LEC ( 2 ) RAD 323

Reconstruction techniques dates back to (1917), when scientist (Radon) developed mathematical solutions to the problem of reconstructing x-ray images from a set of its projections. In 1972, Hounsfield and Cormack contributed to development of reconstruction techniques and produced the first clinically useful CT unit, the EMI scanner. Hounsfield’s images were ‘noisy’, ‘blocky or even ‘ghosty’.

Special algorithms, namely (convolution) or the (Back- Projection) methods were introduced to solve this problem. This approach, beside keen co-work of other scientists, greatly improved image quality and had (speeded) up image processing scan times.

'High resolution' image, from more pixels. 'Blocky' or ' ghost' images, a result of using less number of pixels. Stair-step artifact (blocky image)

Image reconstruction from projections is the process of obtaining a two-dimensional (2D) (distribution) or ( profile) image from the (estimates) of its line integrals (  ) along a finite number of lines of known locations.

Computational problem in CT is to find all values of µ (x,y) from the ray sums for a sufficiently large number of beams (infinite number of lines) passing through object O, (that is for a large set of projections). The ray sums are finally used in the reconstruction of the image.

Formation of CT images by a CT scanner involves three main steps: 1.Data acquisition 2.Image reconstruction 3.Image display, manipulation, storage, recording, and image Communication

In CT imaging, attenuation measurements (projection data) are acquired (collected) from a patient by several schemes based on the geometrical pattern of scanning used: Data acquisition: Method by which a patient is scanned to obtain enough data for image reconstruction. There are 3 data acquisition methods: 1. Parallel beam acquisition. 1. Parallel beam acquisition. 2. Fan beam acquisition. 2. Fan beam acquisition. 3. The spiral beam acquisition. 3. The spiral beam acquisition. Data acquisition system includes the gantry, the x-ray tube, and the detectors. CT (generations) are based on the beam geometry, CT motion, and on the number of detectors used.

THE EARLY EXPERIMENTAL EMI SCANNER : In an early scanner, data acquisition was made by both the x-ray tube and detector moving in a straight line (or translate) across the patient’s head (or any other part of him), collecting a number of transmission measurements (  ) as they move from L to R. Then both tube and detector are rotated ( 1  ) and start again moving across patient’s head, this time from R to L making what is known as (translate-rotate) movement. This was the first scanner of its kind and was used by Hounsfield in his experiments. It was a slow scanner and gave more chances for patient's motion which lead to (motion artifacts). It was known as the EMI scanner.

DETECTORS DATA ACQUISITIONGENERATIONSingle Translate – Rotate  1 st Generation Multiple Array Translate – Rotate  2 nd Generation (Arc) Array Rotate – Rotate  3 rd Generation Circle Array - fixed Rotate Continuously  4 th Generation *  5 th Generation * OTHER ADVANED (5 th GEN.) SCANNERS:  Multiple x-ray tubes.  Electron Beam CT (EBCT), ultrafast scanner.

Parallel beam, single detector, Translate-Rotate

Used a translate-rotate geometry. The patient’s head axial slice was irradiated by highly collimated parallel x-ray beam (3mm thick, 13 mm wide) which was then sampled (digitized) with a single sodium iodide (NaI) scintillation sensor (detector) 160 times to get a single image profile. Single X-ray tube and a single detector were then (coupled) to scan across patient’s head in a linear way. This was then repeated many times at angle increments of 1  up to 180 . Thickness of beam (  ) is equivalent to (slice thickness). The EMI scanner was wholly dedicated to the head, which was immersed in a water-filled bag to serve two purposes:  Reduce and overcome signal dynamic range present in detector, thus allowing for optimization of detector’s sensitivity.  Overcome problems of afterglow in the NaI scintillator.