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Technical Innovation |
1
Department of Radiology, Division of Pediatric Radiology, 1905
McGovern-Davison Children's Health Center, Duke University Medical Center,
Erwin Rd., Durham, NC 27710.
2
Department of Radiology, Children's Hospital and Medical Center, 3333 Burnet
Ave., Cincinnati, OH 45229.
Received August 31, 2000;
accepted after revision December 5, 2000.
Address correspondence to D. P. Frush.
Introduction
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TopIntroductionMaterials and MethodsResultsDiscussionReferences |
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All children received low-osmolar contrast material (Isovue 300 [iopamidol]; Bracco Diagnostics, Princeton, NJ). Contrast material was administered by power injector (Percupump; E-Z-EM, Westbury, NY) at 2.0 mL/sec using either a 20- or 22-gauge angiocatheter placed in an antecubital vein. During our initial scanning experience (group A), a 10-sec scan delay was used. This 10-sec delay was selected on the basis of a recommended scan delay range of 3-12 sec when using a power injector (rate in milliliters per second) for IV contrast material in pediatric helical CT [4]. The contrast dose was either 2.0 or 2.5 mL/kg. Based on suboptimal contrast timing with this initial protocol, we subsequently modified the scan delay to 20 sec in a larger group of children (group B) in whom the contrast dose was 2.0 mL/kg. In group A, there were seven children who underwent seven examinations (mean age, 6.2 years) (2.0 mL/kg: n = 4; 2.5 mg/kg: n = 3). In group B, there were a total of 10 multislice helical CT scans in 10 children (mean age, 5.5 years). One of these children was also included in group A because an initial scan using a 10-sec delay and a subsequent scan with a 20-sec delay were performed (Fig. 1A,1B).
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For the evaluation of hepatic enhancement, CT examinations were analyzed using a method previously described in greater detail [2]. Using an institutionally developed PACS (picture archiving and communication system), regions of interest of 1.0 cm were positioned in three peripheral locations in the liver at three levels. The three regions of interest at each level were combined into a single value of hepatic attenuation for that level. To calculate hepatic enhancement, a single value of unenhanced liver attenuation was subtracted from the calculated attenuation at each level. Because we do not routinely obtain unenhanced examinations of the liver in children, we determined an unenhanced liver attenuation in a group of 20 age-matched children who had undergone unenhanced chest CT. The mean unenhanced attenuation value for the upper liver was 63.6 H (range, 44.8-79.4 H). This value was similar to that obtained (67.6 H) in an unrelated group of children in another investigation [2] (Frush DP, personal communication).
For the qualitative evaluation of groups A and B, two pediatric radiologists with expertise in body CT interpreted the examinations in a random, blinded consensus fashion. Evaluation was focused on assessing the phase of IV contrast enhancement [2]. Splenic enhancement was graded as early (heterogeneous enhancement) or optimal (no heterogenity). Renal enhancement was graded as early (cortical), optimal (nephrographic), or late (excretory). Hepatic enhancement was graded as early (arterial), optimal (portal venous), or late (equilibrium) (Fig. 1A,1B).
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TopIntroductionMaterials and MethodsResultsDiscussionReferences |
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Qualitative assessment of the phase of organ enhancement supported the actual Hounsfield unit difference seen in the liver. Comparing group A with group B revealed the following. Early enhancement of the kidney was present in 71% (5/7) of group A examinations versus 30% (3/10) in group B. Early splenic enhancement was present in 57% (4/7) of group A examinations versus 40% (4/10) of group B. The remainder of children in both groups had homogeneous splenic and nephrographic renal enhancement. All studies in both groups were qualitatively assessed to be in the portal venous phase (Fig. 1A,1B).
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TopIntroductionMaterials and MethodsResultsDiscussionReferences |
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Recently, a summary of pediatric scan delays was presented [2]. Most investigators used contrast administration rates based on a selected milliliter-per-second basis with scanning onset following the completion of contrast material injection [3]. The actual length of this delay, however, is not standard. Recommendations for the scan delay have differed by a factor of 10, ranging from 3 to about 30 sec [2, 4, 5]. Recommended delays using power injectors range from 3 to 12 sec [4, 5].
Our experience with single row detector helical scanning has been that a delay of 20-30 sec is optimal, but these data are based on manual injection [2]. Because we have been using power injection more frequently, in conjunction with multislice scanning, we elected to use the shorter scan delay recommended for power injectors [4, 5]. However, it was clear from our initial experience that this 10-sec delay resulted in scanning too early, with suboptimal splenic and renal enhancement [2]. We could not justify using this delay on the basis of our qualitative and subsequent quantitative comparisons.
The reason for this discrepancy between suggested delays of 3-12 sec [4] and our results indicating improved organ enhancement with a longer 20-sec delay is not entirely clear but may be due to slight differences in injection rates. In the work by Ruess et al. [4], the rates of injection were slightly slower (1.0-1.5 mL/sec) and the time to peak enhancement may be closer to the completion of injection than with faster rates [8]. However, the increased hepatic enhancement we found with an empiric delay of 20 versus 10 sec is in agreement with previous work in which rates of injection ranged from 0.3 to 3.5 mL/sec [2]. The importance of optimal timing of the contrast bolus is made more critical by the rapid scanning speed with multislice CT. Scanning too early in hepatic enhancement with faster multislice CT means all scanning could be completed before optimal enhancement.
This study contains several limitations and considerations. The population size is small; however, it was quickly obvious that a 10-sec delay was too short to have a larger sample size for this group. In addition, the scanning protocol used only one table speed. Although a slower table speed could result in scanning during a time when enhancement is beginning to decline, multislice technology allows faster table speeds. Faster table speeds (at a fixed scan delay) would not change hepatic enhancement values because we did not notice any difference in values of hepatic enhancement from the superior to inferior aspect of the liver at our protocol table speed. The present investigation also dealt with only one rate of IV contrast material administration. As we have discussed, rates different than 2.0 mL/sec could require modifications of empiric scan delays. In addition, differences in physiologic parameters, such as cardiac output or state of hydration, between populations of children may affect the timing of abdominal organ enhancement [3]. In these situations, bolus-tracking technology can individualize scanning, obviating empiric delays [2].
In conclusion, an empiric scan delay of 20 sec after the completion of power-injected contrast material (rate, 2.0 mL/sec; dose, 2.0 mL/kg) provides excellent hepatic enhancement for multislice helical CT in children between 18 months and 12 years old.
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