Gadolinium as a CT Contrast Agent: An Experimental Study for the Effects of Injection Parameters in the Rabbit Brain Model

PURPOSE: We wanted to investigate the use of gadolinium based contrast agent (Gd-DTPA) for computed tomography (CT), and we also wanted to assess the effects of valuable injection parameters on enhancement in an experimental rabbit brain model. MATERIALS AND METHODS: In vitro, attenuation measurements of serial dilutions of Gd-DTPA and iopromide were compared. In five rabbits, single level dynamic gadolinium-enhanced brain CT studies were obtained using different injection parameters. A comparison CT scan after iopromide administration was performed. The time-attenuation curves of the brain vessel and parenchyma were obtained and the magnitude of enhancement (Hmax) and the time to peak enhancement (Tmax) were analyzed. RESULTS: In vitro, the attenuation coefficient of undiluted Gd-DTPA (2,578 HU) was higher than that of iopromide (1,761 HU) at equimolar concentrations. In 5 rabbits, the time-attenuation curve demonstrated a distinct pattern with peak enhancement only in the brain vessel, but not in the brain parenchyma. There was increasing linear relationship between the injection rate of Gd-DTPA and Hmax, and a declining linear relationship with Tmax. The higher the concentration of Gd-DTPA, the higher Hmax was, but no significant difference was found for the Tmax. Higher volumes of Gd-DTPA revealed a higher Hmax and a delayed Tmax. CONCLUSION: Enhancement of the brain parenchyma on gadolinium-enhanced CT is minimal, while enhancement of the brain vessels is distinctive. The most important factor affecting Hmax of the vessel is the concentration of the contrast medium and the most important factor affecting Tmax of the vessel is volume of the contrast medium. The gadolinium-based contrast agent may be an reasonable alternative contrast agent for brain CT, and especially in cerebral vessels, and it may also be advantageous for brain parenchyma of those patients with BBB dysfunction.

Measurement error of spiral CT Volumetry: Influence of Low Dose CT Technique

PURPOSE: To examine the possible measurement errors of lung nodule volumetry at the various scan parameters by using a small nodule phantom. MATERIALS AND METHODS: We obtained images of a nodule phantom using a spiral CT scanner. The nodule phantom was made of paraffin and urethane and its real volume was known. For the CT scanning experiments, we used three different values for both the pitch of the table feed, i.e. 1:1, 1:15 and 1:2, and the tube current, i.e. 40 mA, 80 mA and 120 mA. All of the images acquired through CT scanning were reconstructed three dimensionally and measured with volumetry software. We tested the correlation between the true volume and the measured volume for each set of parameters using linear regression analysis. RESULTS:For the pitches of table feed of 1:1, 1:1.5 and 1:2, the mean relative errors were 23.3%, 22.8% and 22.6%, respectively. There were perfect correlations among the three sets of measurements (Pearson's coefficient = 1.000, p<0.001). For the tube currents of 40 mA, 80 mA and 120 mA, the mean relative errors were 22.6%, 22.6% and 22.9%, respectively. There were perfect correlations among them (Pearson's coefficient = 1.000, p<0.001). CONCLUSION: In the measurement of the volume of the lung nodule using spiral CT, the measurement error was not increased in spite of the tube current being decreased or the pitch of table feed being increased.

The Quality of Reconstructed 3D Images in Multidetector-Row Helical CT: Experimental Study Involving Scan Parameters

OBJECTIVE: To determine which multidetector-row helical CT scanning technique provides the best-quality reconstructed 3D images, and to assess differences in image quality according to the levels of the scanning parameters used. MATERIALS AND METHODS: Four objects with different surfaces and contours were scanned using multidetector-row helical CT at three detector-row collimations (1.25, 2.50, 5.00 mm), two pitches (3.0, 6.0), and three different degrees of overlap between the reconstructed slices (0%, 25%, 50%). Reconstructed 3D images of the resulting 72 sets of data were produced using volumetric rendering. The 72 images were graded on a scale from 1 (worst) to 5 (best) for each of four rating criteria, giving a mean score for each criterion and an overall mean score. Statistical analysis was used to assess differences in image quality according to scanning parameter levels. RESULTS: The mean score for each rating criterion, and the overall mean score, varied significantly according to the scanning parameter levels used. With regard to detector-row collimation and pitch, all levels of scanning parameters gave rise to significant differences, while in the degree of overlap of reconstructed slices, there were significant differences between overlap of 0% and of 50% in all levels of scanning parameters, and between overlap of 25% and of 50% in overall accuracy and overall mean score. Among the 18 scanning sequences, the highest score (4.94) was achieved with 1.25 mm detector-row collimation, 3.0 pitch, and 50% overlap between reconstructed slices. CONCLUSION: Comparison of the quality of reconstructed 3D images obtained using multidetector-row helical CT and various scanning techniques indicated that the 1.25 mm, 3.0, 50% scanning sequence was best. Quality improved as detector-row collimation decreased; as pitch was reduced from 6.0 to 3.0; and as overlap between reconstructed slices increased.

A study of Parameters in Spiral CT Volumetry Using Balloon Phantoms

PURPOSE: To evaluate the effects of threshold values, reconstruction interval, slice thickness and table speed on the spiral CT volumetry. MATERIALS AND METHODS: Two phantoms made of a balloon and diluted contrast media underwent spiral CT scanning with section thicknesses of 5, 7 and 10 mm and table speeds of 5, 8 and 10 mm with scans of 5 mm section thickness, 7, 10, and 14 mm with scans of 7 mm section thickness, and 10, 15, and 20 mm with scans of 10 mm section thickness. The volumetric values of phantom A and B were obtained at varying threshold values and a reconstruction interval of 5 and 10 mm for all scans. Volumes were also determined with the threshold value fixed and a reconstruction interval of 1, 5, 7 and 10 mm, respectively. Three-dimensional display and volumetric measurements were obtained using reconstructed images. The effects of threshold value, reconstruction interval, slice thickness and table speed on volumetry were analyzed. RESULTS: Volumetric values varied according to threshold values. Where a threshold value was low, value increased as pitch increased, but where a the threshold value was high, value decreased as pitch increased. With varying threshold values, measurement errors in hydrostatic volumetry were between 0.19 and 27.98%; at a fixed threshold value, measurement errors in CT volumetry were 1.6 to 9.0%. Volume decreased as reconstruc-tion interval increased. Where the table speed/ slice thickness ratio was constant, volume was constant though slice thickness differed. At fixed threshold values, variation in the reconstruction interval was statistically more significant than variation in slice thickness or table speed ( p<0.05, Kruskal-Wallis one-way ANOVA). CONCLUSION: Among serveral spiral scanning and image reconstruction parameters including threshold value, reconstruction interval, slice thickness, and table speed, threshold value most affected the result obtained. At fixed threshold values, the reconstruction interval usded had more effect on CT volumetry than other parameters.

The Difference of Nodule Detection Rates in the Liver According to the Pitch and Slice Thickness in Spiral CT: Experimental Study by Using Artificial Liver Phantom

PURPOSE: To determine optimal pitch and slice thickness when detecting small hepatoma by spiral CT. MATERIAL AND METHODS: Three types of artificial liver phantom of 45, 65, and 85 HU using agarose and three types ofartifical nodules of cheese with 95 HU of CT attenuation (5, 10 and 15mm in diameter) were prepared. After thethree types of phantom were embedded with three kinds of artificial nodules of different sizes, nine types ofphantom were made. In addition, four more 10-HU artificial liver phantoms embedded with 5-mm nodules were made.After the phantoms were scanned by spiral CT at different slice thicknesses (5, 8 and 10mm) and different pitches(1.0, 1.25, 1.5 and 2.0), nodule detection rates were determined ; these rates were, in addition, determined afteroverlapping reconstruction and changes in CT attenuation according to pitch. RESULT: Regardless of size, pitchand slice thickness, all nodules with more than 30 HU difference between the embedded nodule and artificial liverphantom were detected. The detection rate of 5mm nodules with a density difference of 10 HU decreased at a pitchof 2.0 and at 10mm slice thickness. After overlapping reconstruction, detection rates increased and there were noCT attenuation differences according to pitch. CONCLUSION: Eight-mm slice thickness is preferred and for thedetection of a nodule by spiral CT, pitch should not be greater than 1.5. After overlapping reconstruction,additional nodules were detected.

Biliary Stones: Change of CT Attenuation in Water Soluble Contrast Media

PURPOSE: To investigate change of CT attenuation of biliary stones in water soluble contrast media with time as well as the factors contributing to this change. MATERIALS AND METHODS: Thirty biliary stones were placed within cone-shaped plastic tubes, and as a control study, spiral CT scanning was performed 50 minutes after immersion in normal saline. The stones were rescanned at 5, 10, 20, 30, 45, 60, 90 and 120 minutes after immersion in water soluble contrast media. Mean CT attenuation value and volume of the stones were measured after three-dimensional reconstruction of images. Physical factors such as porosity[(wet weight - dry weight) / wetweight]x100, volume, and cholesterol as a chemical factor were measured. RESULTS: The pattern of change of CT attenuation was classified as one of three types. Fifteen stones (50%) were classified as type 1, in which attenuation increased with immersion time; in ten stones, this increase was rapid, particularly within the first 5 minutes. Twelve (40%) were classified as type 2, in which attenuation showed no significant change. Three stones showed no regular pattern, and these were classified as type 3. The mean porosity of type 1 (median; 32.7, mean+/-SD; 52.83+/-34.48) was greater than that of type 2 (median; 6.7, mean+/-SD; 30.58+/-48.25)(p<.05). The volume and cholesterol fraction of stones were not significantly different between type 1 and 2. CONCLUSION: In some biliary stones, CT attenuation value increases in water-soluble contrast media with time, and porosity is the most important factor in attenuation change.