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Original Research |
1 Departments of Interventional Radiology and Nuclear Medicine, Institute of
Diagnostic Radiology, BG Clinics "Bergmannsheil," Ruhr-University
of Bochum, Buerkle-de-la-Camp Platz 1, D-44789 Bochum, Germany,
2 Department of Medical Informatics, Biometry and Epidemiology, Ruhr-University
of Bochum, Bochum, Germany.
Received November 9, 2007;
accepted after revision January 3, 2008.
Address correspondence to C. M. Heyer
(christoph.heyer{at}rub.de).
Abstract
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SUBJECTS AND METHODS. One hundred twenty-six patients were referred to undergo pulmonary CTA with bolus tracking and ATCM. Besides patient characteristics, the type, position, size, and side of venous catheters were documented. Pulmonary vessel enhancement and image noise were quantified; signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated. Subjective vessel contrast was assessed by two radiologists in consensus.
RESULTS. Patient age showed a moderate but significant positive correlation to vessel enhancement (r = 0.244, p = 0.006), CNR (r = 0.178, p = 0.046), and subjective image quality (r = 0.344, p < 0.001). Patient weight revealed a significant negative correlation to vessel enhancement (r = -0.496, p < 0.001), SNR (r = -0.446, p < 0.001), CNR (r = -0.425, p < 0.001), and subjective image quality (r = -0.422, p < 0.001). In univariate analysis, SNR and CNR were significantly higher in patients who received contrast medium through peripheral catheters (30 ± 13 and 27 ± 13, respectively) than in those in whom central catheters were used (22 ± 8 and 19 ± 7, p = 0.041 and p = 0.029, respectively). Neither patient sex nor catheter size, position, or side had any significant impact on image quality.
CONCLUSION. Patient age and weight showed significant impact on vascular attenuation and image quality in pulmonary CTA with bolus tracking and ATCM, whereas patient sex and different peripheral catheters did not significantly influence image parameters.
Keywords: automated tube current modulation • bolus tracking • CT angiography • MDCT • pulmonary embolism
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One of the major disadvantages of pulmonary CTA is radiation exposure. Results of studies have shown an increase in the number of patients examined with pulmonary CTA for suspected pulmonary embolism and an increase in effective dose per patient [11, 12]. Several investigators have shown that the prevalence of pulmonary embolism among patients evaluated with CT is only 9-35% [9, 13, 14] and that young female patients account for a substantial number of patients undergoing pulmonary CTA for suspected pulmonary embolism [15, 16]. Moreover, studies have revealed that 7-9% of pulmonary CTA studies may be indeterminate or inconclusive [17, 18] and that the nondiagnostic rate is as high as 25% in ICU patients [19]. These facts indicate a need for radiation dose reduction aiming for optimum diagnostic image quality at the lowest possible radiation exposure [20, 21].
Lowering tube voltage has recently been shown to effectively reduce radiation exposure without compromising image quality in pulmonary CTA [22]. Several other scanning parameters affect radiation dose including tube current, beam collimation, pitch, table speed, and gantry rotation time [23]. Tube current is one of the most important factors because there is a linear relationship to the effective radiation dose applied to the patient. Consequently, many of the efforts that have been made to reduce radiation exposure in CT have focused on reduction of tube current levels including automated tube current modulation (ATCM).
The purpose of this study was to compare image quality, patient characteristics, and different types of venous contrast application associated with a standard 16-MDCT pulmonary CTA protocol in patients with sus pected pulmonary embolism performed with auto mated bolus tracking and a z-axis ATCM technique.
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Patient Group
Between June 2006 and September 2007, all consecutive patients who were
transferred to the Institute of Diagnostic Radiology, Departments of
Interventional Radiology and Nuclear Medicine, BG Clinics Bergmannsheil,
Ruhr-University of Bochum, Germany, for pulmonary CTA for suspected pulmonary
embolism were considered for entering the study. The indication for performing
CTA was based on positive results of clinical investigation (determined by a
revised Geneva score [24] or a
Wells' score [25], abnormal
findings of laboratory tests [blood gas analysis, D-dimer level],
abnormal results of echo cardiography or ECG indicative of acute right heart
dysfunction, abnormal findings of lower limb sonography, and results of
conventional radiography suggesting pulmonary embolism). Known allergy to
contrast material, hyperthyroidism, renal insufficiency, pregnancy, and age of
less than 18 years were considered contraindications for entering the study.
Further more, patients who denied consent for the procedure were excluded from
the study group.
In total, 126 patients entered the study. Written informed consent for the pulmonary CTA procedure was given by each patient after the nature of the examination had been fully explained. Body weight, if available, was recorded for every patient. Furthermore, the type (peripheral or central venous line), position (cubital vein or vein on forearm or hand), size (18- or 20-gauge), and side (left, right) of each venous access were documented. All venous lines had been established by the referring physicians before the CT study.
Pulmonary CTA
All scans were obtained using a commercially available 16-MDCT scanner
(Somatom Sensation 16, Siemens Medical Solutions). Patients were examined in a
supine position with both arms extended above the head. A frontal scout view
was acquired at 120 kVp and 50 mA. The angiography scan was obtained in
caudocranial direction during a single inspiratory breath-hold. The scan
volume ranged from the level of the right diaphragm to a level just above the
aortic arch. A standard collimation of 0.75 mm was used with a gantry rotation
speed of 0.5 second and a pitch factor of 1.1. Patients were scanned with a
kilovoltage of 100 kVp and a tube current level of 200 mA (100 mAs) as the
reference value. In all scans, a commercially available z-axis ATCM
technique (CARE Dose 4D, Siemens) was applied.
Vessel opacification was provided by IV injection of 80 mL of iopamidol (Solutrast 300, Altana Pharma) via a peripheral vein (109 patients, 87%) or via a central venous line in the superior vena cava (17 patients, 13%) followed by a saline flush of 40 mL. Flow rate was kept constant at 4 mL/s throughout the procedure. Injections were performed automatically using a commercially available injector (Injektron CT2, Medtron). Individual contrast optimization was based on bolus tracking (CARE Bolus, Siemens) in the right ventricle using a trigger level of 100 H. For measuring early contrast enhancement, a series of dynamic monitoring scans was obtained at 2-second intervals using a low-dose protocol (100 kVp; 20 mAs; slice thickness, 4.5 mm). Bolus tracking was started 4 seconds after injection of the contrast agent had started. Furthermore, an additional delay of 7 seconds was added after bolus tracking before diagnostic pulmonary CTA.
For further postprocessing, thin-slice reconstruction was performed with a slice thickness of 1 mm, an increment of 0.7 mm, and a smooth reconstruction kernel (B30f). Final image analysis was performed on axial images and on coronary maximum intensity projections (MIPs) with slice thicknesses of 3 and 6 mm, respectively.
Assessment of Image Parameters
Scan length (distance covered) was documented for every examination.
Assessment of image material for the presence of pulmonary embolism was
performed by analysis of central and peripheral pulmonary arteries to the
subsegmental level. The presence of endoluminal clots was considered
diagnostic of pulmonary embolism. Signal intensity (SI)—that is, CT
attenuation—measurements were determined at a workstation (Leonardo,
Siemens) along the pulmonary arteries including nine different levels: the
main pulmonary artery, right pulmonary artery, left pulmonary artery, right
upper lobe artery, right middle lobe artery, right lower lobe artery, left
upper lobe artery, left lingula artery, and left lower lobe artery. The
regions of interest used for these measurements were chosen to be as large as
the vessels. Based on these numbers, peak and average pulmonary vessel SIs
were calculated.
If pulmonary embolism was present, average intravascular SI measurement was limited to unaffected vessels. The measurement of background noise was based on assessment of Hounsfield units (SD) within surrounding air at three different regions of interest in front of the patient (central, left, and right) with a size of approximately 1 cm2; averaged values were used for final calculation of background noise. In addition, attenuation of the central parts of the pectoral muscles and attenuation of the deep paraspinal muscles were measured on both sides and averaged (muscle SI). Based on these measurements, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated according to the following equations: SNR = mean pulmonary vessel SI/background noise; and CNR = (mean pulmonary vessel SI - muscle SI)/background noise.
In addition, subjective image quality with regard to pulmonary vessel contrast was assessed based on axial sections and MIP. For evaluation of subjective image quality a 5-point scale (5 = excellent, 4 = good, 3 = moderate, 2 = still diagnostic, 1 = nondiagnostic) was applied.
All measurements and calculations were done by two authors in consensus with 7 and 2 years of experience in chest CT.
Statistical Analysis
Results of image analysis (scanning length, signal measurements, SNR, CNR,
subjective image quality) and patient characteristics (body weight, age) are
expressed as mean values ± SD (range). All data concerning venous
access are calculated as absolute frequencies (percentages). Normality of data
distribution was assessed visually and using the Kolmogorov-Smirnov test. For
comparison of quantitative patient characteristics (age, weight) and imaging
parameters (SNR, CNR, mean pulmonary vessel enhance ment, peak vessel
enhancement, and subjective image contrast), correlations were calculated by
regression analysis using Pearson's correlation coefficient. Display of
calculated data was performed with scatterplots and regression lines based on
univariate linear regression. Imaging parameters were compared between
categoric variables (patient sex, venous access) using the Student's
t test for unpaired samples. Comparison of groups defined by
categoric data was visualized using vertical box plots of quantiles.
Statistical significance of all tests was set at a p value of less
than 0.05. All calculations were performed on a standard PC using SPSS
software (release 11.5.1, SPSS) for Windows (Microsoft).
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Patient Characteristics
In total, 126 patients were included in the study. The mean age of the
study group was 65 ± 15 years (range, 18-89 years). Sixty-five patients
(52%) were women. The mean body weight of the patient group was 79 ± 18
kg (range, 43-131 kg). There were 86 inpatients (68%) and 40 outpatients
(32%). Seventeen patients (13%) were intubated during the CT procedure and
mechanical ventilation was performed. CT evaluation of the pulmonary arteries
revealed pulmonary embolism in 28 patients (22%).
Venous Access
In 109 of 126 patients (87%), contrast material was injected via a
peripheral line. In 60 of 109 patients (55%), an 18-gauge venous line was
used, whereas in 49 patients (45%) contrast material was applied through a
20-gauge line. In 59 of 109 patients (54%), contrast material was injected
through a cubital vein, and in 50 of 109 patients (46%) a peripheral venous
line on the forearm or on the hand was used. A right-sided vein was used in 60
of 109 patients (55%), whereas a left-sided vein was used in 49 patients
(45%). Seventeen of the 126 patients (13%) received IV contrast agent via a
central line placed in the superior vena cava.
Imaging Parameters
The mean scanning length was 260 ± 50 mm (range, 124-360 mm). The
mean pulmonary artery SI in the patient group was 375 ± 105 H (153-612
H) and peak pulmonary artery SI was 431 ± 116 H (190-718 H).
Table 1 displays measurements
of mean and peak artery SIs in the different pulmonary vessels. Mean image
noise was 14 ± 5 H (8-40 H). Calculations of SNR and CNR revealed
average values of 29 ± 13 (9-74) and 26 ± 12 (8-68),
respectively. Mean subjective image contrast was 3.8 ± 1.0 (2-5).
Excellent image contrast (score of 5) was documented in 36 of 126 patients
(29%), whereas good (score of 4) and moderate (score of 3) contrast was
recorded in 44 patients (35%) and 34 patients (27%), respectively. In 12
patients (10%) subjective image quality was rated still diagnostic (score of
2). No examination was rated nondiagnostic (score of 1).
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Table 2 summarizes the comparison of patient sex, venous lines, and imaging parameters.
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Patient Sex and Image Quality
Patient sex showed no significant impact on mean vessel enhancement (male,
345 ± 97 H; female, 403 ± 106 H; p = 0.259), peak
vessel enhancement (male, 398 ± 102 H; female, 462 ± 121 H;
p = 0.120), image noise (male, 14 ± 4 H; female, 15 ± 6
H; p = 0.152), SNR (male, 27 ± 12 H; female, 31 ± 14 H;
p = 0.231), CNR (male, 24 ± 11 H; female, 28 ± 13 H;
p = 0.278), or subjective image quality (male, 3.7 ± 0.9 H;
female, 4.0 ± 1.0 H; p = 0.900).
Patient Age and Image Quality
Patient age showed a significant positive correlation to mean vessel
enhancement (Pearson's coefficient r = 0.266, p = 0.003;
Fig. 1A), peak vessel
enhancement (r = 0.244, p = 0.006;
Fig. 1B), CNR (r =
0.178, p = 0.046; Fig.
1C), and subjective image quality (r = 0.344, p
< 0.001; Fig. 1D), whereas
no significant correlation could be observed to SNR (r = 0.139,
p = 0.120) or to image noise (r = 0.040, p =
0.655).
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Furthermore, the side of peripheral injection of contrast agent did not significantly influence image quality. Patients with a right-sided (left-sided) peripheral catheter showed mean vessel enhancement of 370 ± 105 H (392 ± 111 H; p = 0.857), peak vessel enhancement of 425 ± 114 H (447 ± 123 H; p = 0.782), image noise of 14 ± 5 H (14 ± 4 H; p = 0.330), SNR of 30 ± 13 (31 ± 13; p = 0.973), CNR of 27 ± 13 (28 ± 13; p = 0.985), and subjective image quality of 3.8 ± 1.0 (4.0 ± 0.9; p = 0.419).
Finally, the position of the peripheral catheter did not have any significant impact on image quality. Patients with a venous line in a cubital vein (hand or forearm vein) attained mean vessel enhancement of 392 ± 108 H (365 ± 107 H; p = 0.796), peak vessel enhancement of 445 ± 121 H (423 ± 115 H; p = 0.500), image noise of 14 ± 5 H (13 ± 5 H; p = 0.869), SNR of 31 ± 13 (30 ± 14; p = 0.912), CNR of 28 ± 12 (27 ± 13; p = 0.954), and subjective image quality of 4.0 ± 1.0 (4.0 ± 1.0; p = 0.510).
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In our study, we used bolus tracking in the right ventricle to precisely determine the scan delay for pulmonary CTA. Assuming that a threshold of 100 H within the right ventricle is adequate [9, 27] and that 200 H is the desirable level of contrast enhancement [8, 10, 28, 29], our results indicate that bolus tracking enables adequate and uniform pulmonary vessel enhancement in pulmonary CTA with a mean and peak attenuation of 375 and 431 H, respectively. In subjective image analysis, the quality of the images for 114 patients (90%) was rated excellent, good, or moderate, whereas the quality of the images for only 12 subjects (10%) showed poor but still diagnostic vessel enhancement. None of the investigations performed in our study group were rated as nondiagnostic. Thus, bolus tracking enabled complete avoidance of nondiagnostic studies in our patient group, which is an important observation with regard to overall radiation dose reduction.
In a previous study by Hartmann et al. [30], investigators used a single-detector scanner for pulmonary CTA to compare pulmonary vessel enhancement in 57 patients using bolus tracking and in 50 patients with the scanning delay fixed to 20 seconds. They found no significant difference between the two methods. However, their results are difficult to compare with ours because they used a single-detector scanner and a long injection time (40 seconds). Furthermore, patients with substantial cardiopulmonary impairment and low circulation rates might require long scan delays, which may be missed with a fixed, nonvariable scan delay.
Lee et al. [10] used a study protocol with a 30-second injection of contrast agent followed by a 10-second saline flush. They found an optimal temporal window of 16-41 seconds to achieve pulmonary artery enhancement of greater than 200 H, which underlines the hypothesis that a fixed delay might lead to insufficient vessel attenuation in pulmonary CTA.
To our knowledge, only three studies have systematically evaluated the impact of patient characteristics on vessel enhancement in pulmonary CTA. Bae et al. [31] evaluated the amount of contrast medium required in 16- and 64-MDCT pulmonary CTA in 85 patients. They found that 1.2 mL/kg of body weight of contrast medium was required to achieve 250 H, indicating that body weight had a substantial influence on pulmonary vessel enhancement. That finding is in accordance with our results, which also showed a significant negative dependence of objective and subjective imaging parameters on patient weight.
Besides weight, patient age may have substantial impact on the diagnostic yield of pulmonary CTA. However, in an analysis of 299 patients suspected of having pulmonary embolism, Righini et al. [32] did not find a significant influence of patient age on the sensitivity, specificity, or predictive values of pulmonary CTA comparing three different age groups. In their group of 242 patients investigated, Arakawa and coworkers [2] recently documented that body weight and patient age were independent variables associated with enhancement of the pul mo nary arteries. As in our study, higher patient age was associated with better pulmonary contrast enhancement. The reason for this find ing is unclear, but we speculate that changes that occur in cardiopulmonary circulation with aging, including an increase of pulmonary artery pressure, may be responsible for this observation [2, 30, 33, 34].
Hartmann et al. [30] confirmed that the transit time of a small amount of contrast agent through the pulmonary circulation became significantly longer with increased patient age. Thus, the pulmonary blood pool may be stagnant in older subjects and the contrast medium is better visualized in these patients while performing pulmonary CTA. On the other hand, longer circulation times may explain our finding that bolus tracking does not eliminate the impact of patient age on pulmonary artery attenuation because the time to reach the predefined threshold of vessel enhancement may not necessarily be influenced by the global pulmonary transit time of the contrast medium.
In contradiction to weight and age, patient sex did not significantly influence image quality or pulmonary vessel enhancement in our study group, in accordance with the findings of Arakawa et al. [2].
The data of our study indicate that application of contrast agent through a central venous line might lead to lower vessel enhancement and image quality compared with contrast injection via a peripheral catheter. The reason for this observation is unclear, but direct in jection of contrast material into the superior vena cava and right atrium might lead to im mediate circulation and early pulmonary wash out, resulting in low vessel contrast at the time of image acquisition. We believe that this problem can be solved only by decreasing—as suggested by Schoellnast el al. [8]—or even erasing the additional delay after bolus tracking has started. However, with this technique a command for inspiratory breath-hold cannot be given in some patients, which might cause motion artifacts and impaired image quality. Lee et al. [10] stressed that the triggering delay after the threshold of 100 H should be more than 5 seconds to achieve enhancement of greater than 200 H in the main pulmonary artery. However, in that study contrast material was ad ministered through an antecubital vein in all patients and central catheters were not used.
In our study group, neither position nor size of a peripheral venous catheter showed any significant influence on pulmonary vessel enhancement or image quality when a standard injection protocol was applied. Vessel attenuation is generally determined by the number of iodine molecules administered over time. The iodine flow concentration can be augmented by increasing the injection flow rate or using a contrast agent with a high iodine concentration. In our study, contrast medium was administered at a constant injection rate of 4 mL/s as proposed by other authors [1, 17, 35-39]. Our results indicate that contrast flow profiles of different peri pheral venous access routes are almost similar and that inter-individual differences in the initial rise of pulmonary enhancement are completely compensated by bolus tracking.
ATCM is a technique that automatically adjusts the tube current at different X-ray beam projection angles in each slice location and enables acquisition of a desired image quality at reasonable radiation exposure levels [20, 21, 40-42]. Although previous studies have shown a mean reduction of 20-23% in radi ation exposure for CT examinations perform ed with ATCM [21, 43], Kalra et al. [21] ob served that ATCM resulted in a significant 4-13% increase of radiation exposure in patients with greater body weight. The results of our study also indicate a negative correlation between body weight and pulmonary vessel enhancement and correspondingly image quality in pulmonary CTA. Thus, it has to be considered whether ATCM should be switched off in very heavy patients and fixed tube current levels used to ensure adequate image quality and to avoid unacceptably high radiation dose.
There are some limitations to our study. First, this study is not a prospective randomized trial given that we did not randomly assign patients to different venous access routes. The patients in our study group were examined using a peripheral or central line that had already been placed by the referring physician. Thus, we had only limited influence on the selection of types of venous lines in our study. However, we believe that the distribution of the different types of venous access, including central and peripheral lines of differing locations, made appropriate statistical analysis possible. Moreover, we evaluated patients, including those with severe derangement of cardio pulmonary status, in different hemodynamic conditions. This fact could have affected the time to peak arterial enhancement and pulmonary vessel enhancement, which led to exclusion of those patients in other comparable studies [2].
An additional limitation is that 64-MDCT and dual-source CT scanners are currently state of the art. However, these technologies are not widely implemented and still represent a small percentage of MDCT scanners currently used. Although detector configurations will not vary substantially from 16-MDCT practice, shorter acquisition times may influence contrast infusion and vessel enhancement in pulmonary CTA.
Finally, we assessed only proximal pulmonary arteries for objective and subjective image quality. Thus, a potentially altered SNR or CNR of the peripheral vessels might have been missed. Nevertheless, Prologo and co workers [44] showed that increased visualization of small, more peripheral pulmonary arteries did not affect the clinical outcome of patients with pulmonary embolism.
In conclusion, pulmonary CTA with bolus tracking and ATCM using a standard injection protocol enables adequate and homo geneous pulmonary vessel enhance ment in suspected pulmonary embolism. Patient age and weight showed a significant impact on vascular attenuation and image quality, whereas patient sex and different positions and sizes of peripheral venous lines did not significantly influence image parameters. As a consequence, one must consider further individualization of contrast injection beyond bolus tracking, with a reduction of delay time in young patients and fixed increased tube current in those with a high body weight. Furthermore, contrast injection through a central venous line carries the risk of suboptimal pulmonary artery contrast that might be overcome by a shortened post threshold delay with immediate scanning after bolus tracking or by increasing the time of contrast infusion.
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