Transcript CT Physics Lecture 3

CT Physics
Lecture 3
Brhemstrahlung
Characteristic X-rays
3rd Generation CT
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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 or slice?
What is the difference between a pixel
and a voxel?
Overview of CT system
Overview of CT system
Gantry
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Ring-shaped part of the CT scanner
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Houses
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Slip rings
Generator
Cooling system
X-ray source
Filtration
Collimation
Detectors
Gantry
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Aperture (The Hole)
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Tilt capabilities
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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
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Positioning ISOCENTER
Gantry
X
Y
Z
Gantry
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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
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Carbon Fiber Top
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Strong & rigid
Low absorption
Floats and Rests on pedestal
Vertical and Horizontal
movement
Weight limit
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Generally 450 lbs.
Philips Brilliance Large Bore = 650
lbs.
What is a Slip Ring?
What is a Slip Ring?
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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?
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Brushes enable transmission
of power by sliding in and
out of grooves on the
stationary ring
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2 Brush designs
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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
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AC power delivered to
high voltage generator
Supplies Slip Ring which
powers tube
GENERATOR DOES
NOT ROTATE WITH
TUBE
Low Voltage Slip Ring
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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
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Modern CT units can accommodate 3 – 5 million HU
Heat Unit
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Product of kVp, mA and seconds – the heat generated.
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When heated to capacity machinery will automatically
compensate
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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
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Tube:
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Lead lined cast steel:
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NOT PYREX GLASS
Lead lined to further contain
potential leakage or off focus
radiation
Greater cooling properties
More efficient isolation of high
voltages
Getter: “A deposit of reactive
material that is placed inside a
vacuum”.
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Barium
Helps ensure vacuum by absorbing
any air molecules potentially
released by the target during
operation
CT Unique Components
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Bow Tie Filter
Thicker at ends than in
the middle, help shape the
beam to reduce patient
radiation exposure.
Compensate for uneven
attenuation by the patient.
CT Unique Components
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Collimator
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2 kinds
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Pre-Patient
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Beam Collimation - Single Detector
Detector Collimation – MultiDetector
Mechanical device that restricts
radiation exposure to area of
interest.
Consists of several sections so a
nearly parallel x-ray beam results.
Detector (Pre-detector)
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Assist in shaping the beam
Remove any scattered radiation
from reaching the detectors
Collimation
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Restricts the x-ray beam to a specific area
Reduces scatter radiation
Improves contrast resolution
Decreases patient dose
Single-Detector Row Systems (Beam
Collimation)
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Each detector element is
quite wide in the z
direction
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Opening or closing the
collimator controls the
slice thickness by
controlling the portion of
the detector’s width that is
exposed
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Most scanners before 1990.
Single-Detector Row Systems (cont’d)
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Example… Pre-patient collimation is mechanically set to
4 mm wide along the z-axis. The width of the tissue
exposed at the center of beam rotation is 4 mm. This
means that a beam collimation of 4 mm results in 2D
projections that each represent 4 mm.
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Calculating the area of patient anatomy to be covered
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Simple process of multiplying the slice increment selected by
the number of slices acquired
Review
An examination protocol of the chest calls for
contiguous, 4-mm slices to be taken from the level of
the sternal notch to the lung base; 60 slices are
planned. How much anatomy (in the z direction) will
be covered with cross-sectional slices?
a. 15 mm
b. 64 mm
c. 120 mm
d. 240 mm
Answer
d. 240 mm
4 mm x 60 = 240 mm
Multi-slice Detectors
Multi-row(slice) Detectors
• Purpose = better z-axis
resolution
• IE. 16 slice scanner will be 16
times faster than single slice
 Consists of multiple separate
detector rows which can image
simultaneously up to the
maximum amount of slices per
360 degree rotation
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4 slice scanner = 4 slices per
rotation.
ARRAY TYPES
Multi-slice Detectors
Two Different Types
Matrix Array Detectors
• AKA Fixed Array
All detector cells are equal in dimension along an array
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Perfect Cubes one right after the other
Adaptive Array Detectors
Detector cells have different sizes along an array
Multi-slice Detectors
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Number of slices is dependent
on configuration used
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Configuration can use each
individual detector to produce
that size slice
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Data collection “channels”
Ie. 16-1.25 mm detector can
produce 16-1.25 mm slices
Or….can use combination to
produce larger slices
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Ie. 16-1.25 mm detector can
produce 8-2.5 mm slices or 4-5
mm slices
MDCT Collimation
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MDCT is collimated to the total width of the slices acquired
per rotation.
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Specified in number of detectors (N) x detector width (T).
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Know equation – T (detector size), D (x-ray beam size) and N
is number of detectors.
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D = N xT
Eg. 16 detectors x 1.25 mm thick detectors = 20 mm
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The x-ray beam is limited by the pre-patient collimators to a
maximum dimension of 20 mm along the z-axis
Multi-slice Detectors
Detectors
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Purpose – capture
attenuated radiation
from the patient and
convert it into an
electrical signal
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Signal then converted
into digital data
Desired Detector Characteristics
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High Efficiency – Detector is efficient at absorbing the xrays
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Rapid Signal Decay – Must be capable of excellent
response time and limited after glow.
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Response time – ability of detector to quickly measure x-rays
and then recover before the next measurement.
After-glow – Tendancy of scintillator to glow continuously in
response to x-rays.
Detector Characteristics
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Efficiency
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Geometric Efficiency
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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
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Number of photons absorbed by
detectors
 Atomic #; Density of material; Size
and Thickness of the face
Detector Characteristics
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Stability
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Response Time
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How quickly the detector can detect a
photon and how quick can it recover in
order to record the next event
Dynamic Range
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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
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continuous luminescence after event
Types of Detectors
1.) Scintillation Detectors
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Scintillation crystal coupled to a
photodiode or a PM tube (modern
day is photodiode)
1.
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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
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Solid State Scintillation Crystals coupled with photodiode
Scintillation crystal – calcium tungstate and ceramic to
which the photodiodes are optically coupled
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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
• 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
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Pressure 30 Atm to
increase probability of
event
Excellent stability
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Detection efficiency less
than scintillation detector
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Zero Afterglow
50 – 60 % compared to 95
– 100 %
No longer utilized…
DAS
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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 signal
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
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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
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ADC
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Analog to Digital Converter – divides the electrical signals into
multiple portions
The more portions the better the digital signal…
Optoelectronics
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Series of lens and light diodes which moves data transmissions
from the ADC to the computer at tremendously high speed
50 million bits / sec.
DAS Overview
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DAS consists of electronic components responsible for
measuring the transmitted x-rays.
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Electronic amplifiers condition and boost the signal that the
detectors transmit
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The ADC converts the signal into digital form
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DAS has a maximum number of data channels used to transmit
data from the detectors to the computers
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The number of data channels controls the slices the system
can acquire with one rotation.
References
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Image from of Sprawls.com
Stewart Bushong “Radiologic Science for Technologists”
Bushberg et al., “The Essential Physics of Medical Imaging”
Wikipedia
Impact.org