PATIENT SAFETY Effects of Increased Image Noise on ImageQuality and Quantitative Interpretation in Brain CT Perfusion

BACKGROUNDAND PURPOSE: There is a desire within many institutions to reduce the radiation dose in CTP examinations. The purpose of this study was to simulate dose reduction through the addition of noise in brain CT perfusion examinations and to determine the subsequent effects on quality and quantitative interpretation. MATERIALS ANDMETHODS: A total of 22 consecutive reference CTP scans were identified from an institutional review board–approved prospective clinical trial, all performed at 80 keV and 190 mAs. Lower-dose scans at 188, 177, 167, 127, and 44 mAs were generated through the addition of spatially correlated noise to the reference scans. A standard software package was used to generate CBF, CBV, and MTT maps. Six blinded radiologists determined quality scores of simulated scans on a Likert scale. Quantitative differences were calculated. RESULTS: For qualitative analysis, the correlation coefficients for CBF ( 0.34; P .0001), CBV ( 0.35; P .0001), and MTT ( 0.44; P .0001) were statistically significant. Interobserver agreements in quality for the simulated 188-, 177-, 167-, 127-, and 44-mAs scans for CBF were 0.95, 0.98, 0.98, 0.95, and 0.52, respectively. Interobserver agreements in quality for the simulated CBV were 1, 1, 1, 1, and 0.83, respectively. For MTT, the interobserver agreements were 0.83, 0.86, 0.88, 0.74, and 0.05, respectively. For quantitative analysis, only the lowest simulated dose of 44 mAs showed statistically significant differences from the reference scan values for CBF ( 1.8; P .04), CBV (0.07; P .0001), and MTT (0.46; P .0001). CONCLUSIONS: From a reference CTP study performed at 80 keV and 190mAs, this simulation study demonstrates the potential of a 33% reduction in tube current and dose while maintaining image quality and quantitative interpretations. This work can be used to inform future studies by using true, nonsimulated scans. ABBREVIATIONS: ALARA as low as reasonably achievable; CTDI-vol CT dose index volume; PABAK prevalence-adjusted bias-adjusted values CTP is an imaging technique that allows rapid, noninvasive, quantitative evaluation of the hemodynamic status of the brain by generating parametric maps of CBF, CBV, and MTT. Frequently described applications of CTP in the literature include assessment of acute stroke,delayed cerebral ischemia related to vasospasm, and brain tumor imaging. The technique involves intravenous contrast injection and cine scanning with repetitive imaging through several sections of the brain to track the first pass of the contrast bolus. High radiation doses associated with this technique, often performed in combination with noncontrast CT and CT angiography, have prompted notifications by the US Food and Drug Administration that stress the importance of proper acquisition parameters. As a response to these concerns, the radiology community has heightened global efforts to reduce radiation dose in a variety of radiologic examinations, and CTP in particular, by optimizing imaging protocols under the ALARA principle. Major factors affecting radiation dose during a CT study include tube current, tube rotation time, peak voltage, pitch, and collimation. Changes in all of these parameters have various effects on image quality. Tube current is directly proportional to dose, and reductions in tube current primarily affect image noise without notable effects on other metrics of image quality such as image contrast. Most CTP techniques have a fixed peak voltage at 80 kVp to optimize image contrast but vary in tube current, with some reports as low as 100 –120 mAs. Through a quality improvement program at our institution, we desired to achieve a 33% reduction Received June 29, 2012; accepted after revision October 22. From the Department of Radiology, Weill Cornell Medical College, New YorkPresbyterian Hospital, New York, New York. Please address correspondence to Krishna Juluru, MD, Weill Cornell Medical College, 525 E. 68th St, F-056, New York, NY 10065; e-mail: [email protected] Indicates open access to non-subscribers at www.ajnr.org Indicates article with supplemental on-line figures http://dx.doi.org/10.3174/ajnr.A3448 1506 Juluru Aug 2013 www.ajnr.org in CTP dose by reducing tube current to approximately 125 mAs from our reference protocol performed at 190 mAs. The consequent changes in image quality, particularly from increased image noise, warranted further investigation before this protocol change was implemented. Furthermore, in recent years there has been the introduction of several postprocessing algorithms designed to reduce noise in CT scans acquired at lower doses. The threshold dose at which image quality changes significantly, and at which these algorithms could then subsequently add value, needs assessment. The purpose of this study was to use published techniques for noise addition to CT datasets to simulate dose-reduction to determine the subsequent effects on image quality and quantitative evaluation of CTP examinations. MATERIALS AND METHODS Study Population In this case-control study, consecutive CTP examinations were retrospectively identified from an institutional review board– approved, prospective clinical trial at our institution from August 2007–June 2010. Inclusion criteria for the prospective clinical trial were adult patients ( 18 years) with documented aneurysmal subarachnoid hemorrhage at admission. Aneurysmal subarachnoid hemorrhage was diagnosed on initial noncontrast head CT, CSF analysis, CT angiography, and/or digital subtraction angiography. Exclusion criteria were a history of iodinated contrast allergy, renal impairment, or pregnancy. CTP Scanning Technique We performed CTP examinations by using a standard protocol at our institution, on LightSpeed or Pro-16 scanners (GE Healthcare, Milwaukee, Wisconsin), with cine 4i scanning mode and 45-s acquisition at 1 rotation per second by using 80 kVp and 190 mAs. CTDI-vol was 725.21 mGy, and the doselength product was 1450.42 mGy-cm. Four sections, each at 5-mm thickness, were acquired with the inferior extent at the basal ganglia, above the orbits, to minimize radiation exposure to the lenses. A total of 45 mL of low osmolar (300 mg/mL) or iso-osmolar (320 mg/mL) nonionic iodinated contrast was administered intravenously at 5 mL/s by use of a power injector with a 5-s delay. CTP examinations from all participants were deidentified and were stored in the departmental research PACS in DICOM format. These nonmodified, nonsimulated, and acquired datasets are described in this study as the “reference” datasets. Assessment of the Relationship of Image Noise to Dose on Our Scanning Systems A well-understood relationship exists with image noise and dose, expressed as E1) noise 1/ exposure , in which noise at a given exposure level (E1) is defined as the SD, E1), of the CT numbers (HU) from a uniform ROI. The noise at a second exposure level (E2) can be predicted from a known exposure level E1 by the following relationship: