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Computed Tomography
Chapter 10
115
Medical
Post-Graduate
07/11/2016

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Term
The rotating part of the CT scanner
Definition
gantry
Term
Electrical connections between the rotating gantry and the stationary components are made using ________
Definition
slip rings
Term
CT utilization rates increased starting in the late 80's due to ________ and again in late 90's due to ___________.
Definition
helical scanning, multidetector array
Term
Projection definition and other terms for it.
Definition
The data collected at a specific angle.  Also called profile or view.
Term
Rays
Definition
An individual attenuation measurement that corresponds to a line through the object.  It goes from the source to an individual detector element.
Term
A projection is a collection of ______.
Definition
rays
Term
Range of maximum FOV diameter?
Definition
50-70 cm
Term
Fan beam geometry
Definition
The angle formed from the source as it spreads across the entire row of detectors.
Term
For narrow cone beam geometry, the fan beam is typically ____ degrees and the full cone angle is _____ degrees.
Definition
fan beam - 60 degrees, cone beam 2.4 degrees
Term
Narrow cone beam geometery
Definition
Formed by the small x-ray beam divergence in the z-axis direction.
Term
Full cone beam geometery has a half angle of ____ degrees and typically uses _______ detectors.
Definition
10 degrees, flat panel detectors
Term
Pixel array size for a CT image.
Definition
512x512
Term
The slice thickness is defined where?
Definition
at the isocenter.
Term
Typical power rating for a CT tube?
Definition
5-7 megaJoules (MJ)
Term
How do CT scanners reduce scatter?
Definition
Anti-scatter grid - the grid septa are aligned with the deadspaces between the detector elements.
Term
The anode-cathode axis runs in what direction.
Definition
Along the z-axis.
Term
What advantage does having the anode rotate in the same plane as the gantry have.
Definition
reduces gyroscopic effects
Term
The heel effect occurs in what direction on a CT scanner?
Definition
Along the z-axis, perpendicular to the fan angle.
Term
Typical number of projections in a single rotation.
Definition
1000 - 3000
Term
Typical time for one rotation of the gantry.
Definition
0.3 to 1 sec
Term
The gantry is moving during the time it takes to acquire one projection which can lead to motion blurring and a loss of spatial resolution.  How is this corrected?
Definition
A magnetic steering system is used to guide the electrons as they travel from the cathode to the anode to essentially rotate the focal spot in the opposite direction of the gantry rotation.
Term
Typical tube voltages used in CT.
Definition
80, 100, 120, and 140 kVp.  120 is the most common.
Term
At CT energies, the primary photon interaction is
Definition
Compton Scattering
Term
Compton scattering is primarily affected by
Definition
electron density
Term
The electron density is given by
Definition

ρN(Z/A)

ρ - density, N - Avogadro's #, Z - atomic #, A - atom mass

Term
Hounsfield units (HU) are defined as
Definition
HU = 1000 * (μvoxel - μwater) / μwater
Term
By definition, at any energy the HU for water and air are?
Definition

HUwater = 0

HUair = -1000

Term

HU for the lungs,

HU for adipose (fat) tissue,

HU for most organ parenchyma

Definition

HU lung = -500

HU adipose = -80 to -30

HU organs = 30 to 220

Term

HU for soft tissue with contrast

HU for bones

Definition

HU soft tissue with contrast = 100 to 300

HU bones = 700 to 3000

Term
Maximum HU for a 12 bit scanner
Definition
3095
Term
Most common CT scans are
Definition

Head and torso

Torso can be broken down into chest, abdomen, and pelvis

Term
CT uses a beam shaping filter.  What is it called and what is it used for?
Definition
Bowtie filter - it reduces the intensity of the x-ray beam in the periphery of the x-ray field where the attenuation path through the patient is typically thinner.  This results in the signal levels at the detector being more homogeneous.
Term
The bowtie filter ______ patient dose and ______ image quality.
Definition
reduces, doesn't effect
Term
CT scanners use __________ __________ detectors.
Definition
indirect (scintillating) solid-state
Term
A common scintillation crystal used in a CT scanner.
Definition

Gd2S2O

 

Gadolinium Oxysulfide

Term
What is done to the scintillation crystal to increase density and improve light output?
Definition
Scintering - heating it for long periods of time.
Term
CT detector modules consist of
Definition
Ceramic scintillator coupled to photodiodes that are layered on substrate electronics.
Term
In the old single detector array scanner, the slice thickness was determined by
Definition
the beam collimation
Term
In MDCT, the slice thickness is determined by
Definition
the detector configuation
Term
In MDCT, the beam width is determined by
Definition
the collimator
Term
The overall beam width is given by
Definition

nT

n - number of data slices, T - thickness of a data slice

Term
The advantage of MDCT is
Definition
faster scan times and thinner slices.
Term
The downside to thinner slices is
Definition
they tend to be noiser.
Term
The trade-off in MDCT revolves around
Definition
Image noise, patient dose, and z-axis resolution.
Term
Overbeaming
Definition
Placing the x-ray beam penumbra outside of the active detectors so a relatively constant intensity beam strikes the detectors
Term
Overbeaming results in a loss in ________ ________.
Definition
Geometric efficiency
Term
Geometric efficiency for state of the art MDCT scanners.
Definition
> 95%
Term
Axial slices are typically reconstructed to what thickness for radiologist interpretation?
Definition
5 mm
Term
A CT radiograph is also called
Definition
scout view, topogram, scanogram, or localizer
Term
During a CT radiograph, the x-ray tube and detectors are __________.
Definition
stationary
Term
The CT radiograph is used for
Definition
patient alignment
Term
Describe Axial CT scanning.
Definition
The gantry makes a revolution around the patient with the x-ray tube on.  The tube is turned off and the patient table moves forward about the distance of the beam width.  This process is then repeated.
Term
Axial CT scans require _______ acquisition time than helical CT scans.
Definition
more
Term
Describe helical scanning
Definition
The table continually moves at a constant speed while the gantry rotates around the patient.  The x-ray tube is continually on during the scan.
Term
The pitch of a helical scan is defined as
Definition
P = table travel (feed) distance per full gantry rotation / nominal collimated beam width (nT)
Term
Typical pitch values
Definition
0.75 to 1.5
Term
How does dose vary with pitch.
Definition

inversely proportional

dose ~ 1/pitch

Term
Why do helical scans have to start before an axial scan would start and end after an axial scan would end?
Definition
There is not enough angular coverage at the start and end of a scan to reconstruct the CT image.
Term
Where do helical scans start and end vs an axial scan.
Definition
Start (1/2)nT before the axial and end (1/2)nT after the axial scan.
Term
The wasted dose at the ends of a helical scan increase as a percentage of the overall dose for
Definition
smaller scan lengths and larger collimated beam widths
Term
Advantage and disadvantage of a cone beam acquisition.
Definition
Advantage - A whole organ can be imaged without table motion.  Disadvantage - Increased x-ray scatter and increased cone-beam artifacts.
Term
One cardiac cycle takes about _____ and to freeze cardiac motion requires an image acquisition time of ______ or less.
Definition
1 sec, 100 msec
Term
For true CT reconstruction, how much angular data is required.
Definition
180 degrees plus the fan angle
Term
Cardica CT - describe retrospective gating
Definition
The ECG is acquired with the CT data.  After the scan, the desired time points are selected to reconstruct the image.
Term
Cardiac CT - describe prospective gating
Definition
The ECG is used to trigger the CT scanner so that data is only acquired when the heart is still (at the end of diastole).
Term
What is the benefit of prospective gating over retrospective gating.
Definition
Much lower dose.  The CT scanner is only on at certain points in the cardiac cycle.
Term
A dual source CT was created for ________.
Definition
cardiac imaging.
Term
The dual source CT only has to rotate through ______ degrees and this requires _______ ms.
Definition
90 degrees, 83 msec (1/3 sec ÷ 4)
Term
Dual energy CT can be described as
Definition
a linear combination of low and high energy CT images.
Term
CTA images are often produced using.
Definition
CT Angiography - produced with maximum intensity projection (MIP) techniques.
Term
CT perfusion studies may require how many CT images?
Definition
40
Term
CT perfusion studies result in ________ patient dose.
Definition
very high
Term
What is shuttle mode CT.
Definition
During a profusion study, the table moves back and forth so a CT image can repeatedly be made over an area that is wider than the detector array (nT).
Term
mA modulation allows for
Definition
lower dose levels to be used to achieve the same image quality.
Term
Tube current modulation occurs in what direction.
Definition
mA is modulated as the tube rotates around the patient and also along the z-axis.
Term
Projection measurements undergo _________ and _________ transformation.  Why
Definition

normalization - to a reference detector to account for bowtie filters and different detector responses

logmarithmic transformation - so the measurement is the sum of linear attenuation coefficients instead of an exponent.

Pj = t(μ1 + μ2 + μ3 + .... )

Term
Creating projection data from central measurements is called
Definition
forward projection
Term
Back-projection
Definition
The process of creating the image matrix from the measured projection data.
Term
What problem arises with simple back-projection?
Definition
1/r blurring
Term
What process solves the 1/r blurring found in simple backprojection?
Definition
Filtered backprojection
Term
How does "filtering" work in filtered backprojection.
Definition
A deconvolution kernel is applied to the measured projection data that undoes the effect of the 1/r blur funtion.
Term

How can the convolution of p(x) and h(x) be calculated using the Fourier transform?

Why would you do this?

Definition

Take the FT of each function and multiple the results together.  Then take the inverse FT.

It is faster on a computer.

Term
What is the FT of the deconvolution kernel used in filtered backprojection?
Definition
It is the ramp filter.  It is zero at zero frequency and ramps up linearly with frequency.
Term
What can you do to the ramp filter to remove noise?
Definition
Since noise is a high frequency component, have the ramp filter "roll-off" as frequency increases.
Term
The cone beam reconstruction process is called?
Definition
The Feldkampt algorithm or FDK
Term
How does iterative reconstruction work.
Definition
The CT image is iterated.  At each iteration the forward projection data is calculated.  The difference between the forward projection data and the measured projection data is called the error matrix.  The goal is to construct a CT image that minimizes the error matrix.
Term
What is the downside to iterative reconstruction?
Definition
It is very numerically intense and requires a lot of computer power and time.
Term
What is the advantage to iterative construction over filtered backprojection?
Definition
It results in images with better SNR for the same dose.
Term
The ACR phantom has how many bar patterns and at what frequencies for measuring spatial resolution?
Definition
There are eight bar patterns.  4, 5, 6, 7, 8, 9, 10, and 12 lp/cm
Term
The MTF is affected by the
Definition
reconstruction kernel.
Term
How would you measure the MTF?
Definition
By imaging a wire or plane of metal foil and measuring the line spread function (LSF).
Term
Spatial resolution in the z-axis is measured using the
Definition
slice sensitivity profile (SSP)
Term
How is slice thickness determined from the ACR phantom.
Definition
There are a series of high contrast wires spaced 0.5 mm apart in the z-direction.  You count the number of visible wires and multiple by 0.5 mm.
Term
The MTF in the z-direction is ________ for thinner slices.
Definition
higher
Term
How is dose distribution in the z-axis measured?
Definition
It has to be measured with an external device such as film or a CR plate.
Term
The low-contrast module of the ACR phantom has test objects that have a ____ HU greater density than the surrounding material.
Definition
6
Term
In CT, the direct measurement of noise is given by
Definition
the standard deviation of the HUs
Term
Noise texture refers to
Definition
the frequency dependancy of the noise.
Term
Noise correlation in the (x,y) plane in CT is largely due to
Definition
the filtered backprojection reconstruction kernel.
Term
Primary factors affecting spatial resolution in CT.
Definition
1) focal spot distribution, 2) gantry motion, 3) detector size and sampling, 4) reconstruction filter
Term
Primary factors that affect contrast resolution (noise) in CT.
Definition
1) technique factors; kV, mA, time, and pitch, 2) slice thickness, 3) reconstruction filter (filtered backprojection), 4) reconstruction method - iterative reconstruction significantly reduces image noise
Term
Beam hardening artifacts appear as
Definition
dark webbing between dense regions.
Term
How are beam hardening artifacts reduced?
Definition
By prehardening the beam with added filtration.
Term
When do streak artifacts occur?
Definition
When attenuation levels are excessive such as around metal fillings or cardiac pacemakers.
Term
What amplifies streak artifacts?
Definition
patient motion
Term
View aliasing is caused by
Definition
to few projections acquired for reconstructing high-frequency objects.
Term
What are partial volume artifacts
Definition
When a voxel contains more than one type of tissue.
Term
How are partial volume artifacts avoided?
Definition
By using smaller voxels.
Term
Cone beam artifacts are generally due to
Definition
not acquiring enough data in the cone beam direction.
Term
Describe 1st generation CT
Definition
Pencil beam, translate-rotate
Term
Describe 2nd generation CT
Definition
small fan beam, translate and rotate
Term
Describe 3rd generation CT
Definition
Large fan beam, rotate-rotate - both the x-ray tube and the detector rotate (no more translating of the x-ray tube)
Term
The diameter of the FOV is given by
Definition

FOV = 2S sin (α/2)

S - source to isocenter distance

α - fan angle

Term
Describe a 4th generation CT scanner?
Definition
The x-ray tube rotates, but the detector ring is stationary.  This requires the detector ring to go all the way around.  This is expensive.
Term
What problem from 3rd generation scanners was the 4th generation trying to solve.
Definition
Dead or improperly calibrated detectors would result in ring artifacts.
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