Special issue: Radiation dose optimizationDose optimizationRadiation Dose Optimization for CT-Guided Interventional Procedures in the Abdomen and Pelvis
Introduction
The principle of “as low as reasonably achievable” should be applied to all CT examinations, including CT-guided interventional procedures. Although these examinations represent a small percentage of all CT examinations, the radiation doses are not trivial. For example, in 571 patients undergoing CT-guided interventional procedures at a large academic center, mean effective doses of 119.7 ± 50.3, 25.3 ± 15.4, 20.1 ± 11.0, 13.8 ± 9.2, and 9.1 ± 5.5 mSv were reported for cryoablations, drain placements, aspirations, biopsies, and injections, respectively [1]. Doses can be especially high for CT-guided tumor ablations, with another group reporting a mean effective dose of 72 ± 18 mSv (range, 46–117 mSv) for cryoablation of liver tumors [2]. There is variability in technical skills and both interest and knowledge in optimizing CT dose parameters among radiologists performing these procedures, which results in a wide range of radiation doses for procedures similar in scope and complexity 1, 3. The general absence of defined protocols for CT-guided interventional procedures, a tendency for radiologists and technologists to use default diagnostic CT settings for interventional scans, discomfort among radiologists when reviewing more noisy images, and a perception that the target lesion may be difficult to see at lower radiation doses all contribute to a higher than needed radiation dose. This paper presents an overview of strategies for optimizing dose for CT-guided interventional procedures.
Section snippets
Use of an Alternative Guidance Modality
CT, ultrasound, and fluoroscopy are all used for imaging guidance for interventional procedures. Biopsy of a mass, aspiration or drainage of a fluid collection, and ablation of a tumor are common interventional procedures. The choice of the guidance modality depends on the local expertise, radiologist preference, the availability of equipment, and space on the clinical schedule.
Whenever feasible, ultrasound should be used for guidance because this will result in an overall decrease in the
Tube Current
CT radiation dose is directly proportional to the tube current–time product (expressed as milliampere-seconds), and thus reducing the tube current–time product will reduce the dose. Although decreasing the tube current–time product will increase image noise (and degrade image quality), image quality needs to be just enough to allow visualization of the target and interventional instruments. The tube current–time product can be lowered by decreasing either the tube current or the gantry rotation
Scan Classes in an Interventional Procedure
Scans obtained during the course of a CT-guided interventional procedure include an initial scan for target localization and planning, multiple short-range scans for interventional guidance or monitoring, and in most cases a postprocedural scan to evaluate for therapeutic results or a procedure-related complication.
Target Localization and Planning Scan
The initial scan in a procedure is obtained for localizing the target and planning the interventional path. Because the target spans only a small cranial-caudal distance, the length of this scan should be restricted only to the anatomic location of interventional target. A careful review of prior CT images can help identify the superior and inferior scanning limits to prevent overscanning of unnecessary anatomy. Bony landmarks can be used for this purpose, and these can usually be identified on
Interventional Guidance and Monitoring Scans
Next, a CT-guided interventional procedure entails placement of a needle, guide wire, drainage catheter, ablation electrode, or probe into the target. Repeated scans are often required to accurately guide device placement and avoid injury to adjacent structures. The greater the number of these scans, the higher the dose. Repeat scans result in additive dose accumulation and thus can make a large contribution to the procedural dose. This is especially true for tumor ablations, which additionally
Postprocedural Scan
A postprocedural scan is commonly obtained after the completion of the interventional procedure, to document therapeutic resolve and determine the presence or absence of postprocedural bleeding or hematoma, pneumothorax, or inadvertent injury to nontarget organs. A small amount of bleeding that is clinically insignificant after a procedure is not uncommon. Hence, a postprocedural scan may not be routinely needed in all cases, and its need should be assessed on the basis of the complexity of the
Optimal Tube Current
Patient sizes generally do not vary much over the short cranio-caudal length of interventional scans. Automated dose modulation parameters (such as the noise index or reference tube current–time product) are harder to prescribe when obtaining ultra-low-dose scans. Hence, the use of fixed tube current settings is preferred over dose modulation when obtaining interventional scans.
Studies evaluating optimal dose parameters for CT-guided interventional procedures are lacking. Lucey et al [22]
Improving Target Contrast
Lucey et al [22] reported that technical failure for CT-guided procedures done at low tube current–time product settings of 30 mAs chiefly resulted from the low target-to-background contrast of these lesions, the majority of which were in the liver. Liver and renal lesions can have low inherent lesion-to-background contrast on unenhanced CT, especially if small in size and completely surrounded by parenchyma. Similarly, small lymph nodes in the abdomen and pelvis may be difficult to discern
Conclusions
The approaches outlined in this paper (lowering the tube current, reducing scan length, refining techniques to reduce the number of guidance scans, and using the axial or intermittent fluoroscopy mode for guidance scans) will lower the radiation dose for CT-guided interventional procedures. Adherence to such principles has been shown to result in dramatic dose reductions for CT-guided interventional procedures of the spine, achieving a mean decrease in the procedure dose length product by a
Take-Home Points
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An ionizing radiation–free imaging modality such as ultrasound should be used, whenever feasible, for guiding interventional procedures.
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The cranio-caudal scan length should be restricted to the minimum needed for the planning, guidance and monitoring, and postprocedural scans.
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All scans, especially repeated guidance scans, should be performed at lower tube current–time products compared with the preprocedural diagnostic scan.
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Intermittent-mode CT fluoroscopy and the axial scanning mode are more
References (24)
- et al.
Estimated effective dose of CT-guided percutaneous cryoablation of liver tumors
Eur J Radiol
(2012) - et al.
Percutaneous needle biopsy for small lung nodules beneath the rib under CT scan fluoroscopic guidance with gantry tilt
Chest
(2004) - et al.
Optimizing dose in computed tomographic guided procedures
Techn Vasc Interv Radiol
(2010) - et al.
Radiation dose levels for interventional CT procedures
AJR Am J Roentgenol
(2011) - et al.
Reducing patient radiation dose during CT-guided procedures: demonstration in spinal injections for pain
AJNR Am J Neuroradiol
(2011) - et al.
The challenging image-guided abdominal mass biopsy: established and emerging techniques “if you can see it, you can biopsy it”
Abdom Imaging
(2013) - et al.
Abdominal percutaneous interventional procedures: comparison of CT and US guidance
Radiology
(1998) - et al.
Reducing radiation dose in body CT: a primer on dose metrics and key CT technical parameters
AJR Am J Roentgenol
(2013) - et al.
Optimization of kVp and mAs for pediatric low-dose simulated abdominal CT: is it best to base parameter selection on object circumference?
AJR Am J Roentgenol
(2010) - et al.
Optimization of kilovoltage and tube current-exposure time product based on abdominal circumference: an oval phantom study for pediatric abdominal CT
AJR Am J Roentgenol
(2012)
Using the K-edge to improve contrast conspicuity and to lower radiation dose with a 16-MDCT: a phantom and human study
J Comput Assist Tomogr
Low-dose techniques in CT-guided interventions
Radiographics
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