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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Minimizing risk

Any efforts to minimize risk must start with scanning less frequently. Radiologists should continue to educate referring clinicians regarding imaging appropriateness to prevent unnecessary examinations. In Figure 2, for example, a pregnant woman with a ventriculoperitioneal shunt received 6 plain films as part of a shunt series. Three of the films included the fetus in the direct beam (note the inadequate shielding). This is not ideal given the low diagnostic yield of a shunt series. In a recent review of 192 shunt series, only 1% (2 of 192) shunt series changed clinical management when the head CT was negative or unchanged. 8

Requests for examinations that expose fetuses to radiation should be reviewed by a radiologist to make certain the examination is indicated. When performing an examination with a dose to the fetus <1 rem (10 mGy), there is no appreciable risk of lethality after the first 2 gestational weeks. The fetal risk decreases with increasing gestational age. In the first 2 weeks of gestation, however, radiation doses >1 rem could cause pregnancy loss. High radiation dose to the fetus during the major organogenesis period is more likely to result in congenital malformations than in lethality. 9 The developing brain is most sensitive in weeks 8 to 15 postconception; data from atomic bomb survivors reveals a threshold dose for mental retardation of approximately 0.3 Gy. The dose to a fetus is <0.005 mGy with a head CT and as high as 10 mGy with lumbar spine radiographs. 10

The responsibility for scanning less does not stop with the referring clinicians. Smart scanning is achieved when radiologists factor together the clinical concerns of the referring physicians with the radiation risks for patients. Weighing these issues, radiologists construct protocols that are diagnostic but, when possible, are performed with reduced radiation. Images acquired with less radiation represent a compromise between dose and diagnosis because reducing dose will increase noise. The last 3 decades of CT imaging have been driven by a desire for better resolution. This paradigm is changing as the magnitude of radiation exposure is realized.

Radiologists are familiar with some strategies for dose reduction, such as limiting the length of the scan. Other simple dose-reduction strategies include decreasing the tube current or tube voltage, increasing pitch, and choosing wider collimation. Decreasing tube current is often the most effective step toward dose reduction. Tube current correlates linearly with dose, so that halving tube current (mAs) will halve the dose. However, decreasing tube current will increase image noise. This concept is illustrated in Figure 3. When moving from 200 mAs to 100 mAs, noise increases by the square root of the dose reduction (200 mAs/100 mAs = 2; √2 =1.41). Thus the noise increase when halving the tube current is 41% (1.41 - 1 = 0.41). This noise increase can be mitigated by increasing slice thickness (Figure 4).

Lowering kVp has a greater effect on dose than does lowering tube current, but the loss of tissue contrast typically makes this approach unacceptable. However, low kVp (typically 80 kVp) is useful for all CTA head/neck and CT cerebral perfusion studies, since 80 kVp is near the k-edge of iodine.

Smart scanning avoids unnecessary irradiation of radiosensitive organs, such as the optic lens. There are several strategies to decrease radiation to the lens. When orbital pathology is not suspected clinically, the lens should be kept out of the irradiated field. When scanning a neck, for example, technologists can use the inferior orbital rim on the localizer for the most cranial slice, thereby avoiding the lens. On routine head CT, the lens can be avoided by tucking the patient's chin to the chest or by angling the gantry to exclude the orbits (Figure 5). Unfortunately, there are limitations to the angle of the gantry (30˚ for most MDCT scanners); therefore, the orbits may be exposed if the patient's neck is extended. Bismuth shields have also proven efficacious in reducing lens dose without compromising the evaluation of the brain. 11 Scanning protocols should be implemented to avoid scanning beyond the area of clinical concern. This requires vigilance, since the time cost for additional imaging is nearly imperceptible with today's MDCT scanners.

Sinus CT is low-lying fruit on the path to dose-reduction because high spatial resolution is not needed with such screening examinations. Patients' sinuses may be scanned repeatedly over time, and the optic lens is almost always irradiated. In a study by MacLennan, 12 the mean lens dose varied from 70.3 mGy when scanned at a tube current of 475 mAs, 17.6 mGy at a tube current of 210 mAs, and 4.7 mGy at a tube current of 30 mAs. There have been many approaches to minimizing radiation dose during sinus CT. 13-16 Hojreh et al 15 reported the diagnostic quality of low-dose scans, recommending a tube current of 50 mAs (with 140 kV and a collimation of 16 × 0.75 mm). Figure 6 shows the diagnostic quality of lower-dose sinus scanning.

CT perfusion imaging carries a high radiation cost. When imaging for middle cerebral artery (MCA) territory infarcts, the most caudal slice should be at the superior orbital rim. When performing perfusion on a 40- or 64-detector scanner, most of the MCA territory will be included in the 4-cm slab acquired.

Smart scanning also uses the significant advances that manufacturers are making in reducing radiation dose with MDCT.

Manufacturers have taken several different approaches to reducing dose. GE Medical Systems uses "Auto mA" software. Philips uses "Dose Right." Siemens uses a "CARE Dose" application packet. Toshiba uses the "Real EC." Each of these applications attempts to account for the varied tube current required when imaging patients of different body shapes and composition. When scanning the neck, for example, tube modulation software will reduce tube current when scanning the smallest portions of the upper neck but will increase tube current when scanning through the shoulders at the thoracic inlet (see Figure 7). Tube modulation eliminates the need to scan the entire neck at the high tube current necessary to scan through the shoulders. Modulation can be performed along the z-axis, according to a localizer sequence; or in the angular plane, adjusting with each rotation, from the data of the prior rotation. Such software advances allow a more constant image quality, while reducing unnecessary radiation. In neuroradiology, these should be widely implemented for neck scans and can also be used with most other scans. 17

Conclusion

With knowlege of general MDCT dose guidelines, radiologists can find the necessary balance between maximizing image resolution and minimizing radiation dose. Protocols should be dose conscious. Referring clinicians will need to be included in discussions of radiation risks, or they may balk at changes in image quality. Radiologists' efforts should be wide ranging but can initially be targeted at pediatric patients who are at the greatest risk for stochastic effects. Dose-reduction software from manufacturers can help keep images diagnostic by maintaining an acceptable noise level. During the last 5 years, much has been learned from research on radiation dose and risks. It is time for clinical implementation.

REFERENCES

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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.
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    Figure 2b.
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    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
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  • 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.
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  • 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.
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