AJR 2001; 176:1559-1561
© American Roentgen Ray Society
Effect of Scan Delay on Hepatic Enhancement for Pediatric Abdominal Multislice Helical CT
Donald P. Frush1,
Lane F. Donnelly2 and
George S. Bisset, III1
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
Helical CT has changed the way contrast-enhanced CT is performed in adults
and children [1]. The
increasing speed of multislice CT has created new challenges for optimizing
contrast-enhanced scanning in infants and children
[2,
3]. It is unclear which of
several reported scan delays, ranging from 3 to 29 sec
[1,2,3,4,5],
are appropriate for hepatic multislice scanning. Our objective was to
determine an appropriate scan delay—the period between completion of
contrast administration and scanning onset—in children for multislice
helical CT.
Materials and Methods
Our study period was between October 1998 and September 1999. We included
children between 18 months and 12 years old in the study to represent a large
variation in size. Children younger than 18 months old were excluded because
IV contrast material was usually injected manually. Children older than 12
were excluded because adult scanning techniques could be applicable. All
imaging was performed on a multislice helical CT scanner (LightSpeed QXi;
General Electric Medical Systems, Milwaukee, WI) with a 0.8-sec gantry
rotation duration. The following scanning parameters were constant for every
CT examination: 140 kVp, 80 mA, 3.75-mm detector configuration, 3.75-mm
reconstruction interval with a table speed of 11.25 mm per gantry rotation
(pitch of 3:1). All studies were filmed using a window of 340-360 H and level
of 40-60 H on a 12:1 format.
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).
View larger version (117K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 1A. — Effect of scan delay on hepatic enhancement shown on
multislice helical CT in 4-year-old boy during treatment for rhabdomyosarcoma.
IV contrast material dose was 2.0 mL/kg. Axial contrast-enhanced CT scan of
upper abdomen after 10-sec scan delay shows relatively poor enhancement of
hepatic veins (long straight arrows), early (cortical) enhancement of
left kidney (short straight arrows), and early splenic enhancement
with heterogeneity (curved arrow). Despite slightly early hepatic
enhancement, study was still assessed to be predominantly in portal venous
phase.
|
|
View larger version (115K):
[in this window]
[in a new window]
[as a PowerPoint slide]
|
Fig. 1B. — Effect of scan delay on hepatic enhancement shown on
multislice helical CT in 4-year-old boy during treatment for rhabdomyosarcoma.
IV contrast material dose was 2.0 mL/kg. Axial contrast-enhanced helical CT
scan obtained 4 months after A (at slightly higher level) using
identical scan parameters except for 20-sec delay shows good enhancement of
hepatic veins (arrows), optimal (nephrographic) phase of renal
enhancement, and no splenic enhancement artifacts.
|
|
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).
Results
For comparison of hepatic enhancement, only the four children in group A
who received 2.0 mL/kg of contrast were compared with the 10 children with an
identical dose in group B. In each of these 14 children, there was no
significant difference in liver enhancement among the three levels. For this
reason, the three levels were combined, yielding a single mean enhancement
value for the entire liver for each CT. The mean hepatic enhancement was
higher in the group with a 20-sec delay (47.9 H) than in the group with a
10-sec delay (38.8 H). The small number of patients in group A precluded
statistical analysis. The enhancement values for group A are at the lower end
of the acceptable diagnostic range, whereas those values in group B are in
agreement with other investigations of helical CT and hepatic enhancement in
children
[2,3,4,5,6].
No complications related to the power injection of IV contrast material
occurred.
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).
Discussion
One substantial difference between adult helical CT scanning protocols
[1,
7] and those in children is the
scan delay. In adults, routine delays from the onset of contrast
administration range from 50 to 80 sec using a set rate of administration with
a power injector [1,
7]. No such standard delays are
possible in pediatric scanning, primarily because of the difference in patient
size and the resulting difference in the time needed to inject the variable
volume of contrast material. Therefore, most IV contrast—enhanced
helical CT in children is based on a delay from the completion of the contrast
injection [2].
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.
References
-
Zeman RK, Baron RL, Jeffery RB Jr, Klein J, Siegel MJ, Silverman
PM. Helical body CT: evolution of scanning protocols.
AJR
1998;170:1427
-1438[Free Full Text]
-
Frush DP, Spencer EB, Donnelly LF, Zheng JY, DeLong DM, Bisset GS
III. Optimizing contrast-enhanced abdominal CT in infants and children using
bolus tracking. AJR
1999;172:1007
-1013[Abstract/Free Full Text]
-
Frush DP, Donnelly LF. Helical CT: scanning in
children—technical considerations and body applications.
Radiology
1998;209:37
-48[Free Full Text]
-
Ruess L, Bulas DI, Kushner DC, Silverman PM, Fearon TC. Peak
enhancement of the liver in children using power injection and helical CT.
AJR
1998;170:677
-681[Abstract/Free Full Text]
-
Luker GD, Siegel MJ, Bradley DA, Baty JD. Hepatic spiral CT in
children: scan delay time-enhanced analysis. Pediatr
Radiol 1996;26:337
-340[Medline]
-
Kuhns LR. Optimal timing of abdominal CT in children: relationship
to injection rate. Radiology
1993;189:49
-51[Abstract/Free Full Text]
-
Silverman PM, Kohan L, Ducic I, et al. Imaging of the liver with
helical CT: a survey of scanning techniques. AJR
1998;170:149
-152[Abstract/Free Full Text]
-
Tello R, Seltzer SE, Polger M, Spaulding S, Savci G. A contrast
agent delivery nomogram for hepatic spiral CT. J Comput Assist
Tomogr 1997;21:236
-245[Medline]
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
Top
Introduction
Materials and Methods
