DOI:10.2214/AJR.05.0612
AJR 2006; 187:638-644
© American Roentgen Ray Society
Cardiac 16-MDCT for Anatomic and Functional Analysis: Assessment of a Biphasic Contrast Injection Protocol
Daisuke Utsunomiya1,2,
Kazuo Awai2,
Takashi Sakamoto1,
Taiji Nishiharu1,
Joji Urata1,
Akira Taniguchi3,
Takeshi Nakaura2 and
Yasuyuki Yamashita2
1 Diagnostic Imaging Center, Saiseikai Kumamoto Hospital, 5-3-1 Chikami,
Kumamoto-shi, Kumamoto 861-4193, Japan.
2 Department of Diagnostic Radiology, Graduate School of Medical Sciences,
Kumamoto University, Kumamoto, Japan.
3 CT Systems Division, Toshiba Medical Systems, Tokyo, Japan.
Received April 8, 2005;
accepted after revision August 9, 2005.
Address correspondence to D. Utsunomiya
(d-utsunomiya{at}skh.saiseikai.or.jp).
Abstract
OBJECTIVE. The purpose of this study was to determine the optimal
contrast injection protocol for clear delineation of the endocardial and
epicardial contours and coronary vessels in anatomic and functional imaging
with cardiac 16-MDCT.
SUBJECTS AND METHODS. Thirty-eight patients were allocated to three
groups according to contrast injection protocol: a long-duration biphasic
protocol in which diluted contrast material was used in the latter phase
(protocol A, 13 patients); a uniphasic protocol with saline flush (protocol B,
12 patients); a uniphasic protocol without a flush (protocol C, 13 patients).
Six regions of interest were drawn within the left ventricle (LV), right
ventricle (RV), and interventricular septum along the z-axis. Mean
ventricular attenuation, mean difference between maximum and minimum
ventricular attenuation, and ventricular-myocardial contrast-to-noise ratio
(CNR) were calculated. Attenuation and visualization of the coronary vessels
also were compared.
RESULTS. The difference between maximum and minimum RV attenuation
was significantly smaller in group A (58.1 H) than in groups B (179.5 H) and C
(157.0 H). RV-myocardial CNR was significantly higher in group A (9.0) than in
group B (5.5). The mean LV attenuation, difference between maximum and minimum
LV attenuation, and LV-myocardial CNR were not significantly different among
three groups. In protocol A, both endocardial and epicardial contours were
clearly delineated, and cardiac functional analysis was feasible in all cases.
Average attenuation and visualization of the coronary vessels were not
significantly different among groups. The diagnostic accuracies in detection
of coronary stenosis were 92%, 93%, and 91%, respectively, for protocols A, B,
and C.
CONCLUSION. The long-duration contrast injection protocol with
diluted contrast material is optimal for assessing the coronary vessels and
cardiac function.
Keywords: cardiac imaging • contrast media • MDCT
Introduction
The combination of noninvasive coronary artery imaging and assessment of
cardiac function in a single scan with retrospective ECG-gated MDCT is an
interesting approach to a comprehensive cardiac workup. In cardiac functional
analysis, both left ventricular (LV) ejection fraction and myocardial wall
thickening analysis are clinically important
[1,
2]. Thus the optimal contrast
injection protocol for maintaining homogeneous and adequate enhancement both
in the ventricular cavities and in the coronary vessels should be determined.
Adequate contrast enhancement in the coronary vessels and abatement of
beam-hardening artifacts arising from the presence of dense contrast medium
within the right ventricle (RV) are essential for visualization of the
coronary vessels [3]. On the
other hand, clear delineation of the endocardial and epicardial contours is
essential for assessment of the myocardial wall
[4-6].
A certain amount of contrast medium, however, is needed for visualization of
both the LV and RV cavities, and an increase in contrast medium dose is
undesirable because of the risk of contrast medium-induced nephrotoxicity and
beam-hardening artifacts arising from the presence of dense contrast medium
within the RV [3,
7,
8]. It therefore may be
necessary to modify the traditional uniphasic injection protocol for cardiac
CT. We hypothesize, on the basis of the results of our preliminary study, that
a long-duration biphasic protocol that involves the use of contrast medium
with standard and diluted iodine concentrations may improve enhancement in the
LV and RV cavities during CT without adversely affecting coronary artery
visualization. The objective of this study was to compare a long-duration
biphasic injection protocol in which diluted contrast medium was used with
uniphasic injection protocols for cardiac anatomic and functional 16-MDCT.
Subjects and Methods
Before this clinical trial, a preliminary study with nine patients who had
no history of heart disease was performed to establish the time-density curve
profiles and scan timing for each protocol (Appendix 1). The ethics committee
of our hospital approved both the preliminary and the current study. We
explained the purpose of our study to all patients, and informed consent was
obtained from all patients.
Patients
A total of 45 patients were prospectively enrolled in the study. The
patients referred for cardiac CT presented with atypical chest pain
(n = 8), had undergone coronary intervention (n = 15), or
had findings suggestive of myocardial ischemia on myocardial perfusion SPECT
(n = 22). The exclusion criteria for cardiac CT were known
arrhythmia, renal insufficiency (serum creatinine concentration > 124
mmol/L), known pulmonary disease, and severe heart failure of grade III or
higher according to the New York Heart Association classification. Seven
patients were excluded from the protocol population. Two patients were
excluded because a tube voltage of 135 kVp rather than 120 kVp was used for
scanning. In the cases of the other five patients, image quality was
unacceptable because of irregular heart rate (n = 2), extravasation
of contrast medium (n = 2), or allergic reaction (n = 1).
Thus a total of 38 patients (30 men, eight women; mean age ± SD, 66.9
± 8.7 years; age range, 49-89 years) were included in the study, and
13, 12, and 13 patients were assigned to protocols A, B, and C, respectively,
by use of a random number table. The mean ages of the patients in protocols A,
B, and C were 68.6 ± 8.4 years, 63.9 ± 8.9 years, and 68.0
± 8.8 years, respectively. The mean weights of the patients in
protocols A, B, and C were 59.5 ± 7.0 kg, 62.1 ± 8.5 kg, and
64.3 ± 7.1 kg, respectively. There were no statistically significant
differences in patient age and weight among the three groups (p =
0.35 and 0.28, respectively, one-way analysis of variance).
On the basis of the results of the preliminary study (Appendix 1),
nonionic, low-osmolar iohexol at an iodine concentration of 350 mg I/mL
(Omnipaque 350; Daiichi Pharmaceutical) was injected through a 20-gauge IV
catheter placed in an antecubital vein. In protocol A (long-duration biphasic
protocol), injection of 60 mL of contrast medium (350 mg I/mL) was immediately
followed by injection of 80 mL of diluted contrast medium (175 mg I/mL). In
protocol B (uniphasic protocol with a saline flush), injection of 100 mL of
contrast medium (350 mg I/mL) was followed by a 40-mL saline flush. In
protocol C (uniphasic protocol without flush), 100 mL of contrast medium (350
mg I/mL) was injected without a subsequent saline flush
(Fig. 1). In all protocols, the
injection rate was 3.0 mL/s throughout the injection period. The total
contrast medium volume used in each protocol was 100 mL. Contrast medium was
administered with a dual-head power injector (Dual Shot Type-C,
Nemoto-Kyorindo) that made it possible to inject first contrast medium and
then contrast medium plus saline solution simultaneously. Attached to one side
of the injector was a syringe containing undiluted contrast medium. The other
side was connected to a syringe containing physiologic saline solution. A
Y-shaped tube connected the tips of the two syringes. In protocol A, undiluted
contrast medium was injected during the first phase; the injected contrast
medium was diluted by advancing the plungers of the two syringes at the same
rate during the second phase.
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Fig. 2 —Graph shows left ventricular-myocardial contrast-to-noise
ratio (CNR) profile along z-axis with each protocol. Each profile
shows constant level during CT. Circular regions of interest (ROIs) were
placed in left ventricular cavity and septal myocardial wall from most cranial
(ROI 1) to most caudal (ROI 6) position in each patient.
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Fig. 3 —Graph shows right ventricular-myocardial contrast-to-noise
ratio (CNR) profile along z-axis with each protocol. Profile obtained
with protocol A shows more constant level, whereas profiles obtained with
protocols B and C show decrease at caudal levels in right ventricle. Circular
regions of interest (ROIs) were placed in right ventricular cavity and septal
myocardial wall from most cranial (ROI 1) to most caudal (ROI 6) position in
each patient.
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CT Protocol
CT was performed with a 16-MDCT scanner (Aquilion-16 CFX Edition, Toshiba
Medical Systems). The following scanning protocol was used: 16 rows x
0.5-mm collimation, helical pitch of 0.25, rotation time of 400 milliseconds,
tube voltage of 120 kV, and tube current of 400 mA, with scanning performed in
the craniocaudal direction. Tube current modulation was not applied. The mean
effective dose was 15.3 mSv. ECG was recorded simultaneously with CT for each
patient. Based on anatomic landmarks on the CT scout radiograph, the scan
range was adjusted to include the volume from the tracheal bifurcation to the
base of the heart (= 120 mm). The inflow of contrast medium and the start of
data acquisition were synchronized with a real-time bolus tracking system
(Sure-Start, Toshiba). The trigger threshold was set at 150 H in a region of
interest placed in the midlevel LV cavity. The distance between the monitoring
level and the starting position was approximately 70 mm. Use of the 16-MDCT
scanner made it possible to announce the breath-hold and move the imaging
table to the starting position simultaneously in 4 seconds. Four seconds after
the trigger, CT was started. For image reconstruction, segmented adaptive
cardiac volume reconstruction based on a half-rotation reconstruction
technique was used. Twenty reconstruction sets at 5% intervals over the range
from 0% to 95% of the cardiac cycle were generated from each raw data file.
The sequence showing the least amount of motion was selected for coronary CT
angiography. These 20 data sets also were used for cardiac functional
analysis.
Data Analysis
Images were displayed on a computer monitor with a 1,028 x 1,024
matrix (TWS-5100, Toshiba). The observer who interpreted the images was
blinded to the protocol used. The attenuation values in six circular regions
of interest placed along the z-axis in the LV and RV cavities and
septal myocardial wall were measured at 8-mm intervals from the most cranial
to the most caudal position in each patient. An attempt was made to select a
region of interest of approximately 100 mm2; that is, not so small
as to be affected by pixel variability and not so large as to include the
papillary muscles. For comparison of the degree of contrast enhancement in the
ventricles, the mean attenuation values in the LV and RV cavities along the
z-axis were calculated. For comparison of the uniformity of the
contrast column, the difference between the maximum and minimum attenuation
values along the z-axis was calculated. For myocardial wall
thickening analysis, adequate LV-myocardial contrast and adequate
RV-myocardial contrast are required. Thus the contrast-to-noise ratios (CNRs)
between the ventricular cavity and the myocardium were compared for each
ventricle in assessment of ventricular-myocardial contrast in the three
groups. Ventricular-myocardial CNR was calculated according to the following
equation: CNR = (mean CTDV - mean CTDm) / SDaortic
root; where CTDV is the CT density in the ventricular cavity,
CTDm is the CT density in the myocardium, and SDaortic
root is the SD of the aortic root. The ventricular-myocardial CNR
profile along the z-axis was generated for each LV and RV cavity.
Wall thickening analysis was performed with Cardiac Analysis software
(Toshiba). Systolic wall thickening of the septal myocardium on MDCT was
compared with that on echocardiography. Echocardiography was performed with a
3.0-MHz transducer system (SONOS5500, Philips Medical Systems).
For assessment of coronary arterial enhancement, measurements of
attenuation in the left main coronary artery (LM) and the proximal and middle
segments of the left anterior descending (LAD), left circumflex (LCX), and
right coronary (RCA) arteries were obtained. Because it was difficult to
measure attenuation in the distal coronary arteries, delineation of the distal
coronary arteries was visually graded independently by two radiologists on the
following scale: 1 = excellent, 2 = acceptable, and 3 = poor. Cases scored 1
or 2 were considered assessable. Interobserver variability also was assessed.
Final visual evaluation results were based on consensus between the two
observers. Visual evaluation was performed with volume-rendered and curved
multiplanar reconstruction images in addition to the axial source images. In
cases in which quantitative coronary angiography (QCA) was performed within 2
weeks before or after cardiac MDCT, detection of main coronary arterial
stenosis, greater than 50% on MDCT, including side branches with a diameter
> 2.0 mm, was compared with that on QCA. QCA was performed with a computed
edge-detection program (QCA-CMS version 5; Medis). The presence or absence of
contrast medium inflow artifacts in the RV affecting evaluation of the RCA was
visually evaluated by consensus of two radiologists.
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Fig. 4 —70-year-old man with angina pectoris. Transaxial CT image
obtained with protocol A shows left ventricular endocardial and right
ventricular septal endocardial contours are clearly delineated.
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Statistical Analysis
Before the study, power analysis was performed with 12 additional subjects,
who were not included in the final study population, to determine sample size
for statistical analysis. The differences of mean ventricular attenuation
among contrast injection protocols were approximately one-and-a-half times
greater than their SDs. Therefore a minimum of 30 subjects was considered
appropriate for an intended power of 0.9 or greater at subsequent power
analysis.
All data were recorded as mean ± SD. One-way analysis of variance
was used to assess intergroup differences in mean attenuation in the LV and RV
cavities, the difference between the maximum and minimum ventricular
attenuation values along the z-axis, and ventricular-myocardial CNR.
Two-way analysis of variance was used to assess intergroup differences in
septal wall thickening and to compare septal wall thickening on MDCT with that
on echocardiography. If statistically significant differences were observed,
the Bonferroni-Dunn test was used for post hoc analysis. The Kruskal-Wallis
test was used for verification of visual evaluation results on MDCT images.
Interobserver variability was assessed with kappa statistics. Kappa values
were reported as follows: 0 = agreement was a random effect, < 0.20 = poor
agreement, 0.21-0.40 = fair agreement, 0.41-0.60 = moderate agreement,
0.61-0.80 = substantial agreement, and 0.81-1.00 = almost perfect agreement
[9]. In 24 cases in which QCA
was performed, a chi-square test was used for comparison of diagnostic
accuracies in detection of coronary arterial stenosis. Probability values <
0.05 were considered statistically significant. SAS 8.01 for Windows (SAS
Institute) was used for statistical analysis. Power analysis was performed
with free software (G-Power version 2.1.2) available at
www.psycho.uni-duesseldorf.de/aap/projects/gpower/index.html.
Results
Ventricular Attenuation and Ventricular-Myocardial CNR Measurement
The mean ventricular attenuation values along the z-axis are shown
in Table 1. There were no
statistically significant differences in mean LV attenuation among the three
protocols. The mean RV attenuation obtained with protocol A (335.8 ±
55.0 H) was higher than that obtained with protocols B (265.1 ± 87.4 H)
and C (288.1 ± 71.4 H), but the differences in mean RV attenuation were
not statistically significant among the three groups (p = 0.06). The
differences between the maximum and minimum attenuation values along the
z-axis in the LV and RV cavities are shown in
Table 2. The difference between
the maximum and minimum attenuation values in the RV cavity was significant
(p = 0.005). Post hoc analysis revealed that the difference between
the maximum and minimum RV attenuation obtained with protocol A (58.1 ±
27.3 H) was significantly less than obtained with protocols B (179.5 ±
109.3 H) and C (157.0 ± 115.8 H). A decrease in attenuation in the RV
cavity (200 H or less) was not observed when protocol A was used but was
observed in eight of 12 and six of 13 patients in the latter half of the CT
scanning time in protocols B and C, respectively.
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TABLE 2: Differences Between Maximum and Minimum Attenuation (H) Along z-Axis in
Left and Right Ventricular Cavities
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The ventricular-myocardial CNR calculated from measurements obtained within
the LV and RV cavities and the septal myocardial wall reflected overall
delineation of the LV endocardial and RV septal endocardial contours. The mean
ventricular-myocardial CNR values are shown in
Table 3. There were no
significant differences in the LV-myocardial CNR, but there were significant
differences in RV-myocardial CNR among the three groups. Post hoc analysis
revealed that the RV-myocardial CNR obtained with protocol A (9.0 ±
3.0) was significantly higher than that obtained with protocol B (5.5 ±
2.5) (p = 0.005). The RV-myocardial CNR obtained with protocol A was
higher than that obtained with protocol C (6.8 ± 3.4), but the
difference was not statistically significant (p = 0.07). The
LV-myocardial CNR profile along the z-axis showed a constant level
during CT with each protocol (Fig.
2). The RV-myocardial CNR profile obtained with protocol A showed
a more constant level, whereas the profiles obtained with protocols B and C
showed a decrease at caudal levels in the RV cavity
(Fig. 3).
Evaluation of Cardiac Function
In all patients clear delineation of both LV endocardial and RV septal
endocardial contours was achieved with protocol A
(Fig. 4). In seven of 12 and
five of 13 patients, however, delineation of the RV septal endocardial
contours was difficult to assess with protocols B and C, respectively (Figs.
5A and
5B), and wall thickening
analysis could not be performed with MDCT in these patients. In the other 26
patients (13, five, and eight for protocols A, B, and C, respectively), septal
wall thickening on MDCT was compared with that on echocardiography. The mean
septal wall thickening values were 48.8 ± 8.2% and 50.5 ± 8.0%,
respectively, for MDCT and echocardiography. The results of septal wall
thickening were not significantly associated with contrast medium protocol
(p = 0.94). In comparisons of MDCT with echocardiography, the
Pearson's correlation coefficient and p value were r = 0.958
and p < 0.05, although the value for wall thickening on MDCT was
lower than that on echocardiography.
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Fig. 6A —Time-density curves. PA = pulmonary artery. Graph shows
time-density curve for protocol A. Adequate and uniform attenuation is present
in both aorta and pulmonary artery 20-50 seconds after start of contrast
injection.
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Evaluation of Coronary Arteries
Mean heart rate was 61 ± 11 beats per min, 59 ± 11 beats per
min, and 58 ± 8 beats per min, respectively, with protocols A, B, and
C. There was no significant difference among the three protocols (p =
0.74, one-way analysis of variance). No case was excluded from analysis
because of motion artifacts or calcifications. The mean attenuation values in
the proximal and middle coronary arteries are shown in
Table 4. There were no
statistically significant differences among the three protocols. The results
for visual evaluation of the coronary arteries are shown in
Table 5. The results for visual
evaluation of the coronary arteries were not significantly associated with
contrast medium protocol (p = 0.91) (Figs.
6A,
6B, and
6C). Interobserver agreement
for visual evaluation of coronary vessels was substantial for protocols A
(
= 0.83) and C ( = 0.83) and complete for protocol B.
Coronary MDCT angiograms of 24 patients (nine, seven, and eight for
protocols A, B, and C, respectively) were compared with QCA images, which were
not available in the other 14 cases. On QCA images, 12, 13, and 12 coronary
branches with greater than 50% stenosis were detected with protocols A, B, and
C, respectively. The diagnostic accuracies in detection of coronary arterial
stenosis were 92% (33/36), 93% (26/28), and 91% (29/32), respectively, for
protocols A, B, and C. There was no significant difference among three
protocols (p = 0.95).
Contrast medium inflow artifacts in the RV affecting evaluation of the RCA
were not observed in either protocol.
Discussion
Both LV endocardial and RV septal endocardial contours were clearly
delineated in all patients who underwent CT with a biphasic long-duration
contrast injection protocol in which diluted contrast medium was used in the
latter phase. Because of poor RV-myocardial contrast, RV septal endocardial
contour in the inferior levels of the RV was difficult to delineate in nearly
one half of the patients who underwent CT with a uniphasic protocol with or
without a saline flush. According to previous reports
[3,
10], an attenuation value of
250-300 H is considered optimal vascular and ventricular attenuation. In our
study, optimal attenuation in both ventricular cavities was achieved during CT
performed with a long-duration biphasic protocol. Moreover, attenuation and
visualization of the coronary arteries with our biphasic protocol were
comparable with those obtained with uniphasic protocols (Tables
4 and
5). Nieman et al.
[11] reported that the
accuracy of 16-MDCT in identification of stenosed coronary vessels was 90%.
Our results of visual assessment of coronary vessels were similar to those,
although our study population was relatively small.
With 16-MDCT, the time needed to scan the entire heart with a 0.5-mm
detector row width has been reduced to 25 seconds or less. In general, scan
delay is approximately 25 seconds. Therefore it is important to achieve
adequate and uniform enhancement in the coronary arteries and both ventricular
cavities 25-50 seconds after the start of contrast medium injection. We
performed a preliminary study (Appendix 1) to investigate the pharmacokinetics
of the injected contrast medium. The time difference between arrival of
contrast medium in the pulmonary artery and in the aorta was 5-7 seconds,
which was considered reflective of pulmonary transit time
[12-14].
Thus a decrease in attenuation in the pulmonary artery was observed 5-7
seconds earlier than in the aorta. Attenuation in the pulmonary artery and
aorta was adequate and remained constant 25-50 seconds after the start of
contrast medium administration in the long-duration biphasic protocol
(Appendix 1 and Fig. 6A),
whereas attenuation in the pulmonary artery decreased 30-40 seconds after the
start of contrast medium administration with the uniphasic protocols with or
without saline flush (Appendix 1, Figs.
6B and
6C). The long-duration biphasic
protocol provided adequate and uniform attenuation in both the right and left
sides of the heart. The effects of a saline flush compared with no saline
flush were an increase in peak of maximum enhancement and more prolonged
arterial enhancement above a specified level. Cademartiri et al.
[15] reported similar results
in their study of the parameters affecting bolus geometry in CT angiography.
In our study, clear delineation of both LV endocardial and RV septal
endocardial contours was achieved with the long-duration biphasic protocol,
although delineation of the RV septal endocardial contours was difficult when
the uniphasic protocols were used. These results appear to agree with the
findings of our time-density curve analysis (Figs.
6A,
6B, and
6C).
We believe that subsequent injection of diluted contrast medium in the
long-duration biphasic contrast protocol has two important roles. First,
subsequent injection of diluted contrast medium flushes out the high-density
contrast medium previously injected. The result is greater and more prolonged
arterial enhancement. Second, subsequent injection of diluted contrast medium
ensures that attenuation in the RV remains acceptable throughout the CT
scanning time. Moreover, it is possible to avoid beam-hardening artifacts
arising from the presence of high-density contrast medium in the RV cavity,
which can interfere with visualization of the RCA and clear delineation of the
RV septal endocardial contours.
With regard to analysis of RV function, the use of MDCT in clinical
practice has been limited by the lack of standardized analysis software.
Sophisticated RV analysis software is needed to compare MDCT and MRI
volumetric analysis of the RV. In addition, mixing of higher-density contrast
medium with unenhanced blood in the inferior vena cava caused inhomogeneous
enhancement. This inhomogeneity also was seen in the right side of the heart,
especially the right atrium, making it difficult to delineate the RV contour.
In our study population, the RV contours were distinguishable in all patients
who underwent CT with the biphasic protocol because the degree of
inhomogeneity in the RV was slight. Additional clinical studies are needed to
evaluate whether MDCT is suitable for RV functional analysis.
Our study had several limitations. First, a certain amount of contrast
medium was required for visualization of the four chambers of the heart.
However, the total contrast medium volume used in our biphasic protocol was
100 mL, which was equal to that in the uniphasic protocols. Second, the
injection rate of 3 mL/s was relatively low, and different injection rates
were not applied in this study. The body weight of Asian people is generally
lower than that of people in North America and Europe
[16]. The 3-mL/s injection
rate was considered to provide not only adequate arterial enhancement but also
prolongation of contrast enhancement. Thus, we chose the 3-mL/s rate for this
study. We believe our biphasic injection protocol may correspond to a protocol
in which 80 mL of undiluted contrast medium is followed by 100-120 mL of
diluted contrast medium at 4 mL/s for heavier patients. Most manufacturers
make CT scanners that incorporate 32 or 64 rows of detectors. The higher
efficiency of 64-MDCT may make it possible to reduce the volume of contrast
medium. We think a protocol in which 70 mL of undiluted contrast medium is
followed by 70-80 mL of diluted contrast medium at 4-5 mL/s may be suitable
for 64-MDCT. Third, the number of patients in each group was relatively small.
However, we believe our results were definitive because a minimum of 30
subjects was considered appropriate in a preliminary power analysis with 12
subjects.
In conclusion, a long-duration biphasic contrast protocol for cardiac
16-MDCT is optimal for the evaluation of the coronary vessels and cardiac
function.
APPENDIX 1. Preliminary Study: Time-Density Curve Analysis of the Aorta and the Pulmonary Artery
Nine patients with known malignant disease scheduled to undergo cervical CT
studies for the evaluation of the primary lesion and lymph node metastasis
were randomly assigned to three groups with different injection protocols. The
inclusion criteria were the presence of histologically confirmed primary
malignant disease of the neck or thorax, radiation therapy for the primary
disease, and no known pulmonary or heart disease. These nine patients were not
included in the population of the prospective clinical trial. For measurement
of the time-density curve, CT at a single level (T7) was repeated at
1.5-second intervals of 1.5-60 seconds after the start of injection of
contrast medium. We chose the T7 level because it was easy to monitor both the
ascending aorta and the pulmonary artery. Routine neck and thoracic CT
examination was started after the single-level repeated CT. The X-ray tube
current was reduced to 33 mA to minimize radiation exposure, and the effective
dose was estimated to be approximately 0.7 mSv for single-level repeated CT.
We explained to the subjects that radiation exposure levels would be increased
approximately 10% in this study compared with routine scanning alone. We also
explained the risks associated with radiation exposure with regard to
stochastic effects, and we answered all questions. Informed consent was
obtained from all nine patients.
We measured attenuation values in the ascending aorta and pulmonary artery
by placing a circular region of interest cursor (= 100 mm2) on the
unenhanced image and on all images acquired by single-level repeated CT. The
attenuation values in the vessels were measured by the same radiologist. We
generated the time-density curves for the aorta and pulmonary artery by
averaging the values at each time point of the normalized time-enhancement
curves with each protocol. In general, the attenuation values in the aorta and
pulmonary artery reflect the attenuation values in the LV and RV cavities,
respectively. The time-density curve in protocol A
(Fig. 6A) showed adequate and
uniform attenuation in both the aorta and pulmonary artery 20-50 seconds after
the start of contrast medium injection, whereas those in protocols B
(Fig. 6B) and C
(Fig. 6C) showed a decrease in
the attenuation values in the pulmonary artery approximately 30 seconds after
the start of contrast medium injection.
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Abstract
Introduction
