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.
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.
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
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.
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
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
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.
Hojreh et al
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
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
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
- Valentin J. Relative biological effectiveness (RBE), quality
factor (Q), and radiation weighting factor (wR): ICRP Publication
. 2003; 33(4):93-95.
- Verdun FR, Gutierrez D, Schnyder P, et al. CT dose
optimization when changing to CT multi-detector row technology.
Curr Probl Diagn Radiol.
- McNitt-Gray MF. AAPM/RSNA Physics Tutorial for Residents:
Topics in CT. Radiation dose in CT.
- American College of Radiology. ACR Practice Guideline for
diagnostic reference levels in medical x-ray imaging (Resolution
3). Adopted 2008: 802.
- Mettler FA Jr, Upton AC.
Medical Effects of Ionizing Radiation.
2nd ed. Philadelphia, PA: Saunders;1995.
- Wall BF, Hart D. Revised radiation doses for typical X-ray
examinations. Report on a recent review of doses to patients from
medical X-ray examinations in the UK by NRPB.National
Radiological Protection Board.
Br J Radiol.
- Hadley, JL, Agola J, Wong P. Potential impact of the American
College of Radiology appropriateness criteria on CT for trauma.
Am J Roentgenol.
- Griffey RT, Ledbetter S, Khorasani R. Yield and utility of
radiographic "shunt series" in the evaluation of
ventriculo-peritoneal shunt malfunction in adult emergency
- Valentin J. Biological effects after prenatal irradiation
(embryo and fetus). Publication 90.
- Sharp C, Shrimpton JA, Bury RF. Diagnostic medical exposures:
Exposure to ionising radiation of pregnant women.
- Mukundan S Jr, Wang PI, Frush DP, et al. MOSFET dosimetry for
radiation dose assessment of bismuth shielding of the eye in
AJR Am J Roentgenol.
- MacLennan AC. Radiation dose to the lens from coronal CT
scanning of the sinuses.
- Tack D, Widelec J, De Maertelaer V, et al. Comparison between
low-dose and standard-dose multidetector CT in patients with
suspected chronic sinusitis.
AJR Am J Roentgenol.
- Hagtvedt T,Aalokken TM, Notthelien J, Kolbenstvedt A. A new
low-dose CT examination compared with standard-dose CT in the
diagnosis of acute sinusitis.
- Hojreh A, Czerny C, Kainberger F. Dose classification scheme
for computed tomography of the paranasal sinuses.
Eur J Radiol.
- Bassim MK, Ebert CS, Sit RC, Senior BA. Radiation dose to the
eyes and parotids during CT of the sinuses.
Otolaryngol Head Neck Surg.
- Smith AB, Dillon WP, Gould R, Wintermark M. Radiation
dose-reduction strategies for neuroradiology CT protocols.
AJNR Am J Neuroradiol.