DOI:10.2214/AJR.05.0714
AJR 2006; 187:1074-1078
© American Roentgen Ray Society
MDCT Angiography of the Pulmonary Arteries: Influence of Body Weight, Body Mass Index, and Scan Length on Arterial Enhancement at Different Iodine Flow Rates
Helmut Schoellnast1,
Hannes A. Deutschmann1,
Andrea Berghold2,
Gerald A. Fritz1,
Gottfried J. Schaffler1 and
Manfred Tillich3
1 Department of Radiology, Medical University Graz, 8036 Graz, Austria.
2 Institute for Medical Informatics, Statistics and Documentation, Medical
University Graz, 8036 Graz, Austria.
3 Diagnostikum Graz Sued West, Weblinger Guertel 25, 8054 Graz, Austria.
Received April 27, 2005;
accepted after revision July 25, 2005.
Address correspondence to M. Tillich
(manfred.tillich{at}diagnostikum-graz.at).
Abstract
OBJECTIVE. The purpose of this study was to assess whether body
weight, body mass index, and scan length influence arterial enhancement during
CT angiography (CTA) of the pulmonary arteries at different iodine flow
rates.
MATERIALS AND METHODS. CTA examinations of the pulmonary arteries
performed for routine clinical care of 120 patients between March and December
2003 were retrospectively evaluated. Patients had received either 120 mL of
contrast medium with an iodine concentration of 300 mg I/mL (group A) or 90 mL
of contrast medium with an iodine concentration of 400 mg I/mL (group B). The
iodine dose was 36 g, and the injection rate was 4 mL/s in all examinations.
The iodine flow rate was 1.2 g I/s in group A and 1.6 g I/s in group B.
Arterial attenuation along the z-axis was measured per patient, and
the influence of body weight, body mass index, and scan length on enhancement
of the pulmonary arteries in the two groups was assessed.
RESULTS. In group A and in group B, body weight and body mass index
correlated significantly with mean enhancement along the z-axis
(r = -0.35 and -0.26 for group A and -0.48 and -0.40 for group B).
Scan length showed no correlation with pulmonary attenuation. Mean pulmonary
artery enhancement was significantly higher in group B with a difference of 51
H compared with group A.
CONCLUSION. Pulmonary artery attenuation in CTA of the pulmonary
arteries shows a small but significant correlation with body weight and body
mass index independently of the iodine flow rate used. A higher iodine flow
rate improves pulmonary artery enhancement.
Keywords: arteries • body mass index • body weight • CT angiography • IV contrast agents • thorax
Introduction
Computed tomography angiography (CTA) has become the first-line imaging
technique in daily clinical routine for patients with suspected pulmonary
embolism. The most important advantages of CT over other imaging techniques
are that both mediastinal and parenchymal structures can be evaluated and that
the thrombus can be directly visualized. The conspicuity of the thrombus
depends on the density difference between the thrombus and the
contrast-enhanced artery. The degree of arterial enhancement depends on
injection-related factors and on patient-related factors. Both have been
evaluated for CTA of the aorta, and models have been introduced to overcome
variability between individuals
[1-6].
Few data, however, are available on CTA of the pulmonary arteries. To the best
of our knowledge, no data are available on the influence of body weight, body
mass index, and scan length on enhancement of the pulmonary arteries. The aim
of our study was to assess whether correlation exists between each of these
parameters and pulmonary artery enhancement and whether iodine flow rate
influences the correlation. A further aim was to retrospectively estimate
contrast dose per unit of body weight, a value necessary for reaching an
attenuation threshold of 300 H.
Materials and Methods
Patient Population
In our department, contrast media with different iodine concentrations were
used in clinical routine. The agent used was alternated daily by the
radiologic technician. A retrospective database search yielded 224 patients
who underwent CTA of the pulmonary arteries on the same CT scanner from March
to December 2003. Among these, all patients who underwent CTA of the pulmonary
arteries with contrast medium with an iodine concentration of 300 or 400 mg
I/mL were included for further analysis (199 patients). The CTA images of
these patients were consecutively loaded on a workstation and reviewed by two
experienced radiologists in consensus with respect to evidence of central
pulmonary embolism and cardiac insufficiency. Because coronal and lateral
scout views were available for each patient, the same criteria for evaluation
of cardiac dilatation and pulmonary congestion were used as in interpretation
of radiographs of the chest. Evidence of cardiac insufficiency was defined as
enlargement of the heart in the coronal or lateral scout view or both combined
with dilatation of the pulmonary vessels.
The first 60 patients who underwent CTA of the pulmonary arteries with
contrast medium with an iodine concentration of 300 mg I/mL (iopromide 300,
Ultravist 300, Schering) and who had no radiologic evidence of central
pulmonary embolism or cardiac insufficiency (group A) and the first 60
patients who underwent CTA of the pulmonary arteries with contrast medium with
an iodine concentration of 400 mg I/mL (iomeprol 400, Imeron 400, Bracco) and
who had no radiologic evidence of central pulmonary embolism or cardiac
insufficiency (group B) were included in the final study group (120 patients).
Twenty-four patients were excluded because of evidence of cardiac
insufficiency. Demographics of the final study group are shown in
Table 1. All patients were
referred for CTA because of suspicion of embolism. According to the guidelines
for retrospective studies of our institutional review board, formal approval
and informed consent were not obtained.
Scan Protocol
Patients in group A received a volume of 120 mL of contrast medium, and
patients in group B received 90 mL. The total iodine dose was 36 g, and the
flow rate was 4 mL/s in both groups. The groups differed only in iodine flow
rate, which was 1.2 mg I/s in group A and 1.6 mg I/s in group B. In all
examinations, scan delay was estimated with a semiautomatic bolus tracking
system (SmartPrep, GE Healthcare). The region of interest (ROI) for bolus
tracking was set in the right atrium. Mean scan delay was 14.5 ± 2.8
seconds for group A and 15.1 ± 2.3 seconds for group B. A cubital or
antebrachial vein was punctured with an 18- or 20-gauge needle for venous
access. Before contrast medium was administered, saline injections were
manually administered with the patient's arm in scanning position to ensure
successful cannulation of the vein. After the contrast medium bolus was
injected, a saline flush of 40 mL was administered with a double-syringe power
injector (Missouri CT-Injector XD 2001, Ulrich). Scanning was performed with a
4-MDCT scanner (LightSpeed QXi, GE Healthcare). The scans were obtained with a
detector configuration of 1.25 mm, a pitch of 1.5 (table feed, 7.5 mm per
rotation; rotation time, 0.8 seconds), a reconstruction increment of 0.8 mm,
and a section thickness of 1.25 mm. All scans were performed in a caudal to
cranial direction from the dome of the diaphragm to the lung apex. Mean
scanning time was 19.3 ± 2.6 seconds for group A and 20.7 ± 3.0
seconds for group B. Mean scan length was 17.1 ± 2.4 cm for group A and
18.4 ± 2.6 cm for group B. An X-ray tube voltage of 120 kV and a
current of 230-250 mA were used in all examinations.
Data Acquisition
All CT scans were loaded on a workstation (MagicView, Siemens Medical
Solutions). Quantitative analysis was performed by ROI measurements along the
z-axis. Attenuation measurements were performed along the following
arteries: subsegmental and segmental arteries of the lower lobes, lower lobe
arteries, middle lobe artery, main pulmonary arteries, upper lobe arteries,
and segmental and subsegmental arteries of the upper lobes. When more
measurements were obtained per table position (e.g., subsegmental and
segmental level in the upper and lower lobes) the measurements were averaged.
The distance between measurements was 1.2 cm. An attempt was made to maintain
an ROI including nearly the entire vessel diameter and to localize the ROI in
areas without artifacts. Mean attenuation was recorded for each measurement.
All measurements were performed by one observer.
Data Analysis and Statistical Analysis
Mean pulmonary arterial attenuation was calculated per patient; the
attenuation values obtained were averaged along the z-axis. Mean
pulmonary arterial attenuation was averaged for both groups. Data on body
weight, body mass index, scan length, and mean pulmonary arterial enhancement
are provided as mean ± SD. The significance of differences in body
weight, body mass index, scan length, and pulmonary arterial attenuation
between both groups was tested with Student's t test for unpaired
samples. Mean pulmonary arterial attenuation was correlated with body weight,
body mass index, and scan length. Pearson's product moment correlation
coefficient (r) was used. For all statistical tests, two-sided
p < 0.05 was considered statistically significant. The
administered iodine dose per kilogram of body weight was calculated for both
groups for extrapolation of the dose necessary to reach an attenuation
threshold of 300 H. The software package SPSS version 11.0.5 was used for all
calculations.
Results
There was no statistically significant difference between the groups in
body weight, body length, body mass index, or scan length (p >
0.05, Table 1). Mean pulmonary
arterial attenuation was 274 ± 58 H in group A and 325 ± 87 H in
group B. The difference of 51 H was statistically significant (p <
0.001). The SD for attenuation was higher in group B than in group A. Within
group B, the SD was higher in the upper third of the scan length than in the
lower third (Table 2). In group
A and in group B, body weight correlated significantly with mean pulmonary
artery enhancement (Figs. 1A
and 1B). For group A, Pearson's
correlation coefficient was -0.35 (p < 0.01); for group B,
Pearson's correlation coefficient was -0.48 (p < 0.001). Body mass
index also showed significant correlation with mean attenuation in the two
groups. Pearson's correlation coefficient was -0.26 (p < 0.05) in
group A and -0.40 (p < 0.01) in group B (Figs.
2A and
2B). Scan length showed no
significant correlation with arterial enhancement in group A or in group B.
Pearson's correlation coefficient was 0.24 (p = 0.07) in group A and
-0.06 (p = 0.65) in group B. As shown in Figures
1B and
2B, one patient (body weight,
122 kg; body mass index, 49) in group B had a high deviation in body weight
and body mass index related to the mean value for his group. Excluding this
patient, Pearson's correlation coefficient was -0.44 (p < 0.01)
for body weight and -0.33 (p < 0.05) for body mass index.
Mean administered iodine dose per kilogram of body weight was 0.50 ±
0.11 g I/kg in group A and in group B. In group B, the trend line of the
iodine dose-attenuation curve showed a steeper course than in group A. The
threshold of 300 H was reached at an iodine dose of 0.44 g I/kg body weight in
group B, whereas in group A an iodine dose of 0.68 g I/kg body weight was
necessary (Figs. 3A and
3B).
Discussion
In CTA, the degree of arterial enhancement depends on injection-related
factors such as iodine flow rate and on patient-related factors such as
cardiac output and body weight
[7]. The magnitude of arterial
enhancement is proportional to the iodine flow rate, which can be altered by
changing the injection rate or the concentration of contrast medium. The
higher the iodine flow rate, the faster and higher peak arterial enhancement
occurs [7]. Decreased cardiac
output delays and increases peak arterial enhancement magnitude as a result of
decreased mixing and dilution of the contrast medium. Several strategies have
been introduced to overcome the variability of arterial enhancement due to
differences in cardiac output among patients. Use of a test bolus and use of
bolus-tracking systems have been assessed for adaptation of scan delay to
cardiac output [4,
8,
9]. In our study, a
semiautomatic bolus-tracking system was used. Another important
patient-related factor that influences arterial enhancement is body weight.
High body weight has been found to decrease the magnitude of arterial
enhancement [1,
10], but these factors have
been assessed in CTA of the abdominal aorta. To our knowledge, no data are
available on CTA of the pulmonary arteries.
Our study showed a small but significant negative correlation between body
weight and pulmonary arterial enhancement regardless of the iodine flow rate
used. In addition, body mass index showed a small negative correlation with
mean pulmonary artery enhancement for both iodine flow rates. The correlation
between body mass index and pulmonary artery enhancement was smaller than the
correlation between body weight and pulmonary artery enhancement. These
results are in agreement with the findings of other studies
[1,
10]. Bae et al.
[1] reported that after IV
administration, contrast medium distributes rapidly from the central blood
compartment to the well-perfused extracellular compartment and slowly to the
poorly perfused extracellular compartment. Awai et al.
[10] found that in CTA of the
aorta the poorly perfused extracellular compartment can usually be ignored
because of early data acquisition. One can assume that this finding is
especially valid in CTA of the pulmonary arteries. Consequently, the central
blood compartment, that is, the pulmonary arteries themselves including the
capillary volume, may be the only relevant compartment influencing
distribution of contrast medium. This compartment seems to correlate better
with body weight than with body mass index.
In group B, the SD was higher in the upper third of the scan length than in
the lower third. We assume variabilities in the bolus trigger system caused
this phenomenon. A delayed start of data acquisition showed more effect on
arterial enhancement in the upper third of the scan length in group B than in
group A because of the lower contrast volume and the lower injection time.
The results of this study confirmed the influence of iodine flow rate on
vascular attenuation. Using an iodine flow rate of 1.6 g I/s led to
significantly higher attenuation of the pulmonary arteries than using an
iodine flow rate of 1.2 g I/s (325 vs 274 H). Mean pulmonary artery
enhancement of 300 H, which is suggested for adequate visualization of the
pulmonary arteries [11], was
achieved in terms of an iodine dose of 0.68 g I/kg with an iodine flow rate of
1.2 g I/s. With an iodine flow rate of 1.6 g I/s, the required iodine dose was
0.44 g I/kg. Consequently, an iodine dose of 0.24 g I/kg can be saved when the
iodine flow rate is increased 0.4 g I/s.
Two factors have to be considered when contrast media are tailored to
patient weight. As shown in Table
3, to reach 300 H when an iodine flow rate of 1.2 g I/s is used
(group A), a high volume of contrast medium would be necessary in heavy
patients. Use of this volume would lead to an injection time longer than the
scanning time, which would not make sense. On the other hand, with an iodine
flow rate of 1.6 g I/s (group B), the injection time would be very short in
small patients. It is generally agreed that in pulmonary CTA the injection
time should not be significantly lower than the scanning time to ensure
constant arterial attenuation. Consequently, scanner technology may be the
limiting factor in reduction of iodine dose in small patients when high iodine
flow rates are used because the injection time would be too short for adequate
enhancement over the time needed for 4-MDCT, as was used in our study, to
acquire a complete CTA study. In our study population, mean scan delay was 15
seconds, and mean scanning time was 20 seconds. Consequently, scanning was
finished 35 seconds after initiation of the injection of contrast medium. When
a high iodine flow rate is used (e.g., group B of our study population) and
the contrast medium is tailored to patient weight, injection duration ranges
from 14 seconds for a 50-kg patient to 28 seconds for a 100-kg patient
(Table 3). For small patients,
the injection time would be too short for scanning time because of the small
volume of contrast medium needed. However, this premise is valid only when a
scan protocol similar to the present one is used. These limitations may be
overcome when CTA of the pulmonary arteries is performed with 16-MDCT or
64-MDCT, because these scanners are capable of imaging the entire pulmonary
arterial bed in less than 10 seconds. Therefore, significant reduction of
contrast medium dose may be reached even in small patients when
high-concentration contrast medium is used in combination with 16-MDCT and
higher scanners.
View this table:
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TABLE 3: Calculated Contrast Medium Volume to Reach Pulmonary Arterial
Attenuation of 300 H and Consecutive Injection Time for Patients with
Different Body Weights
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Limitations to our study have to be acknowledged. First, the study was a
retrospective data analysis of routinely performed CT angiographic
examinations of the pulmonary arteries, and data on the iodine dose necessary
to achieve an attenuation threshold of 300 H were retrospectively
extrapolated. Prospective studies are necessary to confirm the data. Second,
no sufficient clinical data were available about the cardiac status of the
patients. However, patients with radiologic evidence of cardiac insufficiency
on CT were excluded. Scan delay was estimated with a semiautomatic tracking
system to minimize differences in arterial enhancement due to differences in
cardiac status. In addition, there were no significant differences in patient
age and body weight or body mass index, and thus the study population was
homogeneous. Third, the viscosity of high-concentration contrast media is
greater than that of conventional contrast media (aside from the iodine load)
(12.6 mPa · second at 37°C for iomeprol 400 vs 4.6 mPa ·
second at 37°C for iopromide 300). Therefore differences in pulmonary
artery enhancement between groups A and B may have been due to differences in
viscosity rather than differences in iodine flow rate. However, it can be
assumed that viscosity is not as important in the contrast dynamics of larger
vessels as it is for the lower-order pulmonary arteries measured in this
study.
In conclusion, pulmonary artery attenuation in CTA of the pulmonary
arteries shows a small but significant correlation with body weight and body
mass index, there being a higher correlation with body weight, independent of
the iodine flow rate used. A higher iodine flow rate improves pulmonary artery
enhancement. Therefore adaption of contrast medium volume to body weight may
help to optimize contrast enhancement. A significant reduction in contrast
medium dose may be achieved with use of high-concentration contrast media in
combination with CT scanners that allow rapid data acquisition. Prospective
studies are needed to confirm the results obtained in this retrospective
analysis.
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Abstract
Introduction
