JACR: Strategies for sub-mSv head CT studies

A medical physics consult in the August issue of the Journal of the American College of Radiology (JACR) outlined the balancing act between dose reduction and image noise in head CT studies, and explained that lower fixed-tube current or automatic exposure control techniques can slash dose for both adult and pediatric head CT, with some studies regularly achieving the sub-mSv threshold.

Although MRI has emerged as the preferred modality for many brain and spinal exams, head CT remains very common, accounting for 28 percent of all CT studies, offered Sarabjeet Singh, MD, MBBS, of Massachusetts General Hospital in Boston, and colleagues.

The authors shared multiple strategies for cutting radiation dose in CT studies of the head, sinuses and spine in adult and pediatric patients, including:

• Ensure appropriate use. Physicians need to ensure that CT is expected to provide information that will affect patient management.
• Avoid guesswork. Save scanning protocols by exam type or clinical indication on the user interface or in explicit documents.
• Benchmark. Sites should benchmark adult protocols to fall below the ACR’s recommended diagnostic CT reference dose of 75 mGy volume dose index.
• Lower the tube current. Studies have indicated that lowering the tube current can halve the dose for head CT while maintaining image quality. For specific clinical indications in pediatric patients, both the peak voltage and tube current can be reduced to obtain diagnostic data at doses in the sub-mSv range.
• Employ automatic exposure control (AEC). Studies have demonstrated that AEC cuts dose. Use of longitudinal AEC dropped volume CT dose index for unenhanced head CT to 36 mGy in adults and 28 mGy in children, shared the authors. Similarly, use of AEC translated into dose reduction up to 38 percent compared with fixed tube scanning for head and neck vascular exams, according to a separate study.
• Optimize repeat or follow-up pediatric studies. “Although initial CT targets subtle findings, repeat CT is generally performed to assess complications or gross morphologic challenges,” wrote the authors. Both tube voltage and tube current can be reduced in studies to evaluate shunt patency, ventricle size, length and location of drain. The result can be a sub-mSv study.
• Consider patient selection. Because comprehensive stroke protocols are associated with fairly high doses (in the 16 to 20 mSv range), users need to weigh the risks and benefits of head perfusion CT in such patients. In addition, all perfusion head CT studies should be performed at 80 kVp to reduce dose and enhance sensitivity.
• Exploit anatomical contrast when possible. “CT of the paranasal sinuses and facial and dental structures should be performed at radiation doses lower than head CT because high inherent contrast between bone, air and soft tissues allows lesion visualization even in the presence of very high noise images,” explained Singh et al. In pediatric patients, doses for sinus CT studies may be on par with standard digital x-ray—0.05 mSv.