Chest Wall Composition Affects Image Noise During Cardiac CT

Becky McCall

March 09, 2011

March 9, 2011 (Vienna, Austria) — Chest wall fat has been shown to have a stronger correlation with image quality on cardiac computed tomography (CT), especially in larger women, than other commonly used conventional biometrics.

"The strongest correlation . . . [was found in women] who have a fat to muscle ratio greater than 1," said Narinder Paul, MD, from the Department of Medical Imaging at the University of Toronto, Ontario, when presenting the data at the European Congress of Radiology 2011.

He drew attention to concerns about radiation, such as those related to cardiac CT and long-term health risks. "In North America, the obesity rate is increasing. As people get bigger, they require more radiation for diagnostic imaging with convention imaging and postprocessing techniques. Clearly, we all want to achieve diagnostic imaging quality at a minimal radiation dose. We feel the best way [to do this] is to tailor exposure parameters to relevant patient biometrics," Dr. Paul explained.

The purpose of the investigation by Dr. Paul and colleagues was to compare the influence of chest wall composition on image quality between cardiac CT and the conventional patient biometrics commonly used in clinical practice. "We looked at the thoracic anteroposterior diameter (APD) of the chest during collaborative research with Charité Hospital in Berlin; comparing it with BMI [body mass index], we found a tighter image quality using APD." Dr. Paul emphasized that subjects with similar thoracic APD might have different chest wall soft tissue thickness, causing differential x-ray absorption, and hence, different image noise.

Data from 100 patients with a coronary artery calcium score were examined. There were 54 males, and mean age was 57.6 years.

"APD does not reflect the parameters of all our patients — in particular, women, often because of the alignment of the breast when women are supine. Also, as most of the cross-sectional chest volume is air, we believe that soft tissue has a significant proportional effect," he pointed out.

"The BMI of 2 people might be the same, but the cross-section of the chest can be very different if one person works out and the other has a sedentary lifestyle." He said that stratification by BMI improved diagnostic imaging somewhat, but he wanted to investigate other patient biometrics for improved image quality.

A region of interest was identified in the descending thoracic aorta at the mid-left ventricle. The standard deviation of the attenuation was taken as representative of image noise. The researchers used 320-slice multidetector computed tomography at 120 kVp, 300 mA, and 0.5 s rotation time.

Patient biometrics, including body weight, BMI, APD, lateral thoracic diameter CT scout parameters, and anteroposterior and lateral thoracic axial CT scan, were all taken. Chest wall composition and all tissues were measured using outer skin contour, muscle contour, and inner chest contour, which provided volumes of fat and muscle and the all-tissues total.

"Finally, we took the image noise and compared it to the parameters mentioned," added Dr. Paul.

Men were heavier, but women had a higher total soft tissue volume because of subcutaneous fat from breast tissue; they also had less muscle and smaller thoracic diameters than men. Patient stratification by BMI and fat to muscle ratio demonstrated the strongest correlations (r > 0.6) between image noise and biometric parameters when the fat to muscle ratio was greater than 1.

The strongest correlation (r = 0.85) with image noise was observed in chest wall composition, cross-sectional area of chest wall fat, women with a BMI above 25 kg/m2, and a fat to muscle ratio greater than 1.

"Patient stratification by BMI demonstrates no strong linear correlation between image noise and any biometric parameter, but if you look at the fat to muscle ratio, we did not find a strong correlation in patients with more muscle than fat. However, the total soft tissue, and particularly fat in the chest wall, had a very strong correlation with image noise," Dr. Paul explained.

Session moderator Dominik Fleischman, MD, from Stanford University School of Medicine in Palo Alto, California, agreed that other patient biometrics were needed in cardiac CT. "Because automated tube-current modulation interferes with cardiac gating, other biometric information is needed to individualize scan parameters in cardiac CT. [These] results show that chest wall composition, notably body fat and it's distribution, is a much better predictor of image noise than BMI or body weight. This may lead to better strategies to individualize the scanning parameter in cardiac CT and optimize image quality in practice," he told Medscape Medical News.

Also commenting on the work, Frank J. Rybicki, MD, PhD, director of the Applied Imaging Science Lab at Brigham and Women's Hospital and Harvard Medical School in Boston, Massachusetts, told Medscape Medical News that image noise in cardiac CT has a dramatic influence on image quality. "To date, this is relatively understudied [compared with the] diagnostic accuracy that is typical of catheter-based techniques used to determine stenosis.

"This work demonstrates that the composition of the chest wall has dramatic influence on image noise, and the fat in the chest wall is the strongest correlation with image noise. The group has also demonstrated that the correlation is strongest in women, particularly larger women, concluded Dr. Rybicki.

This study received no funding. Dr. Paul reports receiving research funding from Toshiba Medical Systems. Dr. Fleischmann and Dr. Rybicki have disclosed no relevant financial relationships.

European Congress of Radiology (ECR) 2011: Abstract B-339. Presented March 4, 2011.