CT Detectors and Generations
Done By/ Bader Nasser Ibrahim AL-Huzaimy
Diagnostic Radiology Department
CT Detectors and generations
*D e t e c t o r s: -
When the x-ray beam travels through the patient, it is attenuated by the anatomical structures it passes through. In conventional radiography we utilize a film-screen system as the primary image receptor to collect the attenuated information. The image receptors that are utilized in CT are referred to as detectors. The CT process essentially relies on collecting attenuated photon energy and converting it to an electrical signal, which will then be converted to a digital signal for computer reconstruction. A detector is a crystal or ionizing gas that when struck by an x-ray photon produces light or electrical energy.
The two types of detectors utilized in CT systems are scintillation or solid state and xenon gas detectors. Scintillation detectors utilize a crystal that fluoresces when struck by an x-ray photon which produces light energy. A photodiode I attached to the scintillation portion of the detector. The photodiode transforms the light energy into electrical or analog energy. The strength of the detector signal is proportional to the number of attenuated photons that are successfully converted to light energy and then to an electrical or analog signal. The most frequently used scintillation crystals are made of Bismuth Germinate (Bi4Ge3012) and Cadmium Tungstate (CdWO4). Earlier designs utilized Sodium and Cesium Iodide as the light producing agent. One of the problems associated with these element was that at times it would fluoresce more than necessary. The after glow problems associated with Sodium and Cesium Iodide altered the strength of the detector signal which could cause inaccuracies during computer reconstruction.The second type of detector utilized for CT imaging system is a gas detector.
The gas detector is usually constructed utilizing a chamber made of a ceramic material with long thin ionization plates usually made of Tungsten submersed in Xenon gas. The long thin tungsten plates act as electron collection plates.
When attenuated photons interact with the charged plates and the xenon gasionization occurs. The ionization of ions produces an electrical current. Xenon gas is the element of choice because of it's ability to remain stable under extreme amounts of pressure. Utilizing more gas in a detector increases the number of molecules that can be ionized therefore, the strength of the detector signal or response is increased. The long thin tungsten plates of the gas detector are highly directional. Ionization of the plates and the resultant detector signal rely on attenuated photons entering the chamber and ionizing the gas. If the xenon gas detectors are not positioned properly there is a chance that the ability of the detector to produce an accurate signal is compromised because the photons may miss the chamber. The xenon gas detectors are generally fixed with the position of the x-ray tube which occurs with 3rd generation scanner geometry designs. The term detector refers to a single element or a single type of detector used in a CT system. The term detector array is used to describe the total number of detectors that a CT system utilizes for collecting attenuated information. 3 rd generation CT imaging systems employ 800-1000 detectors while 4th generation scanners include 4000-5000 individual detectors in a detector array.
*G e n e r a t i o n s:-
- 1st generation: rotate/translate, pencil beam:-
•Only 2 x-ray detectors used (two different slices)
•Parallel ray geometry
•Translated linearly to acquire 160 rays across a 24 cm FOV
•Rotated slightly between translations to acquire 180 projections at 1-degree intervals
•About 4.5 minutes/scan with 1.5 minutes to reconstruct slice
•Large change in signal due to increased x-ray flux outside of head
–Solved by pressing patient’s head into a flexible membrane surrounded by a water bath
•NaI detector signal decayed slowly, affecting measurements made temporally too close together
•Pencil beam geometry allowed very efficient scatter reduction, best of all scanner generations
-2nd generation: rotate/translate, narrow fan beam :-
•Incorporated linear array of 30 detectors
•More data acquired to improve image quality (600 rays x 540 views)
•Shortest scan time was 18 seconds/slice
•Narrow fan beam allows more scattered radiation to be detected
-3rd generation: rotate/rotate, wide fan beam:-
•Number of detectors increased substantially (to more than 800 detectors)
•Angle of fan beam increased to cover entire patient
–Eliminated need for translational motion
•Mechanically joined x-ray tube and detector array rotate together
•Newer systems have scan times of ½ second
- 4th generation: rotate/stationary :-
•Designed to overcome the problem of ring artifacts
•Stationary ring of about 4,800 detectors
3rd vs. 4th generation :-
•3rd generation fan beam geometry has the x-ray tube as the apex of the fan; 4th generation has the individual detector as the apex
5th generation: stationary/stationary :-
•Developed specifically for cardiac tomographic imaging
•No conventional x-ray tube; large arc of tungsten encircles patient and lies directly opposite to the detector ring
•Electron beam steered around the patient to strike the annular tungsten target
•Capable of 50-msec scan times; can produce fast-frame-rate CT movies of the beating heart
6th generation: helical :-
•Helical CT scanners acquire data while the table is moving
•By avoiding the time required to translate the patient table, the total scan time required to image the patient can be much shorter
•Allows the use of less contrast agent and increases patient throughput
•In some instances the entire scan be done within a single breath-hold of the patient
- 7th generation: multiple detector array :-
•When using multiple detector arrays, the collimator spacing is wider and more of the x-rays that are produced by the tube are used in producing image data
–Opening up the collimator in a single array scanner increases the slice thickness, reducing spatial resolution in the slice thickness dimension
–With multiple detector array scanners, slice thickness is determined by detector size, not by the collimator
Done By/ Bader N. AL-Huzaimy
Diagnostic Radiology Department
Done By/ Bader Nasser Ibrahim AL-Huzaimy
Diagnostic Radiology Department
CT Detectors and generations
*D e t e c t o r s: -
When the x-ray beam travels through the patient, it is attenuated by the anatomical structures it passes through. In conventional radiography we utilize a film-screen system as the primary image receptor to collect the attenuated information. The image receptors that are utilized in CT are referred to as detectors. The CT process essentially relies on collecting attenuated photon energy and converting it to an electrical signal, which will then be converted to a digital signal for computer reconstruction. A detector is a crystal or ionizing gas that when struck by an x-ray photon produces light or electrical energy.
The two types of detectors utilized in CT systems are scintillation or solid state and xenon gas detectors. Scintillation detectors utilize a crystal that fluoresces when struck by an x-ray photon which produces light energy. A photodiode I attached to the scintillation portion of the detector. The photodiode transforms the light energy into electrical or analog energy. The strength of the detector signal is proportional to the number of attenuated photons that are successfully converted to light energy and then to an electrical or analog signal. The most frequently used scintillation crystals are made of Bismuth Germinate (Bi4Ge3012) and Cadmium Tungstate (CdWO4). Earlier designs utilized Sodium and Cesium Iodide as the light producing agent. One of the problems associated with these element was that at times it would fluoresce more than necessary. The after glow problems associated with Sodium and Cesium Iodide altered the strength of the detector signal which could cause inaccuracies during computer reconstruction.The second type of detector utilized for CT imaging system is a gas detector.
The gas detector is usually constructed utilizing a chamber made of a ceramic material with long thin ionization plates usually made of Tungsten submersed in Xenon gas. The long thin tungsten plates act as electron collection plates.
When attenuated photons interact with the charged plates and the xenon gasionization occurs. The ionization of ions produces an electrical current. Xenon gas is the element of choice because of it's ability to remain stable under extreme amounts of pressure. Utilizing more gas in a detector increases the number of molecules that can be ionized therefore, the strength of the detector signal or response is increased. The long thin tungsten plates of the gas detector are highly directional. Ionization of the plates and the resultant detector signal rely on attenuated photons entering the chamber and ionizing the gas. If the xenon gas detectors are not positioned properly there is a chance that the ability of the detector to produce an accurate signal is compromised because the photons may miss the chamber. The xenon gas detectors are generally fixed with the position of the x-ray tube which occurs with 3rd generation scanner geometry designs. The term detector refers to a single element or a single type of detector used in a CT system. The term detector array is used to describe the total number of detectors that a CT system utilizes for collecting attenuated information. 3 rd generation CT imaging systems employ 800-1000 detectors while 4th generation scanners include 4000-5000 individual detectors in a detector array.
*G e n e r a t i o n s:-
- 1st generation: rotate/translate, pencil beam:-
•Only 2 x-ray detectors used (two different slices)
•Parallel ray geometry
•Translated linearly to acquire 160 rays across a 24 cm FOV
•Rotated slightly between translations to acquire 180 projections at 1-degree intervals
•About 4.5 minutes/scan with 1.5 minutes to reconstruct slice
•Large change in signal due to increased x-ray flux outside of head
–Solved by pressing patient’s head into a flexible membrane surrounded by a water bath
•NaI detector signal decayed slowly, affecting measurements made temporally too close together
•Pencil beam geometry allowed very efficient scatter reduction, best of all scanner generations
-2nd generation: rotate/translate, narrow fan beam :-
•Incorporated linear array of 30 detectors
•More data acquired to improve image quality (600 rays x 540 views)
•Shortest scan time was 18 seconds/slice
•Narrow fan beam allows more scattered radiation to be detected
-3rd generation: rotate/rotate, wide fan beam:-
•Number of detectors increased substantially (to more than 800 detectors)
•Angle of fan beam increased to cover entire patient
–Eliminated need for translational motion
•Mechanically joined x-ray tube and detector array rotate together
•Newer systems have scan times of ½ second
- 4th generation: rotate/stationary :-
•Designed to overcome the problem of ring artifacts
•Stationary ring of about 4,800 detectors
3rd vs. 4th generation :-
•3rd generation fan beam geometry has the x-ray tube as the apex of the fan; 4th generation has the individual detector as the apex
5th generation: stationary/stationary :-
•Developed specifically for cardiac tomographic imaging
•No conventional x-ray tube; large arc of tungsten encircles patient and lies directly opposite to the detector ring
•Electron beam steered around the patient to strike the annular tungsten target
•Capable of 50-msec scan times; can produce fast-frame-rate CT movies of the beating heart
6th generation: helical :-
•Helical CT scanners acquire data while the table is moving
•By avoiding the time required to translate the patient table, the total scan time required to image the patient can be much shorter
•Allows the use of less contrast agent and increases patient throughput
•In some instances the entire scan be done within a single breath-hold of the patient
- 7th generation: multiple detector array :-
•When using multiple detector arrays, the collimator spacing is wider and more of the x-rays that are produced by the tube are used in producing image data
–Opening up the collimator in a single array scanner increases the slice thickness, reducing spatial resolution in the slice thickness dimension
–With multiple detector array scanners, slice thickness is determined by detector size, not by the collimator
Done By/ Bader N. AL-Huzaimy
Diagnostic Radiology Department
