LESSON 3 PT.1: Components in CT Scanner

1 Gantry
2 Table
3 X-ray tube
4 Detectors
5 Collimators/Filters
6 Computer
7 Display console
8 Image storage units

Components in CT Scanner

Data Acquisition
Image Reconstruction
Image Display And Storage

STEPS INVOLVED IN COMPUTED TOMOGRAPHY SCANNING

Data Acquisition

Inside the Gantry (x-ray tube) -> Patient -> Detector Array

Image Reconstruction

Digital Signal (data) -> ADC -> Electrical Signal -> Algorithm (FFT or RT) -> Mathematical Method -> AP to BP -> Filtration

Image Display

Analog data in the display -> DAC

Data Acquisition

1st step in creating CT Images

Data

__ is acquired when x-rays pass through a patient to strike a detector and are recorded

Gantry
Patient Couch
X-Ray Generator
Computer System
Operators Console

Components of Data Acquisition

Gantry and Patient Table/Couch

Major components for data acquisition

Slice Acquisition Rate (SAR)

A measure of the efficiency of the MSCT imaging system

Slice Acquired / 360 ÷ Rotation time

Formula of Slice Acquisition Rate (SAR)

R
N
T

__ is the slice acquisition rate (slices per second) __ is the number of slices per rotation __ is the rotation time in seconds.

Gantry

receive electronics commands from the operating console and transmit data to the computer for image production and analysis.

50 - 85 cm (20-34")

Gantry Includes __ aperture for the patient.

30 degrees

Gantry Can be angled up to__ to permit positioning for partial coronal images.

white halogen and low-power red laser lights

Gantry: Lights are usually mounted, __ and __ are used for positioning.

Gantry

ring shaped, largest component in CT scanner

HV generator
X-ray tube
Collimators
Detector array
Px support couch
Cooling system

Gantry Subsystems

Aperture

is the diameter of the opening

laser light, gantry tilt, table movement

Control panels of Gantry:

+- 15 to 30 degrees

Tilt range of Gantry

Laser light

light used to position px

Table or Patient Positioning Couch

It is made of carbon graphite fiber to decrease beam attenuation.

Low x-ray absorption
Durable

Why carbon graphite is used for table / patient positioning couch?

high

(high/low) xray absorption will result to too many artifacts or unwanted shadows light weight

Table or Patient Positioning Couch

It must be durable, can withstand warping overtime and can be used with varying sizes

Table or Patient Positioning Couch

must be capable of supporting the entire weight of the patient without sagging when fully extend.

Table or Patient Positioning Couch

should be of automatic indexing Must be capable of horizontal and vertical movement. Capable of uninterrupted movement during spiral/helical scans acquisition (pitch)

Automatic indexing

refers to the precise and controlled movement of the couch in sync with the scanning process.

X-Ray Tubes

High capacity tubes

High speed rotors

are used in most x-ray tubes for the best heat dissipation.

2-5 MHU

MHU capacity of ct scanner (xray tube). It is designed specifically for CT anode heating capacity.

30 exposures

CT Xray Tubes May produce __ exposures per examination.

Xray Tube

Produces x-ray photons to create image

Tungsten

Target material

74

Weight of Tungsten

0.5 mm and 1.0 mm

Common focal spot

Reduces penumbra
better spatial resolution
sharper image

Small focal spot will result to: (penumbra, spatial resolution, image)

Xray Tube

Disadvantage: Cannot tolerate as much heat - concentrate heat into smaller portion of the anode

high: > 120 kvp, 400 ma

Power capacity of CT Xray Tube is __: __ kvp, __ ma

50, 000 exposures

CT xray tubes produces how many exposures in a lifetime?

1 MHU/min

Anode Cooling Rate

Anode disc

has a larger diameter and is thicker

20,000 to 60,000 Heat Units (HU) per minute

Anode HU

Single Phase
Three Phase 6 Pulse
Three Phase 12 Pulse
High Frequency

Heat Units Power Supply

Single Phase

kVp x mA x s

Three Phase 6 Pulse

1.35 x kvp x ma x s

Three Phase 12 Pulse

1.41 x kvp x ma x s

High Frequency

1.45 x kvp x ma x s

Detectors

Should have high capture efficiency, high absorption efficiency, and high conversion efficiency. Should also have high stability, fast response time, and a wide dynamic range.

Stability, Response Time, Dynamic Range

Criteria for CT system X-ray Detectors

Stability

detectors must be recalibrated to meet quality control standards.

Response time

the speed with which the detector can react to recognize an incoming photon and recover for the next input.

Dynamic range

is the ratio of the largest signal that can be measured to the smallest.

1. Scintillation detector

2. Gas or Xenon gas-filled ionization chambers detector

2 classifications of Detectors

Scintillation Detector (Solid State Detectors)

Process: Certain type of material will scintillate when irradiated, they emit a flash of light immediately in response to absorption of ionizing radiation.

Bismuth Germinate and Cesium Iodide

Early detectors contained crystal photomultiplier tube assembly - __ and __

Bismuth Germinate and Cesium Iodide

These detectors Required power supply.

Cadmium Tungstate

Bismuth Germinate and Cesium Iodide were Replaced by __ crystal-photodiode assemblies. (currently used)

Photodiodes (Crystal-Photodiode Assemblies)

are smaller and cheaper and do not require a power supply

Scintillation Detector (Solid State Detectors)

High x-ray detection efficiency, approximately 90% of the x-ray incident on the detector are absorbed and contribute to the output signal.

Solid State Detector Array

Uses crystal that fluoresces when struck by an x-ray photon

Scintillation detectors

high atomic number and high density compared to air

Solid State Detector Array

RATIONALE: higher absorption than xenon gas

Solid State Detector Array

ADVANTAGE: increased absorption efficiency

Solid State Detector Array

DISADVANTAGE: may produce bridge after glow

Sodium Iodide (Nal)
Bismuth germinate and Cesium lodide
Cadmium tungstate and Ceramic Rare Earth (Gd or Y)

Solid State Detector Array Composition:

Sodium Iodide (Nal)

Solid State Detector Array used for the earliest imaging system

Bismuth germinate and Cesium lodide

Quickly replaces Nal crystal (Solid State Detector Array)

Cadmium tungstate and Ceramic Rare Earth (Gd - Gadolinium or Y - Yttrium)

current crystal of choice (Solid State Detector Array)

Photodiodes

Attached to the crystals Transforms light energy into electrical signal

1. Small

2. Cheap

3. Does not require power supply

Characteristics of Photodiodes

Photoconduction

Photodiodes used in CT scanners do not require an external power supply because they operate based on the principle of __, where they generate a current or voltage directly from the incident light (or X-ray photons in this case)

reduces px dose and faster imaging time

High x-ray detection efficiency (90%) results to:

no loss in your front window

High absorption efficiency (nearly 100%) results to:

Photodiodes

Produce afterglow

Photodiodes

More sensitive to temperature fluctuation and moisture

1. The photons interacts with the scintillators crystals

2. The crystals emits light

3. Photodiode converts light into electrical signal

4. Electrical signals are produced (Raw data)

Process of Solid State Detectors

metallic chamber, 1mm

Gas Detectors or Xenon Gas-Filled is constructed by a large __ with baffles spaced at approximately __ intervals.

Gas Detectors or Xenon Gas-Filled

the entire detector array is hermetically sealed and filled under pressure with an inert gas with high atomic number.

45%

__% approximately is the overall total detection efficiency of Gas Detectors or Xenon Gas-Filled

Gas Detectors or Xenon Gas-Filled

Process: Essentially, they measure ionization in air by attracting to an electrode the ions created by x-rays photon in the air.

1. The photon enters the channel

2. Xenon gas is ionized

3. lons are accelerated and amplified by the electric field between the plates

4. Electric current is then processed as raw data

Gas Field Detectors Process

SSD

SSD or GFD: High Photon Absorption

GFD

SSD or GFD: Moderate Photon Absorption

SSD

SSD or GFD: Sensitive to temperature and moisture

GFD

SSD or GFD: Highly Stable

SSD

SSD or GFD: Solid Material

GFD

SSD or GFD: Low Density Material

SSD

SSD or GFD: Can exhibit afterglow

GFD

SSD or GFD: No Afterglow

SSD

SSD or GFD: No Front Window Loss

GFD

SSD or GFD: Losses attributable to front window and the spaces taken up by the plates

1. Geometric Efficiency

2. Quantum Efficiency

3. Conversion Efficiency Additional Efficiency: Capture Efficiency Absorption Efficiency Overall Efficiency

IMPORTANT FACTORS CONTRIBUTING TO DETECTOR EFFICIENCY

Geometric Efficiency

refers to the area of the detectors sensitive to radiation as a fraction of the total exposed area.

Quantum Efficiency

refers to the fraction of incident x-rays on the detector that are absorbed and contribute to the measured signal.

Conversion Efficiency

refers to the ability to accurately convert the absorbed x-ray signal into an electrical signal.

Capture Efficiency

refers to the ability with which the detector obtains photons that have passed through the patient

Absorption Efficiency

refers to the number of photons absorbed by the detector

Overall Efficiency

is the product of the 3 most common efficiency