Pitfalls in urinary stone identification using CT attenuation values: are we getting the same information on different scanner models?

INTRODUCTION: Evaluate the capability of different Computed Tomography scanners to determine urinary stone compositions based on CT attenuation values and to evaluate potential differences between each model. METHODS: 241 human urinary stones were obtained and their biochemical composition determined. Four different CT scanners (Siemens, Philips, GEMS and Toshiba) were evaluated. Mean CT-attenuation values and the standard deviation were recorded separately and compared with a t-paired test. RESULTS: For all tested CT scanners, when the classification of the various types of stones was arranged according to the mean CT-attenuation values and to the confidence interval, large overlappings between stone types were highlighted. The t-paired test showed that most stone types could not be identified. Some types of stones presented mean CT attenuation values significantly different from one CT scanner to another. At 80kV, the mean CT attenuation values obtained with the Toshiba Aquilion were significantly different from those obtained with the Siemens Sensation. On the other hand, mean values obtained with the Philips Brilliance were all significantly equal to those obtained with the Siemens Sensation and with the Toshiba Aquilion. At 120kV mean CT attenuation values of uric acid, cystine and struvite stones obtained with the Philips model are significantly different from those obtained with the Siemens and the Toshiba but equal to those obtained with the GE 64. CONCLUSIONS: According to our study, there is a great variability when different brands and models of scanners are compared directly. Furthermore, the CT scan analysis and HU evaluation appears to gather insufficient information in order to characterize and identify the composition of renal stones.

Characterization of human renal stones with MDCT: advantage of dual energy and limitations due to respiratory motion.

OBJECTIVE: Our aim was to determine, using CT attenuation values, the chemical composition of 241 human renal stones placed in a jelly phantom and to analyze the influence of respiratory motion on the classification. MATERIALS AND METHODS: The stones were placed in a jelly simulating the X-ray attenuation of the kidneys. A dynamic platform was used to apply to the phantom free-breathing motion (sinusoidal motion in z-axis) and motion due to lack of maintenance of a breath-hold (5 mm x s(-1) in z-axis). Determination of the chemical composition was performed with mean CT attenuation values obtained at 80 and 120 kV and with dual-energy CT attenuation values. RESULTS: Two hundred forty-one human urinary stones were classified into six groups: uric acid, cystine, struvite, weddellite (calcium oxalate dihydrate), whewellite (calcium oxalate monohydrate), and brushite. With no motion, the use of dual energy enabled differentiation of all of the types of stones with statistically significant differences. Uric acid (-20 +/- 22 H), cystine (106 +/- 19 H), struvite (271 +/- 16 H), weddellite (323 +/- 5 H), brushite (415 +/- 30 H), and whewellite (510 +/- 17 H) were identified as distinct groups. Motion-induced mean CT attenuation values were significantly different from those obtained with no motion. With motion, dual-energy CT attenuation values did not allow differentiation of all stone types. CONCLUSION: Dual-energy CT attenuation values can be used to predict the chemical composition of stones in vitro. However, when slight motion is applied to renal stones during image acquisition, the values become significantly different from those obtained with no motion. Consequently, confusion arises in differentiating stone types. A perfect breath-hold has to be performed for in vivo use of attenuation value to discern stone type.

Degradation of the z- resolution due to a longitudinal motion with a 64-channel CT scanner.

Isotropic acquisitions are routinely achievable with 64- channel CT scanners,. As it predecessors, it includes MultiPlanar Reformation (MPR) projection for the reconstruction of two-dimensional images and volume rendering for the creation of three dimensional images. The accuracy of images obtained with these postprocessing methods depends on the spatial resolution of image data acquired along the long axis of the patient (ie longitudinal, or z-inis spatial resolution). But physiologic motions can appear during a Computed Tomography (CT) exam and can leacd to a degradation of this spatial resolution. By using two different phantoms and a dynamic platform, we have studied the influence of a z-axis linear motion on the MPR images quality. Our results show that the corruption of the data results in the loss of information about the form, the contrast and/or the size of the scanned object. This corruption of data can lead to diagnostic errors by mimicking diseases or by masking physiologic details.

Influence of a longitudinal motion on image quality with a 64-channel CT scanner.

Physiologic motions can appear during a Computed Tomography (CT) exam. Even if some motions can be prevented, others are unavoidable. To tempt to correct the resulting artifacts, it is necessary to understand their apparition and their influence on the image quality. By using an anthropomorphic phantom and a dynamic platform, we have studied the influence of three different linear speeds on the quality of CT axial images. The results proved that the deformation, the detectability and the contrast of calcifications are of course dependent on the size and on the density of the calcification but also on the movement applied to them. The influence of the motion on the image quality depends on the studied object and cannot be predicted.