Abstract
Manufacturer-recommended exposure mA was typically resulting in 3–5 times greater patient doses for calcium score scans compared with other dedicated CT scanners at Nottingham University Hospitals. Image noise was used as a measure of image quality in phantom and patient data. The noise was quantified from the standard deviation in Hounsfield units within regions of interest in the myocardium. Noise in phantom data was found to vary linearly with the inverse square root of the applied mAs. It was assumed that a linear relationship would also apply to patient data but it was predicted that the linear gradient would vary between patients owing to differing patient size and composition. This noise model was used to calculate the exposure mA required to achieve a target noise level of 25 Hounsfield units in the myocardium for each patient. To maintain the image quality for patients of different sizes, three measures of size, weight, body mass index (BMI) and lateral dimension, were all tested for goodness of fit to the noise model. It was found that BMI correlated best with the noise model for small patients, and therefore, BMI was chosen as a measure of patient size for the revised mA table. Using this methodology, doses to small patients were reduced by a factor of four compared with manufacturer-recommended settings.
Highlights
The National Institute for Health and Care Excellence (NICE) guidelines for patients with chest pain of recent onset was published in 2010
SCALING MA FOR PATIENTS OF DIFFERENT SIZES As patients have different attenuating properties owing to size and composition, the mA will need to be varied for patients of different sizes to obtain the same target noise level of 25 Hounsfield units (HU).[9]
The aim of this study has been to lower the patient doses for performing calcium scoring on the GE Discovery D670 scanner
Summary
The National Institute for Health and Care Excellence (NICE) guidelines for patients with chest pain of recent onset was published in 2010. A threshold of 130 Hounsfield units (HU) is used for defining calcified regions.[3] The Agatston score is a method of quantifying the degree of calcification whereby the area of the lesion is multiplied by a weighting factor (determined by the CT number) and the scores of all the affected slices are summed.[4] The NICE guidelines require classification of patients into three groups; those with a score of 0, who do not need any further tests; those with a score between 1 and 400, who will need to have a CT coronary angiography; and those with a score > 400, who will go on to have conventional angiography. A score of zero has a high negative-predictive value (95%) for excluding significant coronary artery disease.[3]
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