CT PhysicsLecture 3

Data Acquisition: Major Components

Gantry Generator Xray Tube Collimator + Filter Detector Array DAS – Data Acquisition System

Patient table

3rd Generation CT Discussed generations of CT scanners –

modern day scanner is 3rd generation with rotating x-ray tube and detectors.

3rd Generation CT System

What do we mean by cross section?

Gantry Ring-shaped part of the CT scanner

Houses Slip rings Generator Cooling system X-ray source Filtration Collimation Detectors

Gantry

Aperture (The Hole) Commonly 70 cm Large Bore – 80 – 90 cm (Toshiba Acquilon LB)

Tilt capabilities Varies from system to system but usually between

+/- 12 to +/- 30 degrees in 0.5 degree increments Laser

Positioning ISOCENTER

Gantry

XY

Z

Gantry

Important to recognize that especially in the imaging of a bariatric patient that despite a listed aperture size that it does not account for the table height for which has to advance into the bore

Couch

Carbon Fiber Top Strong & rigid Low absorption Floats and Rests on

pedestal Vertical and Horizontal

movement

Weight limit Generally 450 lbs. Philips Brilliance Large

Bore = 650 lbs.

Couch Typical Maximum Scan Range

Not comparable to the length of the couch Generally can cover approximately 162 cm of scan

length Most modern scanners do have a general increase

in scan length

What is a Slip Ring?

Its what made helical scanning possible by providing continuous rotation without conventional cables

Enables the transmission of power and electrical signals from a stationary to a rotating structure.

Consists of conductive rings and brushes which facilitates the transfers

Allows for faster scan times and continuous acquisitions without cable worry

What is a Slip Ring?

Brushes enable transmission of power by sliding in and out of grooves on the stationary ring 2 Brush designs

Wire Brush – Conductive wire as contact

Composite Brush – Conductive material as contact

What is a Slip Ring?

What is a Slip Ring?High Voltage Slip Ring

• AC power delivered to high voltage generator

• Supplies Slip Ring which powers tube

• GENERATOR DOES NOT ROTATE WITH TUBE

Low Voltage Slip Ring

• More Common today• AC Power is

transferred to slip rings by brushes

• Provides power to high voltage (step up) transformer then to tube

• POSITIONED TO ROTATE WITH TUBE

Generator

Produce high voltage for the creation of x-ray photons

Modern scanners use High Frequency Generator

Parameters we can control:kVp (80, 100, 120, 140)mA (25-1000)Time (0.5 – 2 sec)

mA * time = mAs

Heat Capacity Modern CT units can accommodate 3 – 5

million HU Heat Unit

Product of kVp, mA and seconds – the heat generated. Eg 75kVp X 100 mA x 2 s = 15000 heat units

When heated to capacity machinery will automatically compensate Decrease kV Decrease mA Decrease time

End result will result in imaging that is sub-optimal due to the increased presence of noise on the images

X-Ray Source Tube:

Lead lined cast steel: NOT PYREX GLASS

Lead lined to further contain potential leakage or off focus radiation

Greater cooling properties More efficient isolation of

high voltages Getter:

Barium Helps ensure vacuum by

absorbing any air molecules potentially released by the target during operation

CT Unique Components

Bow Tie Filter Shaped in order to

compensate for the attenuations within both the head and the body

Effectively hardens the beam and equalizes the amount of radiation reaching the image receptor

CT Unique Components Collimator

2 kinds Beam Collimation - Single

Detector Detector Collimation – Muti-

Detector Pre-Patient

Adjustable; influenced by focal spot size (Penumbra)

Ensures constant beam width at the detectors

Detector (Pre-detector) Assist in shaping the beam Remove any scattered

radiation from reaching the detectors

Collimation Restricts the x-ray beam to a specific area Reduces scatter radiation Improves contrast resolution Decreases patient dose

Collimation Specified in number of detector rows x

detector width. Eg. 16 x 1.25 (20 mm)

Detectors

Purpose – capture attenuated radiation from the patient and convert it into an electrical signal Signal then converted into

digital data

Detector Characteristics Efficiency• How well the detector receive

attenuated photons Capture Efficiency

Efficiency in which detectors can obtain attenuated photons Influenced by detector size

and distance between detectors

Absorption Efficiency Number of photons absorbed by

detectors Atomic #; Density of material;

Size and Thickness of the face

Detector Characteristics Stability

Detector response How often is calibration needed?

Response Time How quickly the detector can detect a

photon and how quick can it recover in order to record the next event

Dynamic Range Ratio of the largest signal to

measured to the precision of the smallest that is discriminated; …Accuracy…

Product of Capture Efficiency, Absorption Efficiency, and Conversion Efficiency

Afterglow continuous luminescence after event

Types of Detectors

1.) Scintillation Detectors

• Scintillation crystal coupled to a photodiode tube1. X-Ray falls onto crystal which

upon interaction creates photons of light

2. Light gets directed to the photomultiplier increasing the light

3. Light strikes photocathode which emits electrons

4. Electrons pass through dynodes which are arranged and maintained resulting in a signal

Scintillation Detector

Solid State Photodiode Scintillation Crystals (Solid State) Photodiode – calcium tungstate and ceramic to

which the crystals are bonded by fiber optics Allows current flow when exposed to light Current is proportional to the amount of light Extremely fast response time Conversion Efficiency = 99 % Capture Efficiency = 99 % Dynamic Range = 1,000,000 to 1

Types of Detectors2.) Gas Ionization Chamber• Think AEC…• Series of individual highly pressurized gas

(Xenon) filled chambers which are separated by metal plates (Tungsten)

1. X-Ray strikes gas filled chambers2. Gas becomes ionized3. Plates are charged + and -, e- migrate to the

positive plate, positive ions to the negative plate

4. As ions move produces a small signal depending on the number of photons

Gas Ionization Detectors Pressure 30 Atm to

increase probability of event

Excellent stability Zero Afterglow

Detection efficiency less than scintillation detector 50 – 60 % compared

to 95 – 100 % No longer utilized…

Multi-slice DetectorsDual Row• Introduced in 1992• Allowed for faster volume

coverage than single slice• Dynamic Focal Spot –

position of focal spot switched by a computer system during each scan in order to double sampling and measurements• Produced two contiguous

slices with great resolution

con·tig·u·ous adjective \kən-ˈti-

gyə-wəs, -gyü-əs\ —used to describe things that touch each other or are immediately next to each other

Multi-slice DetectorsMulti-row(slice) Detectors• Purpose = increase volume

coverage speed of both single and dual slice scanners• IE. 16 slice scanner will be 16

times faster than single slice• Commonly used for machines

which acquire from 4 to 320 slices per rotation

Consists of multiple separate detector rows which can image simultaneously up to the maximum amount of slices per 360 degree rotation 16 slice scanner = 16 slices per

rotation…Duh

ARRAY TYPES

Multi-slice Detectors

Two Different Types

Matrix Array Detectors• AKA Fixed ArrayAll detector cells are equal in dimension along

an array• Perfect Cubes one right after the other

Adaptive Array DetectorsDetector cells have different sizes along an

array

Multi-slice Detectors

Number of slices is dependent on configuration used Data collection “channels”

Configuration can use each individual detector to produce that size slice Ie. 16-1.25 mm detector can

produce 16-1.25 mm slices Or….can use combination

to produce larger slices Ie. 16-1.25 mm detector can

produce 8-2.5 mm slices or 4-5 mm slices

Multi-slice Detectors

DAS Data Acquisition System Refers to all the electronic components which

lie after the detectors but before the computer

Performs 3 functions1. Measures attenuated radiation2. Converts measurements to digital3. Transmits digital data set to computer

DASDAS Components:• Pre-amplifier – takes the weak electrical signal from

the detectors and boosts it so that it can be more easily converted

• Logarithmic amplifier – performs the conversion of attenuated transmission data to logarithmic data which is sent across to ADC• Conversion of transmission to attenuation and thickness

Attenuation = log transmission * thickness…orµ1 + µ2 + µ3 = (ln I0 / 1) (I / x)

µ = linear attenuation coefficient I0 = Original intensityI = Transmitted intensityx = Thickness of Object

DAS ADC

Analog to Digital Converter – divides the electrical signals into multiple portions

The more portions the better the digital signal…now how do we get more portions again? Hmmmmmm… Help determine the bit depth (grey scale) of image

Optoelectronics Series of lens and light diodes which moves data

transmissions from the ADC to the computer at tremendously high speed

50 million bits / sec.

Data AcquisitionIn order to ensure that an adequate amount of

measurements (samples) are obtained for optimal quality…

1. Utilize thin slices to reduce sample artifact…2. Closely Packed Detector Array – closer they

are, the more are available to receive radiation, the more samples obtained…

3. Quarter Shifted Detector Arc – provides 2 sets of data which can be individually reconstructed• Siemens developed

References Image from of Sprawls.com Stewart Bushong “Radiologic Science for

Technologists” Bushberg et al., “The Essential Physics of

Medical Imaging” Wikipedia Impact.org