Week 3 - Basic Instrumentation Flashcards by Gracie Costello

What are the 3 major components of CT?

-Imaging system -scanner room
-Computer system-computer room
-Display, recording and storage system-operators room

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T/F
There needs to be sufficient space behind the gantry for the table to continue through on a clear path

True

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Mounted framework that surrounds the patient in a vertical plane

Gantry

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What two components are considered the CT scanner?

CT gantry and CT patient table

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What component are mounted on the rotating frame of the gantry?

-generator
-tube
-detectors
-collimators
-DAS

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What is the function of the DAS - data acquisition system?

Converts information (electrical signal/analog information) into digital data

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What type of generator is used in the gantry?

Small, solid state, high frequency

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What is the range of hertz for the high frequency generator?

0.5-25 kHz

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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

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What is the power rating of the high frequency generator?

30-60kW
Allows large selection of exposure techniques

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What is the kV and mA range enabled by the power rating?

80-140kV

20-600mA

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Why are higher kV setting used?

Higher kV reduces heat load due to lower mA used

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T/F
The xray tubes of slip ring scanners require high instantaneous power and have larger anodes

True

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What is the typical diameter of the anode?

5 inch diameter to support tube loading

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What are the sizes of the focal spots used?

Dual - size: 0.5 and 1.0 mm
Angle: small (~12)
3600-10000 RPM

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Why are small focal spots more effective in CT?

Reduce penumbra
Better spatial resolution
*heat stress can occur on small focal spots

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T/F
Thin slices increase spatial resolution

True

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What is the unit of anode heat capacity?

Million Heat Units (MHU)

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What is the maximum anode heat dissipation rate unit?

Thousand Heat Units (KHU)

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What must be taken into consideration when designing protocols as scan time can go above 30 seconds?

Million Heat Units (MHU)

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How many filaments can be positioned in a focusing cup?

1 or 2

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What parts does the anode consist of?

Rotor, disk, bearings

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What are the 3 types of anode disk?

-all metal
-brazed graphite
-chemical Vapor deposition (CVD) graphite disk

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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

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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