Radiologic imaging in cystic fibrosis: cumulative effective dose and changing trends over 2 decades

Scoring of chest CT in children with cystic fibrosis: state of the art

Chest CT has been proposed as a surrogate outcome measure in the evaluation of cystic fibrosis lung disease. Quantitative evaluation of chest CT findings requires application of a scoring system to derive numerical values. Several scoring systems are in use. These mostly rely on a subjective judgement of the severity and extent of various features of cystic fibrosis lung disease, including bronchiectasis, bronchial wall thickening, mucous plugging and air-trapping. Scores can subsequently be added to produce a total score. The precision or reproducibility of scoring systems has been assessed but with heterogeneous statistical approaches. Total scores appear to have high levels of reproducibility, but this might mask poorer levels of agreement for individual observations and component scores. It can also be questioned whether total scores are biologically meaningful, as compared to assessments of individual features. Various studies suggest that CT scores give an accurate indicator of the severity of disease, and CT scores might be the best predictors of long-term outcome, but data in this area are limited. CT scores are more sensitive than traditional lung-function indices such as FEV; however the lung clearance index, by multiple breath washout, appears to offer comparable sensitivity to CT. It is not clear whether CT scores are adequately responsive to changes in disease severity in the short to medium term; this is a challenge to the use of CT as a surrogate outcome measure for clinical trials of therapies specific to cystic fibrosis. Cystic fibrosis scoring would benefit from greater levels of standardisation in terms of CT techniques, scoring system, training of observers and measures of reproducibility. Automated approaches to quantifying CT parameters might also offer improved precision. The benefits of chest CT must be weighed against the principal drawback of radiation exposure. The case for more widespread use of chest CT would be strengthened if precision of CT scoring were improved.

Radiation dose reduction in chest CT--review of available options

Computed tomography currently accounts for the majority of radiation exposure related to medical imaging. Although technological improvement of CT scanners has reduced the radiation dose of individual examinations, the benefit was overshadowed by the rapid increase in the number of CT examinations. Radiation exposure from CT examination should be kept as low as reasonably possible for patient safety. Measures to avoid inappropriate CT examinations are needed. Principles and information on radiation dose reduction in chest CT are reviewed in this article. The reduction of tube current and tube potential are the mainstays of dose reduction methods. Study results indicate that routine protocols with reduced tube current are feasible with diagnostic results comparable to conventional standard dose protocols. Tube current adjustment is facilitated by the advent of automatic tube current modulation systems by setting the appropriate image quality level for the purpose of the examination. Tube potential reduction is an effective method for CT pulmonary angiography. Tube potential reduction often requires higher tube current for satisfactory image quality, but may still contribute to significant radiation dose reduction. Use of lower tube potential also has considerable advantage for smaller patients. Improvement in image production, especially the introduction of iterative reconstruction methods, is expected to lower radiation dose significantly. Radiation dose reduction in CT is a multifaceted issue. Understanding these aspects leads to an optimal solution for various indications of chest CT.

A prospective feasibility study of sub-millisievert abdominopelvic CT using iterative reconstruction in Crohn's disease

OBJECTIVE

Iterative reconstruction (IR) allows diagnostic CT imaging with less radiation exposure than filtered back projection (FBP). We studied an IR low-dose CT abdomen/pelvis (LDCTAP) protocol, designed to image at an effective dose (ED) approximating 1 mSv in patients with Crohn's disease (CD).

METHODS

Forty patients, mean age 37 ± 13.4 years (range 17-69), with CD underwent two synchronous CT protocols (conventional-dose (CDCTAP) and LDCTAP). CDCTAP and LDCTAP images were compared for diagnostic acceptability, yield, image quality and ED (in millisieverts). The optimal level of IR for LDCTAP was also studied.

RESULTS

LDCTAP yielded a mean ED of 1.3 ± 0.8 mSv compared with 4.7 ± 2.9 mSv for CDCTAP, reducing ED by 73.7 ± 3.3 % (mean dose reduction, 3.5 ± 2.1 mSv; P < 0.001) and dose length product by 73.6 ± 2.6 % (P < 0.001). Sub-millisievert (0.84 mSv) imaging was performed for patients with a body mass index (BMI) less than 25 (i.e. 63 % of our cohort). LDCTAP resulted in increased image noise and reduced diagnostic acceptability compared with CDCTAP despite use of IR, but detection of extra-luminal complications was comparable.

CONCLUSION

Patients with suspected active CD can be adequately imaged using LDCTAP, yielding comparable information regarding extent, activity and complications of CD compared with CDCTAP, but with 74 % less dose. LDCTAP at doses equivalent to that of two abdominal radiographs represents a feasible alternative to CDCTAP.

KEY POINTS

• Radiation dose is a concern when imaging patients with Crohn's disease. • New techniques allow low-dose abdominopelvic CT with acceptable image quality. • Using hybrid iterative reconstruction, its diagnostic yield compares well with that of conventional CT. • Sub-millisievert CT of patients with Crohn's disease appears technically and clinically feasible.