CT Number Difference Research Articles

SU‐FF‐I‐06: Determination of Optimal KV for Imaging Iodine and Bone in Computed Tomography

Purpose: To determine the kV that will yield optimal contrast to noise ratio (CNR) at a constant patient dose for iodine/ soft tissue and bone/ soft tissue contrast in head and small body studies. Method and Materials: A head phantom containing water equivalent material and a bone ring to simulate the skull (for head studies) was used to obtain the data. To measure the contrast of iodine to water and bone to water, cylindrical plugs were inserted into the phantom: two plugs contained different dilutions of iodine contrast (Omnipauqe 300mgI/ml), one contained bone equivalent material, and one contained water. This phantom was scanned at kV values ranging from 80 to 140 kV. The contrast was measured as the difference in CT number between the iodine and bone plugs and the water plug. The noise measurements were taken from the water plug. Noise was determined at constant dose (CTDI w), so that the mAs was increased as the kV decreased. Both image noise and image contrast increased as the kV decreased. The relevant parameter for determining optimal kV is the contrast to noise ratio (CNR) at a constant dose as a function of kV. Results: The optimal kV, within the range of 80 to 140 kV, for the imaging of both iodine and bone is 80 kV. Limitations to this kV selection include the possible presence of increased artifacts in bone imaging, and CT scanner limitations in mAs that might not allow low kV with an adequate level of image noise. Conclusion: The common use of 120 to 140 kV in CT scanning may not be optimal for many studies whose primarily objective is the discernment of iodine or bone contrast, such as in brain perfusion studies. The use of lower kV settings may be advantageous in these cases.

TU‐EE‐A3‐06: Sub‐Millimeter Three Dimensional Ventilation Imaging of Rodent Lungs

Purpose: To develop a technique capable of generating a high resolution 3D ventilation image in a rat model from a series of CT images. This technique will be invaluable in developing strategies for radiation treatment optimization and characterization of pulmonary disease states. Method and Materials: A series of three 150gram Fischer 344 rats were euthanized and intubated. The lungs were equilibrated to ambient pressure, then serially inflated and imaged at approximately 1ml increments. A GE flat panel CT scanner capable of 0.15mm resolution was utilized for image acquisition. A deformable image registration algorithm, utilizing 3D optical flow, was applied to each pair of CT images to map, on a voxel by voxel basis, corresponding tissue elements. The difference in average CT number of a nine voxel region surrounding each pair of mapped voxels was then utilized to compute the change in fraction air per voxel (i.e. regional ventilation). This local ventilation measurement was then superimposed onto each voxel of the CT image to generate a 3D ventilation image. The sum of each voxel's ventilation (i.e. total lung ventilation) was then compared to the change in manually segmented lung volumes between each of a series of twenty‐two pair of CT images. Results: Visualization of the 3D ventilation images revealed regional heterogeneity of ventilation throughout the lung fields. The images required smoothing by averaging over a 0.75mm cube surrounding each voxel to minimize artifacts. The calculated total lung ventilation compared favorably with the change in manually segmented lung volumes, with a slope of unity, as expected, and a correlation coefficient of R=0.99. Conclusion: We have demonstrated a unique method of quantifying regional lung ventilation with sub‐millimeter accuracy in a rat model. This model has been developed in order to study lung function and radiation injury.

A method for measurement of cross sectional area, segment length, and branching angle of airway tree structures in situ

Accurate quantitative measurements of airway and vascular dimensions are essential for evaluating function in both the normal and in the diseased lung. This report describes a new integrated method for three-dimensional (3D) extraction and analysis of pulmonary tree structures using data from High Resolution Computed Tomography (HRCT). Serially scanned two-dimensional (2D) slices of the lower left lobe of isolated dog lungs were stacked to create a volume of data. Airway and vascular trees were extracted using a 3D seeded region-growing algorithm based on differences in CT number between wall and lumen. In the region-growing step, voxels in the lumen are tagged with a distance descriptor to identify points along the tree structure equidistant from the seed point. To obtain quantitative data, we reduced each tree to its central axis. From the central axis, branch length was measured as the distance between two successive branch points, branch angle was measured as the angle produced by two daughter branches, and cross-sectional area was measured from a plane perpendicular to the central axis point. Data derived from these methods can be used to localize and quantify structural differences both during different physiologic conditions and in pathologic lungs.

NON-INVASIVE QUANTITATION OF LIVER IRON IN DOGS WITH HEMOCHROMATOSIS USING DUAL ENERGY CT SCANNING

The goal of this project was to quantitate liver iron by CT densitometry, using dual energy scanning technique. Hemochromatosis was produced in two adult dogs by daily injections of iron-dextran solutions, 20 mg/kg over a ten to sixteen week period. Needle liver biopsies were obtained, and preliminary CT at 80 and 120 kV scanning energies, and then every two weeks thereafter. Liver iron concentration was determined by neutron activation analysis. Prior to each CT scan, a dual energy scan of a lucite phantom filled with water containing tubes with iron-dextran solutions of known concentration was obtained. The CT number difference when corrected for the response of water to dual energy scan, reflects the iron content. The graph relating CT number change to iron concentration in the dog liver indicated that for each gram percent of iron, a CT number difference of 24 HU occurred with dual energy scanning. Comparison for the estimated iron content with the measured iron content from biopsy material showed the predicted value to be within 2–20 percent of the measured iron. The long term goal of this study is to use dual energy CT scanning to follow therapy for hemochromatosis in children as a substitute for needle biopsy.

Difference between liver and spleen CT numbers in the normal adult: its usefulness in predicting the presence of diffuse liver disease.

A constant relationship was found between the mean CT numbers of the liver and spleen in 100 normal adults. This relationship was characterized by a mean CT number consistently higher for the liver (24.9 +/- 4.6) than for the spleen (21.1 +/- 4.1). The range for liver CT numbers was 16.7-37.2, and for the spleen it was 14.9-34.3. The mean liver-spleen CT number difference for all subjects was 3.8 +/- 2.1 (p < 0.001); in every instance, the livers exhibiting the high mean CT numbers were in subjects with high mean spleen CT numbers, with the same concordance for low mean CT numbers. This relationship between liver and spleen may be useful in the clinical setting in which a normal liver with low CT numbers must be differentiated from one in which the CT number is low because of fatty infiltration; the fatty liver will exhibit a lower mean CT number than the spleen.