Special issue: Radiation dose optimization
Dose optimization
Radiation Dose Optimization for CT-Guided Interventional Procedures in the Abdomen and Pelvis

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Radiation dose to patients can be high for some CT-guided interventional procedures in the abdomen and pelvis, especially tumor ablations. Strategies for radiation dose reduction include choosing an alternative guidance modality that does not use radiation whenever feasible, restricting the cranio-caudal length of interventional scans to the interventional target, and refinement of technical skills in order to minimize the number of scans acquired for interventional guidance. Dose optimization for these procedures is best achieved by lowering the tube current relative to the prior diagnostic scan, choosing dose efficient scanning modes, and using intermittent-mode, narrowly collimated CT fluoroscopy for interventional guidance.

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

  • An ionizing radiation–free imaging modality such as ultrasound should be used, whenever feasible, for guiding interventional procedures.

  • The cranio-caudal scan length should be restricted to the minimum needed for the planning, guidance and monitoring, and postprocedural scans.

  • All scans, especially repeated guidance scans, should be performed at lower tube current–time products compared with the preprocedural diagnostic scan.

  • Intermittent-mode CT fluoroscopy and the axial scanning mode are more

References (24)

  • B.K. Park et al.

    Estimated effective dose of CT-guided percutaneous cryoablation of liver tumors

    Eur J Radiol

    (2012)
  • T. Yamagami et al.

    Percutaneous needle biopsy for small lung nodules beneath the rib under CT scan fluoroscopic guidance with gantry tilt

    Chest

    (2004)
  • R.G. Dixon et al.

    Optimizing dose in computed tomographic guided procedures

    Techn Vasc Interv Radiol

    (2010)
  • S. Leng et al.

    Radiation dose levels for interventional CT procedures

    AJR Am J Roentgenol

    (2011)
  • T.M. Shepherd et al.

    Reducing patient radiation dose during CT-guided procedures: demonstration in spinal injections for pain

    AJNR Am J Neuroradiol

    (2011)
  • N.I. Sainani 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)
  • D.H. Sheafor et al.

    Abdominal percutaneous interventional procedures: comparison of CT and US guidance

    Radiology

    (1998)
  • P.D. Maldjian et al.

    Reducing radiation dose in body CT: a primer on dose metrics and key CT technical parameters

    AJR Am J Roentgenol

    (2013)
  • J. Reid 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)
  • F. Dong 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)
  • S.P. Kalva et al.

    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

    (2006)
  • M. Sarti et al.

    Low-dose techniques in CT-guided interventions

    Radiographics

    (2012)
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