Minimizing radiation risks with MDCT in neuroradiology

Cari Buckingham, MD, and Megan K. Strother, MD

The medical community and mass media have publicized risks of radiation exposure from CT. It is possible to balance the requirements of maximizing image resolution while minimizing radiation dose. In this article, the authors provide practical recommendations for reducing CT radiation from neuroradiologic imaging.

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Dr. Buckingham is a Fellow and Dr. Strother is an Assistant Professor of Neuroradiology, Department of Radiology, Vanderbilt University Medical Center, Nashville, TN.

In November 2007, the New York Times reported, "Millions of Americans, especially children, are needlessly getting dangerous radiation from 'super X-rays' that raise the risk of cancer and are increasingly used to diagnose medical problems…." This headline is inflammatory; but the core message, coming from the medical community and echoed in the media, is that radiation should be minimized. Success in minimizing radiation will require a multifaceted approach, with a collective effort from radiologists, referring clinicians, and computed tomography (CT) manufacturers. In this article, we provide practical recommendations for reducing CT radiation from neuroradiologic imaging.

Measuring risk

Radiation risks are both stochastic and deterministic. Deterministic effects, such as radiation burns or cataracts, are easy to document and therefore predict. With 0.5 to 2 Gy dose to the lens, for example, patients are at risk for cataracts. 1 Stochastic effects, such as cancer, are much more difficult to predict. They may or may not occur, after a long delay, at a rate nearly imperceptible above baseline, with or without a threshold dose. It is impossible to speak definitively about these risks-an uncertainty that has hampered efforts at risk reduction. Fears regarding stochastic effects are driving the current wave of alarm regarding CT. Because stochastic effects generally occur years after radiation exposure, pediatric patients are at the greatest risk. Pediatric patients' risks are also higher than adult patients' risks because of the proportion of rapidly dividing-and therefore radiosensitive-cells in children. For this reason, efforts at CT dose reduction should start with pediatric patients.

With multidetector CT (MDCT), several parameters are important for understanding radiation dose. Dose efficiency is a measure of how much of the irradiated tissue is included in the final images. With poor efficiency, significant radiation extends beyond the boundary of the imaged area to the area called the penumbra. Thus collimation affects radiation dose. Whereas with a single detector, collimation equals slice thickness, with MDCT, collimation determines the number of detectors exposed to the radiation beam. Each manufacturer has its own detector array and data acquisition systems. The source data acquired defines the width of the smallest section that can be reconstructed (Figure 1). Some collimation/detector protocols maximize coverage (eg, expose the greatest width per 360° scanner rotation); others maximize resolution (eg, expose only the narrowest detectors centrally).

When choosing scanner protocols, radiologists should keep a few general guidelines for dose efficiency in mind. The dose consequence is highest with the smallest beam width; therefore, efficiency improves with the number of detectors exposed. Thus, it is more efficient to expose all detectors than to collimate to the central detectors. Verdun et al 2 reported dose efficiency of 96% when exposing all 64 detectors versus a dose efficiency of only 67% when collimating to the 8 central detectors of a 64-detector scanner. The 64-detector scanner is much more efficient than a 4- or 8-detector scanner at acquiring thin sections for isotropic imaging. With a 4-detector scanner, acquiring images at 1.25-mm section thickness decreases the dose efficiency to 66% and more than doubles the CT dose index volume (CTDI volume, see below). Even on a 64-detector scanner, acquiring isotropic data may lead to increased radiation if the tube current is increased to compensate for the increased noise that plagues thin sections. Reviewing thicker sections makes noise acceptable and, therefore, allows us to limit dose. Thus the helpful dictum, "acquire thin, review thick" when possible.

There are many ways to measure radiation dose. The most helpful clinical parameters are CT dose index volume (CTDI vol ) and the dose-length product (DLP). These are included on most recent MDCT scanners. The CT dose index is a measure of the energy absorbed divided by the unit mass. The weighted CT dose index (CTDI w ) factors in the change in radiation dose across the depth scanned:

CTDI W = 1 ⁄ 3 (CTDI 100 ) center + 2 ⁄ 3 (CTDI 100 ) peripheral

where (CTDI 100 ) peripheral represents an average of 4 different measurements in the periphery of the phantom. For a standard head CT, which is approximated by a 16-cm phantom, the dose does not change significantly from the skin to the center of the patient's head. The CTDI vol is the weighted CT dose index divided by the pitch:

CTDI vol = CTDI W /pitch

Pitch is the distance the table travels per rotation divided by the beam collimation. Dose varies inversely with pitch. For example, if pitch is doubled, the dose is halved. Since CTDI measures are acquired using phantoms, they do not take into account the shape or composition of any individual patient. 3

The DLP is the CTDI vol multiplied by the length of the scan:

DLP = CTDI vol × length of scan (cm)

Both the CTDI volume and the DLP can be used to compare individual scanning protocols with diagnostic reference levels (DRLs). Diagnostic reference levels are intended to represent the 75 th percentile of radiation dose for a given examination. It is acceptable to exceed the DRL for any given patient, but doses should not routinely rise above the DRL. The most recent American College of Radiology guideline, adopted January 2008, recommends a DRL of 75 mGy for the CTDI vol when scanning an adult head. 4 A final useful measure of radiation dose is the effective dose, which attempts to reflect the relative radiosensitivity of organs by incorporating a tissue-weighting factor. Typical effective doses are listed in Table 1. 5-7

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Tables & Figures

  • Figure 1. This diagram illustrates that collimation is not synonymous with slice thickness with multidetector CT. Detector configuration and collimation determine section thickness. Section thickness acquired as source data determines the slice thickness that is possible in the reconstructed images.
    Figure 1.
  • Figure 2a. These (A and B) anteroposterior and (C) lateral radiographs are from an abdominal shunt series performed on a pregnant patient. (B and C) The shielding did not adequately protect the fetus. In the setting of a normal head CT, the very low yield for this study should be balanced against the radiation risk to the fetus.
    Figure 2a.
  • Figure 2b.
    Figure 2b.
  • Figure 2c.
    Figure 2c.
  • Figure 3a. These axial CT images of the head from the same patient were performed with 2 different scanning protocols. (A) This image was acquired in 2004 using 140 kV, 460 mAs, and 5-mm slices. (B) This image was acquired in 2008 using 120 kV, 200 mAs, and 5-mm slices. The scan shown in image A required more than twice the radiation dose than did the later head CT (image B). (B) The increased noise in this image ob-scures the soft tissue contrast delineating the internal capsule and basal ganglia seen in image A. This may be unacceptable noise for delineation of subtle subarachnoid hemorrhage or infarct, but it is certainly acceptable for the “rule-out hydrocephalus” workup that had been requested in this patient.
    Figure 3a.
  • Figure 3b.
    Figure 3b.
  • Figure 4a. Increased slice thickness can compensate (in part) for reduced dose. (A) This scan required twice the radiation dose as (B) the second image. The lower dose increased noise by 41%. This increased noise was mitigated by increasing the slice thickness (5-mm slice thickness in image B compared with 2.5-mm slice thickness in image A).
    Figure 4a.
  • Figure 4b.
    Figure 4b.
  • Figure 5a. The lens can be excluded during many head CTs.  (A) In this scout radiograph from a head CT, the lens was excluded by angling the gantry. (B) The lower-most image was acquired by angling the gantry in this patient. (C) In this scout radiograph from the lowest image in a perfusion CT, the lens was avoided by flexing the patient
    Figure 5a.
  • Figure 5b.
    Figure 5b.
  • Figure 5c.
    Figure 5c.
  • Figure 5d.
    Figure 5d.
  • Figure 6a. (A and C) Axial and (B and D) coronal images from sinus CT of the same patient at different doses. (A and B) These images were obtained with mAs 175. (C and D) These images were obtained with mAs 50. Decreased dose has a negligible effect on image quality of screening sinus CT.
    Figure 6a.
  • Figure 6b.
    Figure 6b.
  • Figure 6c.
    Figure 6c.
  • Figure 6d.
    Figure 6d.
  • Figure 7a. (A through F) Varying the parameters prevents unnecessary radiation dose. These images from a neck CT were acquired with tube-modulation software. The dose is automatically modified by noise level. Thus, the tube current is highest at the level of the shoulders, and lowest as it reaches the thin upper neck.
    Figure 7a.
  • Figure 7b.
    Figure 7b.
  • Figure 7c.
    Figure 7c.
  • Figure 7d.
    Figure 7d.
  • Figure 7e.
    Figure 7e.
  • Figure 7f.
    Figure 7f.