Question

1.)List and explain the two methods used to obtain the two energy xray spectra required in DXA?

2.) Explain the concept og back projection method used in computed tomography?

3.) What are meant by ‘window level’ and ‘window width’

Answer 1

1) Two method of generating DXA have been implemetned using eigther K-edge filters or kVp swiching.

K-edge filters: K-edge absorption filters, made of rare earth material such as cerium and samarium, are ued to split the polyenergetic X-ray bam into high and lowenergy components that mimc the emision from Gd. the 2 component have inherently narrow spectra distribution and hence the problems associated with beam hardening are minimized. Lunar DPX (Lunar Corp Madison) system have a cerium filter and use pulse height analysis at the detector to discriimntate between high and low energy photos. Norland XR ( Atkinson, Wisconsin) system use a saarium filter and seperate detectors for high and low energy X-rays.

Voltage swiching: The second way of producing a dual energy X-ray beam is to switch the high voltage generator between high and low kVp during alternate half cycle of the main supply. Hologic uses this method in its QDR serise of DXA sytem. The spectral distribution is wider than with the k_edge filter method and the cosequent effect of beam hardening is corrected by a rotating calibration wheel containing bone and soft tissue equivalent filters that measure th attenuation coeeficient in given equation below and calibrte the scanned image pixel by pixel. Pulse height analysis is not required giving the instrument an inherently wide dynamic range.

2) Backprojection: The quality techinque of reconstructing CT slices is backprojection.This involves "smearing back" projection across th image at the angle it had been acquired. By smearing back all of the projections, you reconstruct an image. This image appearance almost like the real image however is blurred - we tend to smeared bright pixels across the complete image rather than putting them specifically wherever they belonged. you'll see this resultwithin the machine on the right-most panel.

In order to reconstruct a picture, you would like one hundred eighty degrees of data (* truly one hundred eighty + fan beam angle). Why? The remaining one hundred eighty degrees are merely a mirror image of the primary(because it doesn't matter that approach a photon travels through tissue, it'll be attenuated a similar amount). (Because of the fan beam geometry, you would like to measure an additional amount - equal to the fan angle - to actually get all of the data you would like, however the concept is that the same.)

In a fan-beam geometry, the angle of the fan determines what proportion of the article is included within thereconstructible field of view. a point should be included in all one hundred eighty degrees of projections so as to be reconstructed properly.

3)

1. WINDOW WIDTH : The window width as the name recommend is that the measure of the range of CT numbers that a picture contains. A wider window width (2000), therefore, can show a wider range of CT numbers. Consequently, the transition of dark to lightweight structures can occur over a bigger transition area to that of a narrow window width (<1000).

Accordingly, it's vital to notice, that a considerably wide window displaying all the CT numbers can end in totally different attenuations between soft tissues to become obscured one.

2. WINDOW LEVEL: The window level, typically also stated as window centre, is that the midpoint of the range of the CT numbers displayed. When the window level is reduced the CT image are brighter and vice versa.