INTRODUCTION

With the advent of multislice CT (MSCT), higher resolution and more diagnostic images are now possible.

• By transitioning from the axial to single slice helical to now MSCT (4 Slice, 16 Slice 64 Slice or even 128 Slice) scanners, imaging time has been reduced from minutes to seconds and then to sub seconds (0.33s).
• Increased volume coverage is obtained in lesser time with thinner slices and higher image quality.
• This all is possible because of advances in detector technology and electronics apart from newer x-ray tube designs and slip ring technology.

DETECTOR CHARACTERISTICS

Detectors must be capable of responding with extreme speed to a signal, without lag, must quickly discard the signal, and prepare for the next. They must also respond consistently and be small in size. CT detectors should have high capture efficiency, high absorption efficiency and high conversion efficiency. These three parameters are called the detector dose efficiency

i. The capture efficiency is how well the detectors receive photons from the patient. It is controlled primarily by detector size and the distance between detectors.

ii. Absorption efficiency is how well the detectors convert incoming x-ray photons. It is determined primarily by the materials used ( the scintillation crystal or the gas) as well as the size and thickness of the detector.

iii. Conversion efficiency is determined by how well the detector converts the absorbed photon information to a digital signal for the computer.

CT detectors should also have high stability, fast response time, and a wide dynamic range.

Stability is controlled by how often the detectors must be recalibrated to meet quality control standards.
Response time is the speed with which the detector can react to recognize an incoming photon and recover for the next input.
The dynamic range is the ratio of the largest signal that can be measured to the smallest. Typical modern scanners are capable of dynamic ranges of 1million to 1.

Detector Technology
CT detectors capture the radiation beam from the patient, convert it into electrical signals, which are subsequently converted into binary coded information for onward transmission to computer system for further processing. (Figure 1)

Working Principle
The conversion of x-rays to electrical energy in a detector is based on two fundamental principles.

• Scintillation Crystal detection (Figure 2(a)
• Gas Ionization (Figure 2 (b))

Detector electronics
The data acquisition system (DAS) refers to the detector electronics positioned between the detector array and the computer (Figure 3). It performs three major functions :

• Measures the transmitted radiation beam,
• Encodes these measurements into binary data, and
• Transmits the binary data to the computer.

Types of Detectors:
Three types of detection systems are available for CT machines:

• Multiple scintillation detectors with photo multiplier tubes
• Multiple scintillation detectors with photo-diodes
• A single multi chamber inert gas (xenon) detectors.

Multiple scintillation detectors with photo multiplier tubes
In the past, early scanner used sodium iodide crystals coupled to PM tubes. (Figure 4) Because of after glow problem and limited dynamic range, other crystals such as calcium fluoride and bismuth germinate were used in later scanners.

Multiple Scintillation detectors with photo-diodes
Now a days solid state photodiode multiplier scintillation crystal detectors are used. (Figure 5) Scintillation material used with photodiodes are cadmium tungstate and ceramic material made of high purity rare earth oxides, based on doped rare earth compounds such as yttria and gadolinium oxide ultrafast ceramic (UFC)

A single multi chamber inert gas (xenon) detectors (Figure 6)
Gas ionization detectors consist of the gas chambers separated by tungsten plates, enclosed by a relatively thick ceramic substrate material in which xenon gas in compressed to about 30 atmospheres to increase the no. of gas molecules available for ionization.

Gas (Xenon) Detectors : Advantages and Disadvantages

Advantages

Can be tightly packed thus:

• Increases total detector efficiency to around 50%
• Increases resolution
• Negligible lag time
• Highly directional to receive maximum primary beam

Disadvantages

60-90% efficiency
Highly directional prohibits efficient reception of primary beam from a moving tube

Solid State Detectors : : Advantages and Disadvantages

Advantages
Near 100% efficiency
Capable of reception of primary beam from a moving tube

Disadvantages
Cannot be tightly packed so,
total detector efficiency is around 50%
Resolution is decreased

PLUG IN DETECTOR MODULES
In modern CT scanners the entire array of detectors consist of groupings of detectors with each group known as a detector module which is plugged into a mother board unit of the detection system. (Figure 7)

MULTISLICE DETECTORS AND THEIR DESIGNS
The multislice, multirow detection system has three major detector designs :

Matrix
Adaptive array
Hybrid

The first two support sub- millimeter to 5mm acquisitions mode, whereas, the later supports sub millimeter to 8mm acquisition mode.

Matrix
where all detectors are of the same size (GE CT’s)

Adaptive array
Where they vary from thinner in side to thicker outside (Siemens and Philips CT’s)

Hybrid
where usually two sizes are used with the thinner detectors located centrally (Toshiba CT’s)

Developments in multi detector CTs

Multi-detector CTs debuted in 1992 when Elscint introduced its CT Twin, the first dual-slice scanner.
The first four-slice scanners were presented in 1998, followed by 16-slice systems in 2001; 32 and 40slice scanners followed within a short period. A 64-slice scanner was unveiled during the year 2003.
128 and 256-detector scanners appear to be on the horizon.
Applications include cardiac CT, CT angiography, CT Fluoroscopy, Endoscopic CT and traditional CT.

LATEST INNOVATION
FLAT PANEL DETECTORS FOR CT IMAGING :
Use of FPDs for CT imaging which have been developed for radiographic applications is currently under investigation. FPDs offer excellent performance for the imaging of high contrast structures with high spatial resolution. Low contrast resolution and dose efficiency however, is not at par of dedicated CT detectors. FPDs appear primarily suited for special applications in CT as for 3D angiography or intra operative imaging which also allows improvements in work flow, but at present are not recommended for standard diagnostic CTs due to dose reasons.

CONCLUSION

• During the coming years, cone-beam CT with large-area detectors may allow coverage of entire organs in a single axial scan
• In the meantime, 64-detector systems are the best available technology, and some believe a critical point has been reached.
• The best study obtainable may not be necessary. Thus, protocols are designed to reasonably bridge the possible and the necessary.
• Even so, more advanced systems are fast deluging physicians with incredibly high volumes of CT images.

The respective technical developments in CT detectors will have to be reassessed constantly in the future, whereas the development of detector systems which is equally suited both for radiography and CT is the need of to-day.

Comparison of different CT scanners at PGIMER, Chandigarh, INDIA

References :

• An Introduction to the physics of Draft of the Radiology Christensen, Curry, Dowdy 2nd Edition, 1978, Page 336-339
• Principal of Radiographic imaging - An Art and A Science by Carlton and Adler 3rd Edition 2001 Page 658-660
• Computed Tomography physical principles, clinical applicationa and quality control : Euclid Seeram, 2nd edition Page 88-92
• Siemens Medical Solution CT Detector Technology, June 2004
• Radiologic Sciences for Technologists, Physics, Biology and Protiection, 7th Edition Steward C. Bushong. Page 417-424.