Ct Detectors

Basics of CT detectors for Biomedical Engineers.

1. CT Detectors Nelson Mahat B 11 7 th Sem College of Biomedical Engineering and Applied Sciences 4/29/2020 1

2. Contents Ct Detectors Ideal features of detectors Types of Detectors Gas ionization Detector Scintillating Crystal Detector PMT Based Photo diode Based Conclusion References 4/29/2020 2

3. CT Detector Device which captures transmitted radiation beam from patient to form CT image X-ray beam converted to light photon Light photon converted to amplified electrical signal Electrical signal is then digitized using ADC and sent to computer so that image can be formed 4/29/2020 3

4. Crystal or ionizing gas Produces light photon when x-ray beam strikes Detector array 4/29/2020 4

5. CT Generation No. of Detectors First Generation 1 or 2 Second Generation 5-30 Third Generation 400-1000 Fourth Generation 1200-4800 Fifth Generation many Sixth Generation many Seventh Generation many in multiple rows 4/29/2020 5

6. Ideal Features of Detectors High Detective Quantum Efficiency (DQE) High Quantum Absorption Efficiency (QAE) High Conversion Efficiency (CE) Good Homogeneity: - Purity > 99.99 % Small After Glow: -should be < 0.01 % 100 ms after end of irradiation Fast Response Time Wide Dynamic Range High Packing Density Highly stable Low cost & easily available 4/29/2020 6

7. Types of Detectors Gas Ionization Detectors Scintillating Crystal Detectors PMT Based Photo Diode Based 4/29/2020 7

8. Gas Ionization Detectors Consists of Xenon gas or xenon-krypton mixture gas Have high atomic number Kept at high pressure(25-30 atm) which increases no of gaseous molecules for ionization Gas chamber separated by metallic septa made up of tungsten Hermetically sealed Around 1000 chambers made by this metallic septa with equal volume of gas Keeps sensitivity between each chamber constant 4/29/2020 8

9. Working X-rays interacts with gaseous molecules causing ionization Electric field applied between electrodes Movement of gaseous molecules towards the electrodes where electric charge collected as current Charge proportional to x-ray photons Current is amplified and digitized to form image 4/29/2020 9

10. Advantages Cheap & easily available High dynamic range Thermally stable Rapid response Low afterglow Disadvantages Low DQ E (<50%) Should be maintained at high pressure which may be difficult 4/29/2020 10

11. Scintillating Crystal Detectors Solid state detectors Consists of scintillating crystals coupled with PMT or photodiode X-ray strikes these crystals and visible light is produced Produced visible light reaches PMT or photodiode electrical signal is generated Amount of electrical signal depends on intensity of light incident 4/29/2020 11

12. A. PMT Based Crystal: sodium-iodide, cesium-iodide doped with thallium X-rays strikes the crystal and light generated The light photon strikes the cathode of the PM tube where it is converted into electrons Electrons are then amplified by a chain of dynodes as they pass through the tube Once the electrons bombard the anode, they are converted into an amplified electrical signal which is then processed by the computer 4/29/2020 12

13. Disadvantages Bulky detector geometry Poor packing density High after glow Low dynamic range 4/29/2020 13

14. B. Photo diode based Crystals: Gadolinium Oxisulphide(Gd 2 O 2 S) Ultra Fast Ceramics(UFC) X-rays strikes the crystal and light generated The light is then detected by a Si – Photodiode The resulting electrical current is amplified & converted into a digital signal 4/29/2020 14

15. Advantages Smaller in size Greater stability Relatively high DQE, QAE & CE Reduced noise Cheaper Disadvantages Long afterglow Requires time to time calibration 4/29/2020 15

16. Conclusion In this way detector plays a vital role in determining image quality as well as scan time. Thus, detector to be used in CT scanner must be chosen considering all the different criteria. 4/29/2020 16

17. References https://radiopaedia.org/articles/computed-tomography http://199.116.233.101/index.php/CT_Detectors https://link.springer.com/article/10.1007/s40134-012-0006-4 https://www.researchgate.net/publication/257808890_State_of_the_Art_of_CT_Detectors_and_Sources_A_Literature_Review 4/29/2020 17

18. Thank You !! 4/29/2020 18