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AJR 2000; 175:329-332
© American Roentgen Ray Society

Helical CT of Urinary Calculi

Effect of Stone Composition, Stone Size, and Scan Collimation

K. Chee Saw1, James A. McAteer2, Ashish G. Monga3, Gonzalo T. Chua3, James E. Lingeman1 and James C. Williams, Jr.2

1 Methodist Hospital Institute for Kidney Stone Disease, 1891 N. Senate Blvd., Ste. 700, Indianapolis, IN 46202.
2 Department of Anatomy and Cell Biology, Indiana University School of Medicine, 635 Barnhill Dr., Indianapolis, IN 46202.
3 Department of Radiology, Methodist Hospital, 1701 N. Senate Blvd., Indianapolis, IN 46202.

Received October 19, 1999; accepted after revision January 4, 2000.

 
Supported by National Institutes of Health grant PO1 DK43881 and the Kidney Stone Research Fund, Methodist Hospital of Indiana Institute for Kidney Stone Disease.

Address correspondence to J. C. Williams, Jr.


Abstract
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 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. Helical CT has become the preferred methodology for identifying urinary calculi. However, the ability to predict stone composition, which influences patient treatment, depends on the accurate measurement of the radiographic attenuation of stones. We studied the effects of stone composition, stone size, and scan collimation width on the measurement of attenuation in vitro.

MATERIALS AND METHODS. One hundred twenty-seven human urinary calculi of known composition and size were scanned at 120 kVp, 240 mA, and a 1:1 pitch at different collimations. A model, based on the physics of helical CT, was used to predict the effect of scan collimation width and stone size on measured attenuation.

RESULTS. At a 1-mm collimation, stone groups could be differentiated by attenuation: the attenuation of uric acid was less than that of cystine or struvite, which overlapped; these were less than the attenuation of calcium oxalate monohydrate, which was in turn lower than that of brushite and hydroxyapatite, which overlapped and showed the highest values. At a wider collimation, attenuation was lower and the ability to differentiate stone composition was lost. Attenuation also decreased with smaller stones. At a 10-mm collimation, some uric acid stones (<~6 mm) and other stones (<~4 mm) had very low attenuation, so low that they could remain undetected on helical CT. The model predicted well the degree that attenuation was affected by stone size and collimation width.

CONCLUSION. Stone composition and stone size, relative to CT collimation, independently influenced CT attenuation. The effect of stone size and collimation generally conformed to the model's predictions. We determined that small stones with low attenuation can be overlooked on helical CT.


Introduction
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 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
The introduction of helical CT has heralded a new era in the radiologic assessment of urinary tract calculi. The speed, safety, and accuracy of unenhanced helical CT makes it the method of choice in the assessment of patients with suspected urinary tract calculi [1,2,3,4,5,6,7,8]. Helical CT can reveal urinary calculi more accurately than standard radiography [9], sonography [10, 11], nephrotomography [12], and excretory urography [2, 11, 13, 14]. Furthermore, helical CT can provide an alternative diagnosis in as many as half of patients without stones [1, 4, 5, 7, 8, 11, 15,16,17].

In the treatment of patients with urinary tract calculi, patients' symptoms, stone size, and stone composition are factors that influence therapy [18]. Helical CT can provide helpful information on stone size and stone composition. The ability of helical CT to reveal stone size more accurately than standard radiography and nephrotomography [12] and the ability of conventional CT [19, 20] and helical CT [21] to accurately reveal stone composition in vitro have been reported. Despite these advantages, the optimal parameters for the helical CT examination of patients with acute flank pain have not been determined [22]. To accurately predict stone composition, Mostafavi et al. [21] scanned stones at a 1-mm collimation and a 1:1 pitch. However, to scan the whole urinary tract at a collimation of 1 mm and a pitch of 1:1 is impractical. Typically, helical CT is performed with a wider collimation and a higher pitch [5, 6, 17, 22]. Hu and Fox [23] showed that the measurement of maximum radiologic density of an object (e.g., a stone) on helical CT was affected (to a predictable degree) by CT collimation width and pitch. Logical extension of their model suggests that the measurement of the radiologic density of a stone will be affected to a similar degree when the relative size of the stone and the scanning parameters of CT are considered. Therefore, it may be possible that stones can be quickly scanned (at a wider collimation and a higher pitch) and a model used to predict stone composition. In this article, we investigate the effect of stone size and helical CT parameters on the accuracy of measurements of stone radiologic density. Human urinary calculi of known composition were scanned in vitro and attenuation measurements were compared with those predicted by the model of Hu and Fox.


Materials and Methods
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 Materials and Methods
Results
 Discussion
 References
 
In vivo helical CT scans of patients with multiple stones were studied to determine if stone size and stone composition affected helical CT attenuation. Records were retrospectively obtained for six patients who underwent helical CT (Somatom Plus 4; Siemens, Iselin, NJ) (120 kVp; 240 or 280 mA; 0.75-1.5 sec; collimation, 5 mm; pitch, 1:1) before they underwent percutaneous nephrolithotomy. The stone compositions were determined for each patient, as described next. Stone diameters were measured on helical CT images.

For in vitro studies, human urinary stones were obtained either from a stone analysis laboratory (Beck Analytical Services, Indianapolis, IN) or from patients undergoing nephrolithotomy. The stones were analyzed by microscopic visual inspection, chemical reaction, and, when appropriate, infrared spectroscopy. Only stones containing at least 60% of one stone constituent were used. According to their main stone constituents, the stones were divided into the following groups: brushite (calcium hydrogen phosphate dihydrate), calcium oxalate monohydrate, cystine, hydroxyapatite, struvite (magnesium ammonium phosphate hexahydrate), and uric acid.

The stones were individually placed in water in 15-mL centrifuge tubes (25314-15; Corning, Corning, NY) in a styrofoam rack. Then the stones were viewed with helical CT (Somatom Plus 4; Siemens) (120 kVp; 240 mA; 0.75-1.0 sec; collimation, 1 mm, 3 mm, and 10 mm; pitch, 1:1). The maximum voxel value within a stone was obtained by setting the display level to show the highest attenuation region within the stone, scanning adjacent sections and reading the highest voxel value on the display grid. The average attenuation in a region of interest was obtained by setting the display level to 75% of the maximum attenuation value and then drawing the region of interest by hand in the 75% display area. In this way, the region of interest was drawn to avoid the edges of the stone image in an effort to reduce partial volume effects on measured attenuation.

The effect of beam collimation width, helical pitch, and stone size was modeled using the approach of Hu and Fox [23]. Briefly, the model uses functions representing the shape of the beam profile, the thickness of a slice through a spherical body (the stone), and the helical motion of the CT method. Then the model combines these functions to predict the average radiographic attenuation in a region of interest.

Data were analyzed with JMP software (SAS Institute, Cary, NC), using linear regression or multiple analysis of variance as appropriate. Differences were considered significant when p was less than 0.05. Data are expressed as mean values plus or minus the standard error of the mean.


Results
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After analyzing the helical CT findings of patients with multiple renal calculi, we found that the measured attenuation values of stones declined with stone diameter. This finding suggested that the size of the stone affected the attenuation, as might be expected because of volume averaging. To quantitate the interaction of stone size and beam collimation width in helical CT, we studied the relationship in vitro.

The results for stone attenuation at three beam collimation widths are shown in Table 1, with representative scans of stones in Figure 1A,1B,1C. Using the 1-mm collimation data, one can differentiate the stones into the following groups with little overlap in attenuation: uric acid, cystine and struvite, calcium oxalate monohydrate, and brushite and hydroxyapatite. With increasing collimation width, the ability to differentiate stone compositions was lost, and the attenuation values were consistently lower at larger collimation widths. Furthermore, at a 10-mm collimation, it was not always possible to distinguish the smaller stones from the background. For uric acid stones, which were the least attenuating, even some stones with a diameter of 5-6 mm were indistinguishable from the background. For brushite and hydroxyapatite stones, some were as large as 3-4 mm in diameter and not detectable at a collimation of 10 mm. Therefore, both stone size and CT collimation width affected attenuation.


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  		TABLE 1 Attenuation Values for Human Kidney Stones Determined by Helical CT In Vitro

 


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  		Fig. 1A. —Helical CT scans of a single kidney stone. Scans were obtained at three collimation widths: 1 mm (A), 3 mm (B), and 10 mm (C).

 


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  		Fig. 1B. —Helical CT scans of a single kidney stone. Scans were obtained at three collimation widths: 1 mm (A), 3 mm (B), and 10 mm (C).

 


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  		Fig. 1C. —Helical CT scans of a single kidney stone. Scans were obtained at three collimation widths: 1 mm (A), 3 mm (B), and 10 mm (C).

 

Partial volume effects in the scanning and reconstruction process were likely responsible for the influence of stone size and beam collimation width on attenuation. To test this finding, we compared our data with the helical CT model of Hu and Fox [23], who predicted the effect of object size and helical CT settings on attenuation.

Attenuation data obtained at a 3-mm collimation are shown in Figure 2, compared with the predictions of the model. For this comparison, the 1-mm collimation data were assumed to represent the "true" attenuation values; therefore, the ratio of 3:1-mm attenuation readings is plotted. Note that the model predicted fairly accurately the falloff of attenuation with stone diameter. Somewhat similar results were obtained with the data from a 10-mm collimation width (Fig. 3).



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  		Fig. 2. —Graph shows measured ratio of attenuation (H) for kidney stones scanned with helical CT at 1:1 pitch at 3- and 1-mm collimations (reconstructed at 3- or 1-mm slice thicknesses, respectively). At 3-mm collimation, partial volume effect reduces apparent density, as shown by values less than one. Shaded area indicates range predicted by model, with top of shaded area being maximum value predicted, and bottom of shaded area being minimum value predicted (if stone falls between reconstruction slice locations).

 


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  		Fig. 3. —Graph as in Figure 2, but ratio is 10-mm collimation to 1-mm collimation. Shaded region shows range of ratio that is predicted by model.

 

The range predicted by the model (represented by the vertical dimension of the shaded area in Figs. 2 and 3) was attributable to the fact that a single reconstructed slice will not always be centered on the stone. If the slice reconstruction catches the stone randomly, the model predicts that the measured attenuation of the region of interest will fall somewhere within the shaded area. However, the values falling out of the ranges predicted by the model were almost all below the predicted range. That is, the measured attenuation values at 3- and 10-mm collimations were often lower than what the model would predict. Part of this can be explained by the nonspherical nature of the stones because most of them had some irregularity in shape that would accentuate the partial volume effect. However, the large number of points falling well below the predicted range in Figure 3 suggests that the 10-mm collimation group—in which stone diameter was generally much less than beam collimation width—may be outside the range of applicability of the model.


Discussion
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The general effect of beam collimation width and stone size on CT measurements has been noted before. Parienty et al. [24] found that the attenuation of a 3-mm stone was 312 H when scanned at a 5-mm section thickness and 76 H when scanned at a 10-mm section thickness. These researchers also found that stones less than 5 mm in diameter had a lower attenuation reading (184-300 H) when compared with larger stones (5-9 mm; 300-402 H) of the same composition scanned at the same section thickness. After scanning the same 12 stones at different section thicknesses, Kuwahara et al. [25] reported that attenuation values increased to a maximum as the stone size relative to the section thickness increased.

As for absolute values of attenuation for different types of stones, the results of our study with 127 human stones scanned in vitro confirm the results of Mostafavi et al. [21], who reported that stone composition can be identified using the attenuation value obtained from helical CT. The single-voltage attenuation values reported by these investigators agree well with the data in Table 1 with the exception of struvite stones, which appeared more dense in our study (mean of 1087 H versus 666 H in the study of Mostafavi et al.). However, we observed that struvite stones also displayed the greatest variability in attenuation values compared with all other stone types. Perhaps this variability (among studies and within the present study) is a reflection of the variability of the constituent components of different struvites. However, our overall results suggest that helical CT attenuation can be used to identify the probable composition of a urinary stone.

Our results showed a definite effect of stone size on attenuation measurements and a dramatic effect of beam collimation width on the measured attenuation of a stone. The mathematical model of Hu and Fox [23] provides a detailed prediction of the interaction between the size of the imaged object and the helical CT parameters in determining the partial volume effects, which lead to a diminution in the measured value of attenuation. The fit of data in the present study is a test of the model of Hu and Fox with clinically relevant objects that roughly match the spherical shape assumed for the mathematical development of the model. The results of the fit of the 3-mm collimation data to the model (Fig. 2) are promising, suggesting that the model is good at predicting the effect of beam collimation and stone size in the range tested. The fit is not as good with the 10-mm collimation, suggesting that the parameters used for those scans may have been outside the range of assumptions appropriate for the model. Further testing of the model would be required, but the data in Figures 2 and 3 suggest that if the beam collimation width exceeds twice the diameter of the stone, the model of Hu and Fox generally underestimates the error in attenuation.

The vertical range predicted by the model of Hu and Fox [23] (Figs. 2 and 3) is attributable to the fact that on reconstruction of image slices in helical CT the stone may not fall in the middle of a reconstruction slice; therefore, the attenuation values measured will be reduced from what would be seen if a slice were centered on the stone. This means that some of the error seen with increasing beam collimation can be eliminated by reconstructing the images with slice widths finer than the beam collimation. Thus, a patient could be imaged at a 5-mm collimation, which can be accomplished with a single breath-hold, and the images reconstructed at 1 mm to lessen the error in attenuation determination. However, the model predicts that some error will persist, as shown by the top of the range in Figures 2 and 3.

It is likely that the technology of helical CT will continue to improve, and in the future, reconstruction at finer slice widths could be performed rapidly. Further development will allow stone size to be calculated automatically, and correction of attenuation, using something like the algorithm that we used, could be automatic as well. Therefore, it seems reasonable to suggest that in the future, it will be possible to perform helical CT analysis of renal calculi rapidly (with a wide collimation and a high pitch) while retaining the ability to accurately identify stone composition.

An important observation from this study is that small stones may be difficult to visualize when scanned at a wide collimation. We found that some smaller stones of all types were not distinguishable from the background when they were scanned at a 10-mm collimation. The possibility of overlooking a small stone has been recognized in the helical CT literature. Smith et al. [15] described patients who subsequently passed a stone that was overlooked on helical CT, and they suggested that, because of volume averaging, a small stone with a low attenuation value might be overlooked. The observation of Smith et al. has been supported by other researchers, with false-negative rates ranging from 2% to 7% [1, 4, 10, 11, 13, 26, 27]. Recently, indinavir stones, which are relatively radiolucent, have been reported to be undetectable on CT [27]. All these findings are consistent with our data, showing that small stones with a low attenuation value can be overlooked on helical CT.

In conclusion, helical CT attenuation can be used to predict stone composition in vitro. However, stone size and CT collimation affect the accurate measurement of attenuation. The degree to which CT attenuation will be affected can be accurately predicted by a simple algorithm. However, some stones may not be detectable on helical CT when a wide-beam collimation is used.


Acknowledgments
 
We thank William McGinty for excellent technical assistance.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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