Download 1199 CT Physics

CT Physics
Lecture 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)



Tilt capabilities


Commonly 70 cm
Large Bore – 80 – 90 cm (Toshiba Acquilon LB)
Varies from system to system but usually between +/- 12 to +/30 degrees in 0.5 degree increments
Laser

Positioning ISOCENTER
Gantry
X
Y
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.


When heated to capacity machinery will automatically
compensate




Eg 75kVp X 100 mA x 2 s = 15000 heat units
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



Pre-Patient



Beam Collimation - Single
Detector
Detector Collimation – MutiDetector
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
•

Capture Efficiency


How well the detector receive attenuated
photons
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



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



Detector response
How often is calibration needed?
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 tube
1.
2.
3.
4.
X-Ray falls onto crystal which
upon interaction creates photons
of light
Light gets directed to the
photomultiplier increasing the
light
Light strikes photocathode which
emits electrons
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 Detectors
2.) 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 chambers
2. Gas becomes ionized
3. 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


Detection efficiency less
than scintillation detector


Zero Afterglow
50 – 60 % compared to 95
– 100 %
No longer utilized…
Multi-slice Detectors
Dual 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 Detectors
Multi-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 Array
All detector cells are equal in dimension along an array
•
Perfect Cubes one right after the other
Adaptive Array Detectors
Detector cells have different sizes along an array
Multi-slice Detectors

Number of slices is
dependent on configuration
used


Configuration can use each
individual detector to
produce that size slice


Data collection “channels”
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 functions
1.
2.
3.
Measures attenuated radiation
Converts measurements to digital
Transmits digital data set to computer
DAS
DAS 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…or
µ1 + µ2 + µ3 = (ln I0 / 1) (I / x)
µ = linear attenuation coefficient
I0 = Original intensity
I = Transmitted intensity
x = 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 Acquisition
In 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