Week 3 - Basic Instrumentation Flashcards
What are the 3 major components of CT?
-Imaging system -scanner room
-Computer system-computer room
-Display, recording and storage system-operators room
T/F
There needs to be sufficient space behind the gantry for the table to continue through on a clear path
True
Mounted framework that surrounds the patient in a vertical plane
Gantry
What two components are considered the CT scanner?
CT gantry and CT patient table
What component are mounted on the rotating frame of the gantry?
-generator
-tube
-detectors
-collimators
-DAS
What is the function of the DAS - data acquisition system?
Converts information (electrical signal/analog information) into digital data
What type of generator is used in the gantry?
Small, solid state, high frequency
What is the range of hertz for the high frequency generator?
0.5-25 kHz
T/F
Because the generator is located close to the clay tube, only a short high tension cable is required to couple the xray tube and generator
True
What is the power rating of the high frequency generator?
30-60kW
Allows large selection of exposure techniques
What is the kV and mA range enabled by the power rating?
80-140kV
20-600mA
Why are higher kV setting used?
Higher kV reduces heat load due to lower mA used
T/F
The xray tubes of slip ring scanners require high instantaneous power and have larger anodes
True
What is the typical diameter of the anode?
5 inch diameter to support tube loading
What are the sizes of the focal spots used?
Dual - size: 0.5 and 1.0 mm
Angle: small (~12)
3600-10000 RPM
Why are small focal spots more effective in CT?
Reduce penumbra
Better spatial resolution
*heat stress can occur on small focal spots
T/F
Thin slices increase spatial resolution
True
What is the unit of anode heat capacity?
Million Heat Units (MHU)
What is the maximum anode heat dissipation rate unit?
Thousand Heat Units (KHU)
What must be taken into consideration when designing protocols as scan time can go above 30 seconds?
Million Heat Units (MHU)
How many filaments can be positioned in a focusing cup?
1 or 2
What parts does the anode consist of?
Rotor, disk, bearings
What are the 3 types of anode disk?
-all metal
-brazed graphite
-chemical Vapor deposition (CVD) graphite disk
Consists of base body made of titanium, zirconium, molybdenum (TMZ)
Can transfer heat from focal track very quickly
All metal disk
**not ideal for spiral scanners - very heavy
Tungsten-rhenium focal track brazed with graphite at the base
Graphite has 10x thermal capacity than that of tungsten
Tubes with spiral scan use this type
Brazed graphite disk
Tungsten-rhenium layer deposited by chemical Vapor process
Graphite based
Large, lightweight, large heat capacity, fast cooling
Also ideal for spiral scan
CVD graphite disk
What is the purpose of filtration?
Remove long wavelength X-rays contributing to dose and artifacts
Shapes the energy distribution across regions of the body
What dose a ‘hard’ beam mean?
Beam is more penetrating as lower energy xray photons have been removed
What type of filtration takes into account differential thickness?
And is useful for areas with different thicknesses, shows more uniform
Bowtie filter
T/F
A filter will be less effective if a patient is not properly isocentered
True
Suboptimal image will be seen, can also over radiate the patient (increased dose)
T/F
Long wavelength X-rays are readily absorbed by the patient
True
They do not contribute to the image, just patient dose
Low energy, remain in patient
When is isocentering done?
Before a scout is done
What is the purpose of a bow tie filter?
Useful for areas with differential thickness, shows more uniform on image
Why is filtration used?
It is essential that the polychromatic beam has the appearance of a monochromatic beam to satisfy the requirements of the reconstruction process
T/F
A ‘harder’ beam can cause beam hardening artifacts
True
What can beam hardening artifacts be referred to as?
Spectral shaping
Desiring the beam to the anatomy of interest
Reduce scatter radiation
Decrease dose
Improve image quality
Improve contrast resolution
Collimation
*narrowing(decreases slice thickness)
*widening(increases slice thickness)
T/F
Collimation is based on scout images to ensure no clipping of anatomy
True
T/F
Which scout is required for a head CT? Why?
Lateral scout
AP not required because head is in holder
Determines slice thickness
Determines patient dose across slice
Pre patient collimator
Prevents scatter from reaching detectors
Post patient or predetector collimation
T/F
Both pre and post patient collimators control slice thickness and help shape the beam
True
What s the size of the aperture on most CT scanners
70cm
To what degree must the gantry be able to tilt?
+/-30 degrees
(12-30)
Usually in 5 degree increments
What are the laser beams used for on the gantry?
Positioning the patient in all 3 axes
Measures intensity of transmitted rays from the xray source to a particular detector
Detector array
What are the essential detector characteristics for CT image production
-efficiency
-response time
-dynamic range
-high reproducibility
-stability
Efficiency to which the detectors can obtain photons transmitted from patient
Detector capture efficiency
Number of photons absorbed by detector, which depends on
-atomic number
-physical density
-size and thickness of detector face
Detector Absorption efficiency
Speed of detector detecting an xray and recovering to detect another xray
- should be very short
- defined as brief, persistent flash of scintillation (flashes of light)
Detector response time
Ratio of the largest signal to be measured to the precision of the smallest signal to be discriminated
- 1M:1 in most CT scanners
Detector dynamic range
Refers to the steadiness of the detector
Allows a system to be used without interruption of frequent calibration
Detector stability
What is the function of a detector?
Capture xray photons and convert them into electrical signals (analog information)
What are the 2 types of detectors?
Scintillation
Gas ionization
T/F
Photodiodes do not exist in gas ionization detectors
True
What size of detector is ideal for spatial resolution and scatter rejection?
Small detector
What type of detector is used in modern CT?
Xenon gas detector
T/F
The photodiode/scintillation component is eliminated in the xenon gas detector
True
What type of electrical supply did early CT scanners use? What were the setbacks?
Recoiling system cable
-limited rotation up to 360
-limited scan/acquisition to step and shoot
-increased scanning time
(Only axial scanning, not helical)
Single slice
What causes the gantry frame to be able to rotate continuously?
Slip ring technology
-electromechanical device
-circular electrical conductive rings
-brushes that transmit electrical energy across a rotating interface
What are some advantages to slip ring technology?
-minimal inters can delay
-eliminates need to straighten cables
-eliminates start and stop process
-capacity for continuous acquisition protocols
What is the table/patient couch made of?
Carbon fibre
-low absorption value
-excellent vibration-damping feature
-strength to support patient weight
What is the scannable range of the patient table?
170-200cm
Reference point : 0
-centering
-landmark must be consistent*
Center point of the body is in the Center of the gantry
Isocentering
What is the function of the computers?
-image reconstruction
-image manipulation
-visualization
Dedicated electronic circuit of high speed calculations needed in CT
Array processor
Completed the computationally intensive reconstruction operations needed to provide the final image
Reduces the processing requirements of the computer
Graphic processing unit
Contains algorithms for building up images from raw data
Reconstruction software
Corrects data collected from detectors prior to being sent to reconstruction CPU
Preprocessing software
Operate on reconstructed images to facilitate viewing
Image postprocessing software
What is the typical image matrix size?
512x512
To speed up transmission of information and storage requirements
Lossless: no info lost
Lossy: some info lost
Compression
Controls the entire system and enables operations of various functions
Interested console
T/F
Helical CT table incrementation is quantified in mm/s because the table continues to move throughout the scan
True
