Prospectively gated cardiac computed tomography.

A fourth-generation scanner has been modified to perform prospectively gated cardiac computed tomography (CT). A computer program monitors the electrocardiogram (ECG) and predicts when to initiate the next scan in a gated series in order to acquire all projection data for a desired phase of the heart cycle. The system has been tested with dogs and has produced cross-sectional images of all phases of the cardiac cycle. Eight to ten scans per series were sufficient to obtain reproducible images of each transverse section in the end-diastolic and end-systolic phases. The radiation dose to the skin was approximately 1.4 cGy per scan. The prospectively gated system is more than twice as efficient as a retrospectively gated system in obtaining complete angular projection data for a 10% heart cycle window. A temporal smoothing technique to suppress reconstruction artifacts due to sorting inconsistent projection data was developed and evaluated. Image noise was reduced by averaging together any overlapping projection data. Prospectively gated cardiac CT has also been used to demonstrate that the error in attenuation measured with a single nongated CT scan through the heart can be as large as 50-60 CT numbers outside the heart in the lung field.

Computed tomographic analysis of meteorite inclusions.

The discovery of isotopic anomalies in the calcium- and aluminum-rich inclusions of the Allende meteorite has improved our knowledge of the origin of the solar system. Inability to find more inclusions without destroying the meteorite has hampered further study. By using a fourth-generation computed tomographic scanner with modifications to the software only, the interior of heterogeneous materials such as Allende can be nondestructively probed. The regions of material with high and low atomic numbers are displayed quickly. The object can then be cut to obtain for analysis just the areas of interest.

Measurements of computed tomography x-ray fields utilizing the partial volume effect.

Due to the x-ray field geometry, exposure measurements on computed tomographic (CT) scanners require modification of widely used measurement techniques. Ionization chambers have geometries which do not allow measurement of narrow rotating x-ray fields. However, by utilizing the partial volume effect, an ion chamber can be calibrated to measure narrow x-ray fields with a spatial resolution of down to 1 mm. The number of scans required to reconstruct a complete profile depends on the resolution required, and is typically on the order of 10 to 20 scans. This is accomplished using a series of attenuating sleeves over the ionization chamber. The chamber and sleeves are calibrated against a standard chamber using large field geometries. Comparisons with TLD measurements are shown to verify the accuracy of the technique.