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Clinical Observations |
1 All authors: Department of Radiology, Centre Chirurgical Marie Lannelongue, 133 Avenue de la Résistance, 92350 Le Plessis-Robinson, France.
Received February 8, 2005;
accepted after revision April 29, 2005.
Address correspondence to J. F. Paul.
Abstract
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CONCLUSION. This study showed the combined effect of lowering the kilovoltage setting (80 kV) and using an automatic modulation technique (ECG-pulsed tube current modulation) for coronary CT. Radiation dose exposure can be reduced by up to 88% for slim patients without impairing image quality.
Keywords: cardiac imaging • coronary arteries • CT coronary arteriography • MDCT • radiation dose
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Our purpose was to investigate the feasibility and benefits of combining ECG pulsing and lowering the kilovoltage setting to achieve the ALARA (as low as reasonably achievable) concept in slim patients.
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Study Population
Eleven consecutive slim patients (weight, < 60 kg; mean age, 40 years;
age range, 13-77 years) requiring coronary CT examination were evaluated using
a low-kilovoltage protocol. All examinations were performed for clinical
reasons. The indications for coronary CT were atypical chest pain (n
= 7), heart transplantation (n = 2), or suspicion of anomalous
coronary artery (n = 2).
This study followed the ALARA principle using a step-by-step approach based on substantial previous experience.
MDCT Protocol
All CT examinations were performed on a 64-MDCT unit (Sensation 64, Siemens
Medical Solutions) with a rotation time of 330 msec, collimation of 0.75, tube
voltage of 80 kV, and tube current of 520 mAs. The weighted CT dose index
(CTDIw) was 11 mGy. The CTDIw is the weighted average
dose at the center and peripheral locations calculated on a phantom.
Contrast medium (400 mg/mL iomeprol [Iomeron, Bracco]) was injected into an antecubital vein (70 mL at 4 mL/sec). A CARE bolus was used to determine the beginning of CT acquisition by monitoring the signal density of contrast medium in the ascending aorta. CT data were acquired when the attenuation value of 100 HU was reached.
A ß-blocker (metoprolol) at a dose of 100 mg was given orally 45-60 min before CT examination if the patient's heart rate exceeded 65 beats per minute. Data sets were reconstructed with retrospective ECG gating with a medium-smooth filter (B30 f) at the diastolic phase.
Image Analysis
Two senior radiologists performed a qualitative consensus analysis of the
hard-copy film CT images. The variables assessed during the CT image
interpretation sessions were overall quality of axial slices and of
multiplanar and maximum-intensity-projection (MIP) reconstructions. Overall
image quality was subjectively rated on a 5-point scale as follows: a score of
1 was considered unacceptable; 2, suboptimal; 3, adequate; 4, good; and 5,
excellent. Diagnostic quality was considered to have been achieved for images
with a score of 3 or higher. Analysis was performed of only a hard copy of the
selected images. A consensus was easily achieved because image quality was
good in all cases. There was no case of disagreement.
Image noise, defined as the SD of the mean enhancement, determined using region-of-interest (ROI) methodology, was calculated in a standard 1 cm2 circular ROI in the aorta at the origin of the left anterior coronary artery. The signal-to-noise ratio (SNR) was calculated for each ROI considered. The dose-length product (DLP) for a complete examination was recorded at the end of each CT examination.
Statistical Analysis
Results are reported as means and SDs.
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MDCT revealed a severe stenosis of the left mainstem coronary artery (Figs. 1A, 1B, and 1C) in one patient and normal coronary arteries (Figs. 2A, 2B, and 2C) in the 10 remaining patients.
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The DLP value of 1,000 mGy x cm corresponds to an estimated radiation dose exposure of 16 mSv, illustrating the high radiation dose often associated with cardiac MDCT. As a consequence, radiologists should assess the clinical impact of each indication before performing cardiac CT examinations.
Our study was conducted because of the concerns about overexposure of women with atypical chest pain and of children with congenital heart disease or a heart transplant who are routinely examined in our institution.
Coronary MDCT appears to be an alternative diagnostic approach that can be offered to women with atypical chest pain to avoid more aggressive tests. A quick and effective evaluation of the coronary arteries is possible because of the excellent negative predictive value of MDCT for coronary artery disease [6, 7].
Screening slim women for coronary artery disease using coronary MDCT also requires a reduction of exposure to radiation, particularly breast exposure. Patients with cardiac transplants are exposed to coronary vasculopathy. Annual conventional coronary angiography is required to assess for coronary stenosis in denervated heart patients who are asymptomatic [8]. MDCT has been shown to be effective for ruling out coronary stenosis and is a less invasive technique than coronary angiography [9]. A reduction in radiation dose exposure is required because the patients concerned are young and repeated CT examinations are necessary.
The idea of decreasing radiation doses associated with CT led manufacturers to develop various automatic techniques based on tube current modulation (angular and z-axis tube current modulation). One of the most recently developed methods, from Siemens Medical Solutions (CARE dose 4D), combines the effects of angular and z-axis modulation techniques and seems promising [10]. Virtually all anatomic regions in the thorax, abdomen, and pelvis have benefited from these sophisticated techniques that result in considerable dose reduction. The z-axis modulation of CARE dose 4D is not compatible with ECG pulsing.
ECG-pulsed tube current modulation is the most significant improvement in minimizing radiation from CT technology and is the only one dedicated to cardiac imaging. ECG pulsing is performed online during cardiac CT examination and allows a decrease in radiation exposure of between 30% and 50% by modulation of tube current output to decrease the dose given during systole [3]. The nominal tube current output is applied only during the diastolic phase of the cardiac cycle, at which time images are most likely to be reconstructed. However, this method is dependent on the patient's heart rate and requires a regular sinus rhythm to prevent compromising image quality. Furthermore, despite the improvement, this technique alone still leaves pediatric and slim patients exposed to substantial radiation doses. Indeed, dose exposure estimated from the DLP provided by the manufacturer is a calculation made on an acrylic phantom of 32 cm; thus, the actual radiation dose delivered is always higher than that provided by the manufacturer in slim patients.
The protocol we used allowed a large decrease in the DLP. Despite the substantial dose reduction, overall diagnostic image quality was preserved. This illustrates that the high radiation dose with a standard protocol is unnecessary and shows that patients undergoing the MDCT with the standard protocol are currently being overexposed.
The use of weight-adapted low-kilovoltage settings was first described for the chest, where the benefits for lower-weight patients were shown using 80 kV without impairment of diagnostic image quality [4]. A recent application of the 80-kV protocol in cardiac MDCT was studied for analysis of left ventricle postinfarction changes [11].
In our study, the mean SNR was above 10 and a good image quality was preserved as a result of the parallel elevation of signal and noise. However, image noise was high, and we cannot exclude blurring at the edge of the coronary artery wall, making assessment of coronary plaque more difficult using a low kilovoltage setting. Also, stent visualization could be substantially less satisfactory using the 80-kV protocol. These situations were not in the scope of our work and require specific evaluation.
Direct comparison of our protocol with the standard protocol was not performed. However, because the DLP is predictable and can be calculated on the basis of the known CT parameters, we estimated the percentage of radiation savings when compared with the one used in our study. Using 120 kV, 800 mAs, with a 16 cm length, we obtain an approximate DLP of 1,000 mGy x cm (the exact value given by the CT is 956 mGy x cm).
In conclusion, this study highlights the culmination of the beneficial effects of the combination of lowering the kilovoltage setting (80 kV) and using an automatic modulation technique (ECG-pulsed tube current modulation) for coronary CT. Substantial lowering of radiation exposure, reaching a decrease of up to 88% compared with standard settings, was achieved without impairment of image quality in a group of patients with particular characteristics. Attempts to reduce dose should be considered before each use of coronary CT based on patient morphology and the clinical indications.
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