Radiation Dose Reduction Strategies in Cardiac Computed Tomography

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CHAPTER 10 Radiation Dose Reduction Strategies in Cardiac Computed Tomography

Sandra Simon Halliburton

Rapid technical advances in CT and increased availability of cardiac-capable CT systems have led to a sharp increase in the number of cardiac CT examinations performed during the last decade. Some technical advances (smaller detectors, faster gantry rotation) have necessitated increased x-ray exposure to the patient to maintain image noise. Concern regarding the resulting increase in radiation dose to the population from cardiac CT and the associated biologic risk has motivated critical assessment of dose-related imaging parameters.

A strategy for reducing radiation dose to the patient undergoing cardiac imaging should be employed particularly for patients at greatest risk for harm from x-ray exposure to the chest, young patients, and female patients.¹ This plan should include educating patients on the risks of exposure to ionizing radiation, seeking alternative studies that do not rely on ionizing radiation (e.g., MRI or ultrasonography) when appropriate, assessing the risk/benefit ratio of CT for the individual patient, and applying the as low as reasonably achievable (ALARA) principle to the selection of CT scan parameters.

TECHNICAL REQUIREMENTS

This chapter focuses on the selection of critical scan parameters directly or indirectly affecting radiation dose. Numerous modifiable scan parameters, including x-ray tube current, peak x-ray tube voltage, pitch (for helical scanning) or image acquisition spacing (for axial scanning), scan length, and, on some systems, scan field of view [SFOV], directly influence radiation dose by altering the number or energy of the x-ray photons interacting with the patient. Other scan parameters, including reconstructed slice thickness, indirectly affect dose by changing image quality and prompting the scanner operator to change parameters directly influencing dose. Strategies for customizing these parameters for the individual patient and minimizing radiation dose are presented in general terms to benefit users of any CT system.

TECHNIQUE TO REDUCE X-RAY TUBE CURRENT

ECG-Based Reduction in Tube Current

Acquisition of data using a lower dose, axial mode is indicated for contrast-enhanced evaluation of the coronary arteries in patients with low, stable heart rates; non–contrast-enhanced evaluation of coronary artery calcium (also known as calcium scoring), and evaluation of less mobile cardiovascular anatomy (e.g., aorta) if the z coverage of the scanner allows a reasonable breath-hold time. Helical data acquisition is typically required for contrast-enhanced coronary evaluation in patients with high or irregular heart rates. Online, ECG-based modulation of the tube current should be used with helical techniques to the extent possible for a given scanner.

Size-Based Reduction in Tube Current

Reduction of the nominal x-ray tube current before scanning is indicated in thin patients imaged with axial or helical techniques to reduce radiation dose. Online, anatomic-based tube current modulation during scanning can be used for all patients imaged with axial and non–ECG-gated, helical techniques.

Technique Description

X-rays used for CT imaging are generated when accelerated electrons strike a tungsten target. The number of electrons striking the target per unit time is described as the x-ray tube current and expressed in units of milliamperes (mA). A decrease in the x-ray tube current decreases the number of electrons striking the target and, subsequently, the number of x-rays produced per unit time.

Radiation dose decreases linearly with a decrease in tube current such that a 20% reduction in tube current results in a 20% reduction in dose. Dose savings is achieved at the expense, however, of increased image noise because decreasing the number of x-ray photons produced per unit time decreases the probable number of photons penetrating the patient and reaching the detector array. Image noise is proportional to such that a 20% reduction in tube current results in a 12% increase in image noise.

The parameter manipulated on clinically available CT scanners is often the product of the tube current and the exposure time per rotation with units of milliampere-second (mAs). The tube current × time product determines the number of x-ray photons produced per rotation. Additionally, some manufacturers automatically normalize the tube current × time product to pitch for helical scanning and define the resulting value in units of effective mAs or mAs/slice.

ECG-Based Reduction in Tube Current

During cardiac imaging, the x-ray tube current can be switched off or greatly reduced during systolic phases of the cardiac cycle significantly decreasing radiation dose. In patients with low, stable heart rates, data acquisition can be confined to a limited portion of the RR interval using prospectively ECG-triggered axial techniques. Average effective radiation doses of 2 to 3 mSv have been reported using these techniques to image the coronary arteries.²^,³

Axial imaging is associated with increased sensitivity to arrhythmia, owing to prospective referencing of the ECG signal, and increased examination times, owing to incrementing the patient table between acquisitions. It is often necessary to acquire data during the entire cardiac cycle using a helical mode and retrospectively reference image reconstruction to a simultaneously recorded ECG signal. Although ECG-gated helical techniques require continuous x-ray exposure, tube current outside the phase of interest can be reduced to decrease patient radiation dose significantly. Effective radiation doses can easily exceed 15 mSv ⁴ using helical imaging without ECG-based dose modulation, but can be reduced 50% or more depending on patient heart rate,⁵ the minimum tube current value,⁶ and the duration of the full tube current window.⁷

Size-Based Reduction in Tube Current

The x-ray tube current can be reduced for slimmer patients. Attenuation of the incident x-ray beam decreases with the thickness of the tissue between the x-ray source and the detector such that less radiation exposure is required to penetrate thinner tissues and achieve desired image noise (Fig. 10-1). Patients can be assigned to size categories based on visual inspection, weight, body mass index, or cross-sectional body measurements from scout images,⁸ and the tube current can be adjusted manually to a predefined value. Weight-adapted tube current protocols were shown to reduce coronary CT angiography dose by 18% in men and 26% in women⁹ at one institution.

FIGURE 10-1 A and B, Axial images from a larger patient (lateral width at top of liver 40 cm) (A) and a smaller patient (lateral width 27 cm) (B). To achieve noise equal to approximately 25 HU, 400 mAs/rotation was required in the larger patient, whereas only 320 mAs/rotation was required to achieve the same noise level in the smaller patient. Data acquisition with a lower tube current and resultant decrease in patient x-ray exposure is acceptable for imaging of smaller compared with larger patients without increasing image noise.

Automatic methods of online adaptation of tube current to patient size can also be used to reduce dose. The tube current can be modulated along the x, y, and z directions during scanning based on local tissue thickness without sacrificing image noise. Tube current is reduced at projection angles and table positions requiring less x-ray penetration. Online, anatomic-based tube current modulation has been shown to reduce radiation exposure to the thorax by 20% compared with a fixed tube current while maintaining image noise.¹⁰

Pitfalls and Solutions

ECG-Based Reduction in Tube Current

Axial imaging is vulnerable to cardiac motion artifacts, particularly in patients with high or irregular heart rates, according to prospective data collection. Some additional data beyond the minimum required for image reconstruction can be acquired (also known as padding) to permit minor retrospective adjustments of the reconstruction window and, potentially, to reduce cardiac motion artifacts.

Helical data acquisition with retrospective ECG gating is less susceptible to cardiac motion artifacts and provides an alternative to axial imaging at high or irregular heart rates. ECG-based tube current modulation is prescribed before scanning, however, so changes in heart rate could result in unintended reduction of the tube current during a desired phase of reconstruction for a given cardiac cycle. Some CT systems allow adjustment of the full tube current duration, increasing the utility of ECG-based tube current modulation for patients with high or irregular heart rates because the optimal reconstruction phase is less predictable.⁷ For patients with severe arrhythmia, some systems temporarily suspend or permanently switch off ECG-based tube current modulation if beat-to-beat variation exceeds a threshold value during data acquisition, virtually eliminating the risk of improperly timed downward modulation of the tube current.

Another consideration when employing ECG-based tube current modulation with helical imaging is the availability of multiphase data for functional evaluation because of increased image noise during periods of low tube current. Although reconstruction of thin slices (<1 mm) is confined to a small portion of the cardiac cycle, the reconstruction of thick slices (>5 mm) useful for functional evaluation may still be possible because noise decreases as reconstruction section thickness increases.

Size-Based Reduction in Tube Current

Weight-based approaches to categorizing patients are limited for cardiac imaging because attenuation depends on the thickness of the chest, which can vary substantially among patients of similar weight. Even body mass index cannot always adequately account for differences in chest size, particularly for women. Measurement of cross-sectional body dimensions from scout images or even visual inspection may be more appropriate for size categorization of cardiac patients and determination of the tube current necessary to achieve the desired image noise.

Online, anatomic-based modulation of the tube current in the z-direction is limited in combination with ECG-based modulation. Because position and time change along the z-axis, anatomic-based modulation in this direction competes with ECG-based modulation. In practice, ECG-based tube current modulation is given priority, and anatomic-based modulation serves only to determine the optimal nominal tube current value necessary to achieve the desired noise based on the scout image.

TECHNIQUE TO REDUCE TUBE VOLTAGE

Indications

Selection of a reduced x-ray tube voltage is indicated in thin patients to reduce radiation dose.

Technique Description

Electrons striking a tungsten target are accelerated by applying a potential difference between the positive and negative electrodes of an x-ray tube. The potential difference is described as the x-ray tube voltage in units of kilovolts (kV), and determines the energy of the accelerated electrons. Discrete values for peak x-ray tube voltage, the maximum voltage across the tube, are selectable on clinical CT scanners: 80 kVp, 100 kVp, 120 kVp, 135 kVp, and 140 kVp (specific tube voltages available vary with scanner type). A tube voltage of 120 kVp is standard for cardiac imaging and is suitable for most patients. Decreasing the peak tube voltage (kVp) decreases the energy of electrons striking the target and, subsequently, the energy and number of the x-rays produced.

Radiation dose with CT is approximately proportional to the square of the tube voltage such that a reduction in tube voltage from 120 to 100 kVp results in a 31% reduction in dose—assuming no other changes to dose related parameters are made. An additional benefit of decreased tube voltage is increased image contrast. Within the diagnostic CT energy range, lower energies produce greater differences in attenuation among body tissues. Decreased radiation dose and increased image contrast are achieved at the expense of increased image noise, however, because decreasing the peak tube voltage decreases the probable number of photons penetrating the patient and reaching the detector array. Image noise is proportional to 1/tube voltage such that a reduction in tube voltage from 120 to 100 kVp results in a 20% increase in image noise. In practice, selection of a lower tube voltage typically results in an automatic increase in tube current to minimize the negative impact on image noise, but with a net decrease in x-ray exposure.

As with x-ray tube current, x-ray tube voltage can be reduced according to patient size to avoid unnecessary exposure to slimmer patients, while still achieving acceptable image noise. Size can be assessed as described previously, and tube voltage can be adjusted accordingly. A more recent study showed a tube voltage of 100 kVp could be used to obtain diagnostic quality coronary CT angiography images from patients of normal weight with a 25% decrease in average effective dose compared with a tube voltage of 120 kVp; the average effective dose was decreased by 50% and diagnostic image quality was maintained when tube current was also reduced.¹¹

Pitfalls and Solutions

Hounsfield unit (HU)–dependent analysis of cardiac CT images is affected by changes in tube voltage because of associated changes in tissue attenuation characteristics. Automatic bolus timing methods rely on the attenuation of enhanced blood within a specified region of interest to reach a predefined threshold before initiating the scan. Because iodine absorption increases with a decrease in tube voltage, iodinated contrast material appears brighter on 100 kVp images compared with 120 kVp images. This may necessitate a higher threshold with automatic bolus timing to prevent premature initiation of the scan if the contrast injection protocol is unchanged. Alternatively, it may be possible to reduce iodinated contrast media volume at 100 kVp compared with 120 kVp, but maintain blood pool opacification.¹² Additionally, attenuation-based characterization of tissues typically relies on HU thresholds or ranges established with 120 kVp imaging that may not be appropriate at 100 kVp. Affected cardiac applications include, but are not limited to, quantification of coronary artery calcium, characterization of coronary artery plaque, and differentiation of simple fluid and blood products or soft tissue.

TECHNIQUE TO SHORTEN SCAN LENGTH

Indications

Scan length should be optimized for all patients to avoid unnecessary x-ray exposure.

Technique Description

The length of the scan in the z-direction (direction of patient movement) is defined using scout images. Increasing the scan length increases the irradiated portion of the body. Therefore, scan length is directly proportional to patient radiation exposure. Optimization of scan length from the scout image can be inexact, particularly when imaging the chest because of variation in the position of the heart with respiratory state. The scan start can be more accurately determined by performing an additional low-dose, cross-sectional scan before the diagnostic scan to verify anatomic location. The scan end should be carefully specified using standard landmarks on the scout image. If allowed by the scanner, manual stoppage of the scan should be initiated if the desired anatomy is covered before reaching the programmed value for the scan end.

Pitfalls and Solutions

For coronary artery imaging, the scan is typically started just below the tracheal carina. In patients with a large abdomen, the heart may shift upward significantly during a breath-hold, changing its position relative to the carina such that a scan start at the standard location would miss critical anatomy. Minimization of the scan range in these patients may be secondary to ensuring sufficient coverage and avoiding repeat scanning.

TECHNIQUE TO INCREASE BEAM PITCH (FOR HELICAL SCANNING)

Indications

With some CT scanners, an increased beam pitch may be indicated in cardiac patients with higher heart rates to reduce dose.

Technique Development

The spatial distribution of individual scans during helical imaging is described by the beam pitch. Specifically, beam pitch is defined as the table feed per gantry rotation (mm)/beam collimation (mm). An increase in the beam pitch results in less overlap between successive data acquisitions.

Radiation dose is inversely proportional to beam pitch such that a twofold increase in pitch results in a 50% reduction in dose. An increase in beam pitch causes degradation of the slice sensitivity profile and decreased spatial resolution, however, because data are averaged over a greater distance along the z-axis. In addition, increased pitch results in increased noise because of less data sampling. The exception is scanners that normalize tube current values to beam pitch (e.g., effective mAs, mAs/slice). In this case, x-ray flux is changed with beam pitch to maintain image noise, and dose savings is not realized. Some manufacturers fix beam pitch for a given clinical application to ensure image quality standards are maintained.

A low beam pitch (e.g., 0.2) or highly overlapping scans are required for imaging with fast gantry rotation to ensure continuous anatomic coverage with reconstruction of data from successive cardiac cycles. If the pitch is too high for the patient’s heart rate, the table moves too far between consecutive cardiac cycles to sample the data adequately, resulting in gaps in the imaged volume. This situation is of particular concern for multisegment reconstruction algorithms that use data from two or more consecutive cardiac cycles (rather than a single cardiac cycle) to reconstruct each image. Higher beam pitch (e.g., 0.2 to 0.5) can be used to image patients with higher heart rates, particularly when single-segment reconstruction algorithms are used.

Pitfalls and Solutions

A decrease in heart rate is sometimes observed after initiation of a breath-hold. If beam pitch is too high for the resulting heart rate, portions of the desired anatomy are missing from the reconstructed images. One solution is to practice breath-holding with the patient and decrease the pitch before scanning if a decrease in heart rate is observed. The pitch should also be lowered below the recommended value for the patient’s average heart rate if any heart rate irregularities are noted before scanning or during the practice breath-hold. Selecting a lower pitch results in increased dose, but ensures adequate anatomic coverage.

TECHNIQUE TO INCREASE BEAM FILTRATION

Indications

With some CT scanners, restriction of in-plane x-ray beam dimensions through selection of the SFOV may be indicated in smaller patients to reduce radiation dose.

Technique Development

X-rays generated for CT imaging are filtered before reaching the patient to limit x-ray exposure. Highly attenuating material is placed in the path of the x-ray beam to absorb unhelpful low-energy photons and to restrict the x-ray beam to the minimum required spatial dimensions. On some CT systems, additional in-plane beam filtration can be applied by the user through selection of the SFOV. On these systems, smaller SFOV can be selected for smaller patients to reduce radiation dose. With other scanners, data are obtained from the entire field, and FOV is a reconstruction parameter used to control the anatomic area displayed. When FOV is designated as a reconstruction parameter, it has no impact on in-plane beam filtration, and no impact on radiation dose.

Pitfalls and Solutions

If an additional filter is placed between the x-ray beam and the patient by selection of a smaller SFOV, any information outside the SFOV is unavailable for image reconstruction. Care must be taken to ensure that the entire region of interest is included within the SFOV. Lung fields may be excluded with a small SFOV for some cardiac patients.

TECHNIQUE TO INCREASE RECONSTRUCTED SLICE THICKNESS

Indications

The reconstruction of thicker images is indicated for non–contrast-enhanced evaluation of coronary calcium and the evaluation of larger cardiovascular anatomy (e.g., aorta, pulmonary vein). Thicker slices permit a reduction in x-ray tube current and, subsequently, a reduction in patient radiation dose.

Technique Development

The reconstructed slice thickness determines the number of absorbed x-ray photons contributing to the CT image. An increase in the reconstructed slice thickness increases the number of x-ray photons contributing to each image allowing a decrease in the number of absorbed x-ray photons (i.e., decrease in radiation dose) to achieve the same image noise (Fig. 10-2). Noise is proportional to such that a 3 mm thick image has 73% less noise than a 1 mm thick image. The decrease in tube current and radiation dose made possible by an increase in reconstructed slice thickness comes at a cost of decreased spatial resolution. The reconstruction of thicker slices is inappropriate for evaluation of small cardiac anatomy such as the coronary arteries.

FIGURE 10-2 A and B, Axial images from two different patients of similar size (lateral width at top of liver 39 cm for both) reconstructed with 0.75 mm slice thickness (A) and 1.5 mm slice thickness (B). To achieve noise equal to approximately 32 HU, 350 mAs/rotation was required in 0.75 mm thick images, but only 280 mAs/rotation was required to achieve the same noise level in 1.5 mm thick images. Reconstruction of thicker images is acceptable for evaluation of pulmonary vein anatomy, permitting data acquisition with a lower tube current and a resultant decrease in patient x-ray exposure compared with reconstruction of thinner images.

Pitfalls and Solutions

Increasing reconstructed slice thickness (a reconstruction parameter) has no direct effect on x-ray exposure. It is incumbent on the user also to decrease the x-ray tube current (a data acquisition parameter) to reduce patient radiation dose.

KEY POINTS

Reducing x-ray tube current is the most straightforward method to reduce radiation dose.

ECG-based tube current modulation can be used most aggressively in patients with low, stable heart rates.

The nominal tube current can be reduced for smaller patients before imaging, but anatomic-based, online modulation has limited utility for cardiac imaging.

Decreasing the tube voltage can reduce radiation dose and is appropriate in smaller patients, but affects the attenuation properties of tissues.

The scan length should be optimized in all patients to reduce x-ray exposure.

Beam pitch can be increased for patients with higher heart rates when single-segment reconstruction is employed, but the option is currently available from only one manufacturer.

An increase in reconstructed slice thickness permits a decrease in x-ray tube current for evaluation of larger cardiac anatomy.

SUGGESTED READINGS

Kalara MK, Maher MM, Toth TL, et al. Strategies for CT radiation dose optimization. Radiology. 2004;230:619-628.

Kubo T, Lin PP, Stiller W, et al. Radiation dose reduction in chest CT: a review. AJR Am J Roentgenol. 2008;190:335-343.

McNitt-Gray MF. AAPM/RSNA physics tutorial for residents: topics in CT. Radiation dose in CT. RadioGraphics. 2002;22:1541-1553.

Paul JF, Abada HT. Strategies for reduction of radiation dose in cardiac multislice CT. Eur Radiol. 2007;17:2028-2037.

Primack AN, McCollough CH, Bruesewitz MR, et al. Relationship between noise, dose, and pitch in cardiac multi-detector row CT. RadioGraphics. 2006;26:1785-1794.

1 Einstein AJ, Henzlova MJ, Rajagopalan S. Estimating risk of cancer associated with radiation exposure from 64-slice computed tomography coronary angiography. JAMA. 2007;298:317-323.

2 Earls JP, Berman EL, Urban BA, et al. Prospectively gated transverse coronary CT angiography versus retrospectively gated helical technique: improved image quality and reduced radiation dose. Radiology. 2008;246:742-753.

3 Husmann L, Valenta I, Gaemperli O, et al. Feasibility of low-dose coronary CT angiography: first experience with prospective ECG-gating. Eur Heart J. 2008;29:191-197.

4 Hausleiter J, Meyer T, Hermann F, et al. Estimated radiation dose associated with cardiac CT angiography. JAMA. 2009;301:500-507.

5 Jakobs T, Becker CR, Ohnesorge B, et al. Multislice helical CT of the heart with retrospective ECG gating: reduction of radiation exposure by ECG-controlled tube current modulation. Eur Radiol. 2002;12:1081-1086.

6 Stolzmann P, Scheffel H, Schertler T, et al. Radiation dose estimates in dual-source computed tomography coronary angiography. Eur Radiol. 2008;18:592-599.

7 Leschka S, Scheffel H, Desbiolles L, et al. Image quality and reconstruction intervals of dual-source CT coronary angiography: recommendations for ECG-pulsing windowing. Invest Radiol. 2007;42:543-549.

8 McCollough CH, Ulzheimer S, Halliburton SS, et al. A multi-institutional, multi-manufacturer, international standard for the quantification of coronary artery calcium using cardiac CT. Radiology. 2007;243:527-538.

9 Jung B, Mahnken AH, Stargardt A, et al. Individually weight-adapted examination protocol in retrospectively ECG-gated MSCT of the heart. Eur Radiol. 2003;13:2560-2566.

10 Mulkens TH, Bellinck P, Baeyaert M, et al. Use of an automatic exposure control mechanism for dose optimization in multi-detector row CT examinations: clinical evaluation. Radiology. 2005;237:213-223.

11 Leschka S, Stolzmann P, Schmid FT, et al. Low kilovoltage cardiac dual-source CT: attenuation, noise, and radiation dose. Eur Radiol. 2008;18:1809-1817.

12 Sigal-Cinqualbre AB, Hennequin R, Abada HT, et al. Low-kilovoltage multi-detector row chest CT in adults: feasibility and effect on image quality and iodine dose. Radiology. 2004;231:169-174.

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