Fig 4.18: spiral CT
Also, a three-dimensional vascular image dataset could be acquired very shortly after injection of an iodinated contrast agent, resulting in a significant increase in the SNR of the angiograms. Incorporation of this technology has resulted in three-dimensional CT angiography becoming the method of choice of diagnosing disease in the arteries and has been used in neurosurgery.
A number of data acquisition parameters are under operator control, and one of which is called the spiral pitch p. The spiral pitch for multislice CT is defined as the ratio of table feed per rotation of X-ray source to the single-slice collimated beam width. The spiral pitch can be selected based on the application.
In case of multislice spiral CT the number of detector arrays increases like for 4-slice 4 arrays of CT detectors are used. Now, the new generation CT scanner include 256 slice CT. As the number of slices increases the three-dimensional reconstruction will be better.
Image reconstruction takes place in parallel with data acquisition in order to minimize the delay between the end of data acquisition and the display of the images on the operator’s console. For each projection, the signal intensity of each detector depends on the attenuation coefficient and the thickness of each tissue that lies between the X-ray source and the particular detector and based on this the attenuation coefficient is assumed form the received intensity.
The image reconstruction from a series of projections is done based on Radon transform. Each X-ray projection is expressed in terms of the Radon transform of the object being studied and the inverse Radon transform is used to reconstruct the image. The most common methods for implementing the inverse Radon transform use backprojection or filtered backprojection algorithms. The image is reconstructed as shown in Fig 4.19.
Fig 4.19 Image reconstruction
After reconstruction, the image is displayed as a map of the tissue CT number, which is defined by
Where CT0 is the CT number in Hounsfield units and μ0 is the linear attenuation coefficient of the tissue. The information from a slice is used to create a two-dimenasional image and a full volume data set is used to create three-dimesnional images.
The reconstructed image consists of CT numbers varying from +3000 to -1000. The image display screen typically has only gray levels and thus some form of nonlinear image windowing is used to display the image. Standard sets of contrast and window parameters exist for different types of scan. The Fig 4.20 represents the windowing method and two of the window parameters are window level (WL) and window width (WW).
Fig 4.20: WL and WW
The term ‘window level’ represents the central Hounsfield unit of all the numbers within the window width. The window width covers the HU of all the tissues of interest and these are displayed as various shade of grey. Tissues with CT numbers outside this range are displayed as either black or white. Both the WL and WW can be set independently on the computer console and their respective settings affect the final displayed image. Fig 4.21 represents the same image data at different WL and WW.
Fig 4.21 an image with different WL and WW