [95% CI, 45.3–51.9 kg]) and contrast volumes (30–80 mL; median, 80.0 mL) were observed. The median attenuations were 149.0 HU (141.0–160.0 HU) for the aorta and 113.5 HU (109.5–120.0 HU) for the liver. Moderately high correlations were observed between iodine mass body index and aortic (Spearman's rho [rs] = 0.60 [0.45–0.72]; p < 0.001) and hepatic (rs = 0.60 [0.42–0.70]; p < 0.001) attenuations. The regression formulae for aortic attenuation (58.4 + 176.3 x iodine mass body index [p < 0.001]) and hepatic attenuation (58.7 + 108.5 x iodine mass body index [p < 0.001]) indicate that 1.5 and 1.8 mL/kg (350 mg I/mL) of contrast media are required to achieve 116 and 127 HU, respectively, of contrast-enhanced attenuation in the liver.

CONCLUSION. In our study, using abdominal 64-MDCT in pediatric patients, we found that approximately 1.5 mL/kg, or 0.525 g I/kg, yields 116 HU of hepatic attenuation or 50–55 HU of hepatic enhancement.

Keywords: abdominal CT • contrast agents • CT • IV • pediatric patients


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Insufficient hepatic parenchymal enhancement in abdominal CT results in diminished lesion conspicuity [1–3]. A number of studies have addressed the issue of what level of hepatic enhancement constitutes the minimum acceptable for liver imaging [4–13]. In general, 50 HU is considered to be a diagnostically appropriate level of hepatic enhancement for abdominal CT in adult patients [14–17]. To our knowledge, no study that addresses an appropriate level of hepatic enhancement in pediatric patients has been reported.

A patient's body weight and the amount of contrast medium are closely related to the degree of aortic and hepatic contrast enhancement [1, 14, 18–24]. When consistent contrast enhancement is desired, the amount of the administered iodine dose should be adjusted according to the patient's body weight. One common scheme is to increase or decrease the amount of administered iodine mass linearly in proportion to the body weight. For example, approximately 0.5 g I/kg is needed to achieve the maximum hepatic enhancement of 50 HU; that is, 35 g I for a 70-kg patient [14].

Adjusting the iodine dose for body weight is particularly crucial in children because of the wide range of body sizes. The traditional dose of contrast medium administered in children is 2 mL/kg, with a maximum dose of 150 mL [25]. This scheme, which was based on use of low-concentration contrast media (240–300 mg I/mL), has been commonly practiced since early CT more than 30 years ago and is widely used even in the current era of fast MDCT and the general use of higher-concentration contrast media. Short scanning times offered by fast MDCT allow improved contrast enhancement and more efficient use of contrast media [24, 26]. With MDCT, the amount of contrast medium for some clinical applications may be reduced without decreasing contrast enhancement. Furthermore, for a given requirement of iodine mass, the use of higher-concentration contrast media allows us to reduce the volume of contrast media compared with the use of lower-concentration contrast media. To our knowledge, these considerations for determining optimal iodine dose in pediatric patients with abdominal 64-MDCT, including the effect of body weight, have not been addressed in previous studies. Thus, the purpose of this study was to evaluate the effect of body weight and iodine dose on aortic and hepatic contrast enhancement in pediatric patients who underwent 64-MDCT of the abdomen and pelvis.


Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Patients
Institutional review board approval was obtained, and informed consent was not required for our HIPAA-compliant retrospective study. Over a 5-month period, the list of consecutive pediatric patients who underwent standard abdominopelvic CT with a 64-MDCT scanner (LightSpeed VCT, GE Healthcare) was reviewed. We used our radiology department's informatics database (with entries made by CT technologists and nurses) to obtain the patients' body weight, sex, age, and volume and injection rate of contrast medium. For small children (typically 3 years or younger), contrast medium was commonly injected manually or at slow rates. To reduce the effect of injection rate on contrast enhancement, we excluded patients for whom IV contrast medium was administered manually or at an injection rate other than 2 mL/s. No injections more rapid than 2 mL/s were used for abdominopelvic CT in children in our department. During the image review, we also excluded patients who had undergone liver transplantation or had extensive liver masses because of a concern that normal hemodynamics and physiology would likely have been greatly disturbed in these pa tients. The final study group consisted of 87 children (50 boys and 37 girls; age range, 3.8–17.6 years; median age, 12.1 years [95% CI, 11.4–13.3 years]).

CT Imaging
All CT examinations were performed with a 64-MDCT scanner (LightSpeed VCT). Scanning parameters were detector collimation, 64 x 0.625 mm; total z-axis coverage, 40 mm per rotation; gantry rotation speed, 0.4 second; tube voltage, 120–140 kV; and tube current with automatic dose modulation ranging from 150 to 320 mA. The mean craniocaudal coverage of CT scans was 374.2 ± 48.1 mm (SD) with a range of 265–485 mm. Section thickness of the reconstructed CT images was 2.5 mm for children of 7.5–9.0 kg body weight, 3.75 mm for children 18.5–22.0 kg, and 5.0 mm for children 40.5 kg and above.

Contrast enhancement was achieved by administering 350 mg I/mL of ioversol contrast medium (Optiray 350, Mallinckrodt Imaging) into an antecubital vein. Contrast medium was injected using a power injector at an injection rate of 2 mL/s, followed by 15–20 mL of saline flush. According to our departmental CT protocol, the volume of contrast medium administered for abdominopelvic 64-MDCT was approximately 1.8 mL/kg of body weight, up to the maximum volume of 80 mL. The volumes of contrast medium used for CT, however, frequently deviated from that prescribed and estimated from the patient's weight according to the protocol. After each scan, the actual volume of administered contrast medium was documented in the radiology informatics database by a CT technologist or nurse. The CT examination from the dome of the diaphragm to the pubic symphysis was initiated 60 seconds after the start of the contrast medium injection.

Aortic and Hepatic Attenuation Measurement
CT images were retrieved from the institutional PACS and displayed and reviewed on a clinical workstation. Mean attenuations of the aorta and liver were measured using a circular region-of-interest (ROI) cursor by radiologists with experience in interpreting body CT images. The size of the ROI was approximately half the diameter of the aorta (typical ROI diameter, 5–7 mm). Approximately the same size ROI was used for both aortic and hepatic attenuation measurements. CT attenuation values of the abdominal aorta were measured in areas just above the celiac arterial level, and values for hepatic parenchyma were measured in two areas (right posterior and left lateral segments) and averaged. Any focal parenchymal lesion, blood vessel, or artifact was carefully excluded from hepatic attenuation measurement areas.

Statistical Analysis
The patient data for the following variables were fitted with normal distributions and tested for normality with Shapiro-Wilk W tests: patient age and body weight, contrast volume, and the CT attenuation values of the aorta and the liver. Means were calculated for normally distributed data and medians for nonnormally distributed data; 95% CIs were calculated for the means and medians.

CT attenuation values of the aorta and the liver were plotted against the body weight to illustrate the trend of CT attenuation values varying with the patients' body weights. Because the maximum amount of contrast medium was set at 80 mL in our CT protocol, patients with large body weights tended to receive contrast medium volume of less than 1.8 mL/kg of body weight compared with patients who had smaller body weights. We used the Pearson product–moment correlation coefficient (r) to assess the strengths of associations involving normal data distributions and the Spearman's rho (rs) correlation coefficient to assess strengths of associations involving nonnormal data distributions.

To evaluate the amount of injected contrast medium relative to the patient's body weight, we introduced and calculated the injected contrast medium iodine mass per body weight index (g I/kg) (hereafter, iodine mass body index) for each patient. Contrast medium iodine mass was calculated as the product of the volume of injected contrast medium and a fixed concentration of contrast medium (i.e., 350 mg I/mL) and divided by body weight to yield the iodine mass body index. CT attenuation values of the aorta and the liver were plotted against the iodine mass body index, and linear regression analyses were performed between the iodine mass body index and CT attenuation values. Ninety-five percent CIs were plotted for the regression lines. Alpha was set at 0.05. Statistical analyses were performed with JMP Statistical Software, version 6.0.2 (SAS Institute), and Statistics for Biomedical Research, version 8.1.00 (MedCalc Software).


Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Data Distribution for Age, Weight, Contrast Volume, Iodine Mass Body Index, and Attenuation
Data were normally distributed for body weight and iodine mass body index (Shapiro-Wilk W test, p > 0.05). Age, contrast volume, and aortic and hepatic CT attenuation were nonnormally distributed (Shapiro-Wilk W test, p < 0.05). A wide range of patient weights (19–82 kg; mean, 48.6 kg [95% CI 45.3–51.9 kg]) and iodine mass body index (0.32–0.70 g I/kg; mean, 0.54 [0.51–0.56 g I/kg]) were observed. The range of contrast volume was 30–80 mL (median, 80.0 [80.0–80.0 mL]). The median aortic attenuation was 149 HU [141.0–160.0 HU], with a range of 83–265 HU. The median hepatic attenuation was 113.5 HU [109.5–120.0 HU], with a range of 72–179 HU. Abdominal CT images from three patients (small, medium, and large body sizes) are presented in Figure 1A, 1B, 1C to illustrate the different degrees of contrast enhancement.


Figure 1
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  		Fig. 1A —Transverse CT images in patients with three different body sizes (small, medium, and large) in whom different degrees of aortic and hepatic attenuations were seen. (Image display window width, 300 HU; center, 30 HU.) 6-year-old girl who weighed 20 kg (small body size) and received 40 mL of contrast medium (contrast medium iodine mass per body weight index, 0.70) had 195 HU of aortic and 134.5 HU of hepatic attenuation.

 

Figure 2
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  		Fig. 1B —Transverse CT images in patients with three different body sizes (small, medium, and large) in whom different degrees of aortic and hepatic attenuations were seen. (Image display window width, 300 HU; center, 30 HU.) 12-year-old boy who weighed 50 kg (medium body size) and received 80 mL of contrast medium (contrast medium iodine mass per body weight index, 0.56) had 131 HU of aortic and 111 HU of hepatic attenuation.

 

Figure 3
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  		Fig. 1C —Transverse CT images in patients with three different body sizes (small, medium, and large) in whom different degrees of aortic and hepatic attenuations were seen. (Image display window width, 300 HU; center, 30 HU.) 17-year-old boy who weighed 75 kg (large body size) and received 80 mL of contrast medium (contrast medium iodine mass per body weight index, 0.37) had 104 HU of aortic and 72 HU of hepatic attenuation.

 
Effect of Body Weight on Aortic and Hepatic Attenuations
Moderately strong inverse correlations were noted between the body weight and aortic (rs = –0.52 [–0.66 to –0.35], p < 0.001) and hepatic (rs = –0.49 [–0.63 to –0.31], p < 0.001) attenuations, indicating reduced aortic and hepatic attenuations in heavier patients. Aortic and hepatic attenuations and iodine mass body index (g I/kg) data are plotted in Figure 2A, 2B. Moderately strong correlations were observed between iodine mass body index and aortic (rs = 0.60 [0.45–0.72], p < 0.001) and hepatic (rs = 0.60 [0.42–0.70], p < 0.001) attenuations. The regression formula for aortic attenuation is 58.4 + 176.3 x iodine mass body index (p < 0.001), and the regression formula for hepatic attenuation is 58.7 + 108.5 x iodine mass body index (p < 0.001). The hepatic attenuation formula indicates that 1.5 mL/kg and 1.8 mL/kg of 350 mg I/mL of contrast medium (i.e., 0.525 g I/kg and 0.63 g I/kg) are required to achieve 116 HU and 127 HU, respectively, of contrast-enhanced attenuation in the liver.


Figure 4
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  		Fig. 2A —Graphs of attenuation against contrast medium iodine mass per body weight index with 95% CI fit to regression lines. Plots of aortic (A) and hepatic (B) attenuation against contrast medium iodine mass per body weight index show moderately strong correlations existed between index and aortic (Spearman's rho [rs] = 0.60 [95% CI, 0.45–0.72], p < 0.001) and hepatic (rs = 0.60 [0.42–0.70], p < 0.001) attenuations. Regression formulae are aortic attenuation = 58.4 + 176.3 x contrast medium iodine mass per body weight index (p < 0.001), and hepatic attenuation = 58.7 + 108.5 x contrast medium iodine mass per body weight index (p < 0.001).

 

Figure 5
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  		Fig. 2B —Graphs of attenuation against contrast medium iodine mass per body weight index with 95% CI fit to regression lines. Plots of aortic (A) and hepatic (B) attenuation against contrast medium iodine mass per body weight index show moderately strong correlations existed between index and aortic (Spearman's rho [rs] = 0.60 [95% CI, 0.45–0.72], p < 0.001) and hepatic (rs = 0.60 [0.42–0.70], p < 0.001) attenuations. Regression formulae are aortic attenuation = 58.4 + 176.3 x contrast medium iodine mass per body weight index (p < 0.001), and hepatic attenuation = 58.7 + 108.5 x contrast medium iodine mass per body weight index (p < 0.001).

 

Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
It is well known with pediatric CT that the dose of contrast medium should be adjusted for body weight to achieve consistent contrast enhancement. Despite recent marked advances in CT technology and iodinated contrast media, there is a relative paucity of studies for optimization of contrast medium administration in children compared with the plethora of studies dealing with adult patients. Difficulties of conducting contrast medium administration studies in children include a large variation in body weights and use of nonstandardized contrast administration techniques. For example, IV contrast medium is usually injected manually in small children. IV access, the amount of contrast medium, and the timing for scanning initiation are difficult to standardize in children.

With the availability of new CT technology and new contrast media, there is a strong clinical need to revisit and reassess the traditional approach of administrating contrast media in children. The results from our study confirm that to achieve consistent aortic or hepatic contrast enhancement in pediatric patients in 64-MDCT, the amount of contrast medium should be adjusted to the patient's body weight. We found approximately 1.5 mL/kg (0.525 g I/kg) to yield 116 HU of hepatic attenuation. Because we do not routinely perform unenhanced examinations of the liver in children, we could not determine net contrast enhancement (i.e., contrast-enhanced minus unenhanced attenuations).

A previous study [27] reported that the mean unenhanced attenuation value for the upper liver in children was 63.6 HU (range, 44.8–79.4 HU). Thus, 116 HU of hepatic attenuation seems to correspond to approximately 50–55 HU of hepatic enhancement. This degree of hepatic contrast enhancement in response to the amount of administered iodine dose is compatible to that reported in adult patients—that is, 50 HU of hepatic enhancement with approximately 0.5 g I/kg [14, 28–31]. To our knowledge, no study has been reported regarding an appropriate level of hepatic enhancement in pediatric patients. To address this, a prospective study that assesses the diagnostic image quality at different enhancement levels in pediatric patients would be required.

Although 1.8 mL/kg is the standard amount of contrast medium recommended according to our departmental CT protocol, this dosage was not rigorously followed and was frequently deviated from, as represented by the iodine mass body index data. Furthermore, we set the maximum amount of contrast medium at 80 mL. This implies that patients whose weights are larger than 44.4 kg would receive less than 1.8 mL/kg because of this upper limit of contrast medium volume. Our data show that CT attenuations in these larger patients were significantly lower than those in smaller patients. With the study results in hand, we have subsequently revised our CT protocol: The standard amount of contrast medium for pediatric abdominal CT is 1.5 mL/kg (or 0.525 g I/kg) without an arbitrarily set upper limit of contrast medium. In a recent article [32], 1.5 mL/kg of contrast medium was also recommended for pediatric thoracic CT angiography but with a 120-mL maximum dose for adult-sized children.

It has been shown that fast MDCT allows us to substantially reduce the amount of required contrast medium without a significant reduction in contrast enhancement for CT angiography in adult patients [26, 33]. Unlike CT angiography, however, a substantial reduction of contrast medium may not be achievable in abdominal CT even with fast MDCT in either adult or pediatric patients [24]. This is because contrast enhancement in CT angiography is highly dependent on the delivery rate of contrast medium, whereas contrast enhancement in the hepatic parenchyma is mainly affected by the amount of total iodine mass administered (i.e., total contrast medium volume times concentration) [3, 11, 14, 18, 20, 21, 29, 34–39]. Body weight (more precisely, g I/kg of body weight) is the most important patient-related factor affecting the magnitude of hepatic enhancement [3, 14, 19]. The magnitude of hepatic parenchymal enhancement decreases linearly with increasing patient weight. Therefore, when imaging large patients, the total iodine load should be increased to achieve a constant degree of hepatic enhancement. The iodine load can be increased by increasing the contrast medium concentration, volume, or injection rate [16, 29, 38].

In this study, contrast medium was administered at a fixed injection rate of 2 mL/s. We excluded CT scans acquired with different injection rates to reduce the effect of injection rate on contrast enhancement. Although hepatic parenchymal enhancement increases mildly with an increase in injection rate, it is apparent only at relatively low injection rates [3, 12, 38]. Fast injections are known to be useful in multiphase hepatic imaging and in detection of hypervascular liver masses [3, 12, 39–42]; however, this is not so in routine abdominal CT. An injection rate of 2 mL/s is commonly used for routine abdominal imaging for children and adult patients.

The timing of the onset of scanning or scanning delay is a crucial factor for optimal contrast enhancement. It is particularly critical for CT angiography when the CT is conducted during the first pass of contrast medium and for fast MDCT, for which the CT time window is very short. Fast CT, such as 64-MDCT, allows us to scan the abdomen within a few seconds. As a result, scan timing becomes far more critical and challenging than with older and slower CT scanners [26, 43–45]. We used a fixed scanning delay of 60 seconds. Although a 60-second delay from the start of the injection was advocated for pediatric abdominal CT by some investigators [46], it may not be optimal for small children who receive small amounts of contrast media injected with short injection durations.

Recently, Frush et al. [27] recommended an empiric scanning delay of 20 seconds after the completion of power injection of contrast media for abdominal MDCT imaging in children between 18 months and 12 years. This scheme of a 20-second postinjection delay is equivalent to a scanning delay of 60 seconds from the start of the injection only when the injection duration is 40 seconds (for example, injection of 80 mL at 2 mL/s). For injections of smaller volumes of contrast media for which the injection durations are shorter than 40 seconds, 20-second postinjection delays will result in scanning delays shorter than 60 seconds from the start of the injection.

Although we used a fixed 60-second delay from the start of the injection, a 20-second postinjection delay seems a reasonable approach, particularly in smaller children who require less contrast media and have shorter portal venous circulation of the injected contrast media and thus shorter scanning delays. For hepatic imaging with MDCT, scanning too early likely results in inadequate hepatic enhancement or arterial phase enhancement. Although bolus-tracking technology is available to individualize the scanning onset [47], it may not be practical for routine abdominal CT. This was not specifically investigated in the current study.

There are limitations to this study. First, the study was retrospective and not specifically designed to test the clinical impact or relevance of the different degrees of contrast enhancement on the sensitivity and specificity for diagnosing aortic or hepatic abnormality. No follow-up or further clinical evaluation was performed to investigate the extent to which the image (diagnostic quality) was associated with the degree of contrast enhancement. Second, the amount of contrast medium adjusted for body weight was neither rigorously controlled nor uniformly distributed in this retrospective study. The maximum allowed volume of contrast medium was 80 mL. This volume cap, which constituted the majority of contrast medium volumes used in the study, skewed the volume distribution and restricted the range of contrast media volume comparison. Even for the patients who received less than 80 mL of contrast medium, there was a considerable variation in the amount of administered contrast medium. The exact causes of this variation are not available in this retrospective study. We postulate that the variation is related to the relative newness of the 64-MDCT system to our pediatric radiology department, and technologists were still learning and becoming familiar with the new system and protocols. Third, although children at 3 years or younger are an important group of patients in a pediatric practice, they were excluded from our study because contrast media in this group were administered manually or at injection rates other than 2 mL/s. Fourth, we used a fixed scanning delay without accounting for individual patient variations in circulation. A difference in circulation likely affects the magnitude of aortic and hepatic contrast enhancement. The bolus-tracking technique, however, is not routinely used for abdominal CT in pediatric patients in our institution. Finally, the findings of our study are based on our study protocol. The study results may vary depending on the types of CT scanners, scanning parameters, and contrast medium administration protocols.

In conclusion, to achieve consistent aortic or hepatic contrast enhancement in pediatric patients with abdominal 64-MDCT, the amount of contrast medium should be adjusted to the patient's body weight for all ages of pediatric patients: approximately 1.5 mL/kg, or 0.525 g I/kg, to yield 116 HU of hepatic attenuation or 50–55 HU of hepatic enhancement.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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