Object Size Determination at Computed Tomography

To investigate whether large differences in attenuation between an object and its background at CT examination alter the apparent size of the object on the image compared with the conditions with a small attenuation difference, phantom experiments were performed. A t the same time the influence of large attenuation differences on the geometric resolution was examined. The results show that neither the size reproduction of the object nor the geometric resolution is altered by variations in attenuation differences. Especially with large differences in attenuation between the object and background, however, the window centre setting was of greatest importance for correct reproduction of the object.


INTRODUCTION
In computed tomography (CT) sharp borderlines between structures of different density are reproduced as blurred contours. The degree of blurring depends, among other things, on the X-ray beam width, the dimensions and properties of the detectors, and the reconstruction algorithms employed. This blurring is of importance when measuring the size of structures in the image

(1,3).
When an object which differs markedly in attenuation from its background is examined by CT, its apparent size varies with the viewing conditions. A decisive factor in this context is the question of which part of the CT-calculated attenuation profile is utilized to produce the image. This is determined by the setting of the window centre (WC) and window width (WW) (Fig. 1).
When patients have undergone CT examination both before and after bipedal lymphography, the lymph nodes have appeared larger after the contrast filling, when the image has been viewed with the window setting normally used for evaluation of the retroperitoneal space. This enlargement has considerably exceeded that caused by the contrast filling in itself ( 2 , 4 ) .
It is not clear whether the apparent enlargement is solely an effect of the WW and WC settings or whether it is due to an error of measurement or of reconstruction. An investigation was therefore undertaken to elucidate the way in which the interpretation of the CT image is influenced by the difference in attenuation between an object and its background, different reconstruction algorithms and different window settings. In addition, the geometric resolution with various attenuation differences and window settings was examined.

MATERIAL AND METHODS
A Siemens Somatom DR2 whole-body scanner with a 256 x 256 matrix was used for the investigation. The exposure data were 120 kV, 0.518 As, exposure To examine the effect of different attenuation differences and window settings on the geometric resolution, a phantom of parallel plexiglass rods with a square 5 x 5 mm cross-section was used. The distance between the rods was 1 mm. The plexiglass rods were examined both in air and when surrounded by water. The attenuation value for plexiglass was 120 H U , which means that the difference in attenuation between plexiglass and air was 1120 H U and between plexiglass and water 120 HU. The CT slice was perpendicular to the longitudinal axis of the rods.

RESULTS
The attenuation profiles for the cylindrical hole when it was filled with Gastrografin of different concentrations, are given in Fig. 2 . When the standard algorithm was used for reconstruction, the width of the profile was independent of the difference in attenuation between the object and background ( Table 1 )       When the high-density object was viewed with a large WW, so that the entire attenuation difference between the object and background was covered by WW, the border of the object against the background became diffuse and it was difficult to define any exact measurement points for the diameter in the image (Fig. 5).
The WC setting also influenced the reproduction of an object which differed relatively little in attenuation from its background (by 85 H U ) , but only when WW was narrow (Fig. 6). When WW was smaller than the object-background attenuation difference, the apparent size of the object varied with different settings of WC, and this size variation increased with decreasing WW.
When, on the other hand, WW was larger than the attenuation difference, the size was not affected by changes in WC as long as the entire attenuation profile lay within the WW limits.
On examination of the plexiglass phantom with parallel rods with a square cross-section, in air and in water, parts of the attenuation profiles of the two objects coincided under all tested conditions (Fig. 7 ) . The lowest attenuation value for the "space" between the rods, when the images were reconstructed with a standard algorithm, was found to be higher than the attenuation value of the background by 30% of the object-background attenuation difference (Fig. 7 a and c ) . This finding was independent of the magnitude of the attenuation difference and of the zoom factors employed. On examination both in air and in water, too low values (20 and 110 HU) were obtained for the maxima of the profiles, thus 100 and 10 H U , respectively, below the true attenuation value for plexiglass (120 H U ) . On reconstruction with an edge-enhancing algorithm, the attenuation values for the "space" were lower than when a standard algorithm was used; thus it was now higher than the background value by only 15% of the object-background attenuation difference, both when the examination was performed in air and in water (Fig. 7 b and d ) .
The position of WC was of importance for the geometric resolution, especially on reconstruction with a standard algorithm. When the plexiglass rods, surrounded by air, were viewed with a WC of -1000 HU and a WW of 400 H U , they appeared to form one figure (Fig. 8 a ) ; when WC was increased stepwise while WW was kept constant, the rods w e r e reproduced as one unit (Fig. 8 b) until the upper limit of WW exceeded the lowest attenuation value for the "spa-ce" between the rods. Thereafter the space began to appear in grey tones. The space increased in width with increasing WC up to about -500 HU, and parflllel with the increase in width of the space the size of the rods decreased (Fig. 8 c   and d ) . When WC was so high that the entire WW lay above the lowest attenuation value for the space, this appeared black (Fig. 8 e). The same effect was observed when the image of the plexiglass rods surrounded b y water was viewed with varying WC. As long as WC lay within the values for the attenuation profile, 0-110 HU (Fig. 7 a ) , however, WW had to be relatively smallno greater than 60 HU at a WC of 0 HU, for the two rods to converge. However, with the window settings that are normally used at abdominal examinations, 400 HU for WW and 100 H U for WC, the rods appeared clearly separated.  (1) found that when WC was set at half the attenuation difference between the object and background, the size of the object was reproduced correctly, a finding in accordance with the present results (Fig. 4 ) . When WC is set at the level of the background attenuation on examination of an object of high attenuation, the apparent size of the object is greater than its real size, and its greatest part appears completely white, as this part of the attenuation profile lies above the upper limit of WW with WW unchanged, only the maximum of the attenuation profile will give an image, which will mean that the apparent size will be smaller than the real one, at the same time as the image will appear in grey tones (Fig. 1 b ) .

DISCUSSION
For the object of low attenuation, the WW setting also influenced the apparent size in those cases where WW was smaller than the object-background attenuation difference (Fig. 6 ) . With decreasing WW the object became more and more distinctly outlined in the image, but in parallel with this the size reproduction became more sensitive to changes in WC.
When closely adjacent objects of high attenuation were examined and WC was set at a level with the attenuation of the background, at the same time as the selected WW (400 HU) was considerably smaller than the object-background attenuation difference (1110 HU), the geometric resolution was poor. On CT examination of closely adjacent contrast-filled lymph nodes, these nodes, with WC and WW settings normal for abdominal examinations (100 and 400 HU respectively), may converge and be interpreted as one large single node (Fig. 9 ) .
On reconstruction with the edge-enhancing algorithm, the geometric resolution was improved at low WC values. This means that in clinical examinations of structures differing largely in attenuation, an edge-enhancing algorithm should be used. When WC was set at a level corresponding to half the attenuation difference, good resolution was obtained. The maximal attenuation values for the plexiglass rods, when examined in air and water and with use of a standard algorithm for the reconstruction, lay 100 and 10 HU, respectively, below the real attenuation value for plexiglass. This is explained by the fact that the spatial resolution for the algorithm used is insufficient for a correct reproduction of the attenuation values of such small objects.

CONCLUSIONS
The difference in attenuation between an object and its background has very little influence on the reproduced size of the object and on the geometric resolution with the CT scanner used for this investigation.
For correct reproduction of the size of an object and for good geometric resolution in the monitor image, the WC setting is of the greatest importance. A correct reproduction of the object of interest was achieved when WC was set at a value in the middle of the attenuation values of the object cand the background.