Detectors and Detector arrays:

Detectors and Detector arrays:

Xenon Detectors:

1)       High pressure – about 25mmHg nonradioactive xenon gas, in long thin cells between metal plates

2)       Gas detectors are poorer detector efficiency then solid detector – need thicker gas detector to compensate for poor detector efficiency

3)       Metal septa separate the Xenon detectors,

4)       Long thin ionisation plates of Xenon detectors are directional – must be positioned in a fixed orientation wrt the x-ray source. Cant be used for 4^thgeneration scans – can only be used for 3^rd generation CT

5)        Xenon detectors are Ionisation detectors – ionised particles create an electronic signal -  and its numerical value is proportional to the to the x-ray intensity striking the xenon detector. 

6)       Surpassed by solid state detectors

Solid State Detectors

1)       Solid state CT detector is composed of a scintillator coupled tightly to a photodetector.

2)       The scintillator emits light when struck by an x-ray

3)       Light reaches a photodetector – typically a photodiode, which converts light intensity into electrical signal proportional to light intensity

4)       Solid state detectors have a better x-ray absorption efficiency then pressurised Xenon gas detectors,

5)       Small gap between the solid state detectors is needed to reduce cross talk between the detectors, this reduces the geometric efficiency somewhat.

6)       Required for 4^th Generation and higher CT scanners

Multiple detector arrays

1)       Multiple detector array are a set of several linear detector, tightly abutted. An assembly of multiple detector array modules.

2)       Slice width is determined by the detectors – not by the collimators – by adjusting detector width. Grouping one or more detectors in a multislice CT.

3)       Multiple detector array CT scanners make use of solid state detectors

4)       All multiple array scanners use 3^rd generation geometry – as they need less detectors than 4^th generation geometry.

Details of Acquisition

 Slice thickness:

1)       Single detector array scanners

a)        Slice thickness determined by physical collimation of the incident x-ray beam with 2 lead jaws – the wider the lead jaws the ticker the slice. Opening the collimation beyond a point will do nothing to increase the slice thickness, but would increase both the dose to the patient and the amount of scatter radiation.

b)       For the same kV and mAs, the number of detected x-ray photons increase linearlywith slice thickness. The signal to noise ratio increases by , therefore increase from 5 to 10mm doubles the detected x-ray photons and increases the SNR by 2 = 41%

c)         Larger slices increase the contrast resolution but the spatial resolution in the slice thickness is reduced. Thin slices improve the spatial resolution in the thickness dimension and reduce the partial volume averaging. For thin slices the mAs is usually increases to compensate for loss of x-ray photons resulting from the increased collimation. 

d)       Slice sensitivity profile:

2)       Multiple detector array scanners

Detector pitch and Collimator Pitch

CT Christianson

CT Numbers:

7)         

8)