CT Scan Data Acquisition

1. CT Scan Data Acquisition
3 1 A u g u s t , 2 0 2 2
Ali Asghar Ayaz
King Saud Bin Abdul-Aziz University of Health Sciences
Radiological Sciences Program
CT Scan Physics (RADS-413)
4th Year

2. Data Acquisition in CT
• The systematic collection of information from the patient to
produce the CT image.
• Data acquisition systems, abbreviated by the acronyms DAS.
• In CT Scan, Data acquisition is the process of sampling
electrical signal and converting the resulting samples into
digital numeric values that can be manipulated by a
computer.
• The functions of data acquisition systems include:
• Measure transmitted radiation beam
• Encodes measurement to binary data
• Transmits binary data to computer

3. Data Collection Basics
1. X-ray tube & detectors collect information systematically.
2. X-ray source & detector must be in & stay in alignment.
3. Beam moves (scans) around patient and takes many X-ray transmission measurements.
Patient
X-Ray beams
4. Pre-patient beam is collimated to pass only through slice of
interest.
5. It is shaped by special filter (bow-tie) for uniformity.
6. Beam is attenuated by patient and transmitted photons are
detected by scanner.
7. Detected photon intensity converted to electrical signal (analog)
by detectors.
8. Electrical signal converted to digital value by A to D converter.
9. Digital value sent to reconstruction computer.

4. Data Collection Basics
CT “Ray”
• That part of beam falling onto a single detector.
• attenuated by patient
• projected onto one detector
• detector produces electrical signal
• produces single data sample
Ray
# of simultaneously
collected rays

5. Detectors
• Capture radiation from patient
• Ability to absorb & convert x-ray photons to electrical signals.
CT Detector Characteristics
1. Efficiency
2. Response time
3. Dynamic range
4. Reproducibility and Stability

6. Detectors
1. Efficiency
Ability to capture, absorb & convert x-ray photons to
electrical signals.
a. Capture efficiency
• Efficiency of detector to obtain transmitted photons from
patient
• Size of detector area, distance between 2 detectors
b. Absorption efficiency
• no. of photons absorbed
• Z , density, size, thickness of detector
c. Conversion efficiency
• fraction of absorbed energy which produce signal
Overall Detector
Efficiency
Capture Efficiency
X
Absorption Efficiency
X
Conversion Efficiency

7. Detectors
2. Response Time
• Speed with which detector can detect an x ray event and recover to detect the next
one.
• Minimum time after detection of 1st event when detector can detect 2nd event.
• If time between events shorter than response time, second event may not be
detected.
• Shorter response time better.

8. Detectors
3. Dynamic Range
• Ability to faithfully detect large range of intensities.
• “Ratio of largest signal to be measured to the precision of the smallest
signal to be detected”
• Typical dynamic range: 1,000,000:1
• much better than film.
4. Stability
• “Steadiness” of detector system.
• Consistency of detector signal over time.
• The less stable, the more frequently calibration required.

9. Photomultiplier Tubes
X-Rays Light
Scintillation
Crystal
PM
Tube
Photocathode
(Photodiode)
-
es
es
es
es
es
Dynodes
• Photomultiplier tubes (Photomultipliers or PMTs for short), members of
the class of vacuum tubes, and more specifically vacuum phototubes, are
extremely sensitive detectors of light.
• Electrons attracted to series of dynodes each dynode slightly more
amplified than last one.

10. Dynodes
Photocathode
PM Tube
Photomultiplier Tubes

11. Photodiode Solid State Detectors
• A photodiode is a semiconductor device that converts light into an electrical current.
• The current is generated when photons are absorbed in the photodiode and is proportional to the
incident light.
• Benefits of Solid State Detectors
• Output electrical signal amplified.
• Fast response time.
• Large dynamic range.
• Almost 100% conversion & photon capture efficiency.
• Scintillation materials
• cadmium tungstate
• high-purity ceramic material
X-Rays Light
Photodiode
Semiconductor
Electrical
Signal

12. Components of DAS
1. Amplifier
2. Log Amplifier
3. Analog to Digital Converter (digital data)
4. Digital Transmission to computer
Amplifier
Analog to Digital Converter
Logarithmic Amplifier
From
Detector
To
Computer
Compresses
dynamic range;
Converts
transmission
intensity into
attenuation data
Increases signal
strength for later
processing

13. Logarithmic Amplification
1,000
1 10 100 1000 1 10 100 1000
100,000
10,000
1,000
100
10
1
5
4
3
2
1
0
Input Logarithm
Hard to
distinguish
between 1 & 10
here
Difference between
1 & 10 the same as
between 100 & 1000
Logarithms stretch low
end of scale; compress
high end
• Accepts widely varying input
• Takes logarithm of input
• Amplifies logarithm
• Logarithm output dynamic range now appropriate
for A/D conversion

14. Analog-Digital-Converter
• In CT scan, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system
that converts an analog signal, such as any electrical signal picked up from
the signal-amplifier into a digital values, which can be converted into a
digital image via computer.

15. DAS

16. Thank You For Listening
• If there are any questions or concerns, feel welcome to contact
me via the following:
• E-mail: ayazal@ngha.med.sa
• Phone Extension: 18109