06.CT6.pptx the best lecturer in CT technical

‫الرحيم‬ ‫الرحمن‬ ‫الله‬ ‫بسم‬
ALZAEM ALAZHARI UNIVERSITY
Faculty of radiological sciences and
medical imaging
Quality management
Lec (6)
Computed tomography QC

INTRODUCTION
• A current computed tomography (CT) system is
composed of numerous electronic parts and
computers that generate and process huge
amounts of data.
• Because of the system's complexity, a quality
control program is essential to ensure optimal
system performance and image quality with the
least amount of radiation dose to the patient.

Movement of the x-ray tube is not helical . It just appears
that way because the patient moves through the plane of
rotation during imaging.

• A, During multislice helical computed tomography,
image data are continuously sampled.
• B, Interpolation of data is performed to reconstruct
the image in any transverse plane.

ACCEPTANCE TESTING
• Typically, the installation of most CT units is
immediately followed by extensive acceptance
testing by qualified physicists.
• Acceptance testing consists of measuring
radiologic and electromechanical performance,
analyzing image performance, and evaluating
the system components.

ACCEPTANCE TESTING
• The results of the acceptance tests are used to
identify system components that may need only
slight adjustments and defective parts that
should be replaced.
• At the conclusion of the acceptance testing,
scans are taken of standard objects so that their
images, CT numbers, and standard deviations
can be recorded as a baseline for future
measurements of the system's performance.

ROUTINE TESTING
• To establish more consistency in the performance
measurements of CT scanners, federal performance
standards state that the vendors of CT systems
manufactured after September 1985 are required to
supply the following:
1. Instructions for performing quality control tests.
2. Schedule for testing.
3. Allowable variations for the indicated parameters.
4. Method to store and record the QC data.
5. Dose information in the form of a CT dose index.

ROUTINE TESTING
• In addition, each vendor is required to
supply phantoms capable of testing the
following parameters:
1. contrast scale, noise, slice
thickness.
2. spatial resolution
capabilities for both high-
and low-contrast objects.
3. The mean CT number of
water or other reference
material.

COUCH INCREMENTATION
• Because the couch (table) moves through the gantry for CT
examinations, the couch incrementation must be precise to ensure
accurate patient position. All that is needed for this test is a
measuring tape.
• Procedure:
1. Before scanning the patient, note the starting
position of the couch.
2. Scan the patient as usual and note the end position
of the couch.
3. Using the measuring tape, measure the distance the
couch moved and compare with the intended couch
movement.

CONTRAST SCALE AND THE MEAN CT
NUMBER OF WATER
• Contrast scale is defined as the change in linear attenuation
coefficient per CT number relative to water.
• The contrast scale is determined by the CT numbers for air (-1000
HU) and water (0 HU).
• This test is done to determine if the scanner is assigning CT numbers
that correspond to the appropriate tissue.
• This is important because many radiologists use CT numbers to
identify suspected pathology in the image.
• To measure contrast scale it is necessary to calculate the CT number
of known materials.

CONTRAST SCALE AND THE MEAN CT NUMBER OF
WATER
• Water is the reference material used to
determine CT numbers because it
constitutes up the most of soft tissue mass,
is easy to obtain, and is completely
reproducible.
• Because water has a CT number value of
zero, tissues with densities greater than
water will have positive CT numbers, and
those with densities less than water will
have negative CT numbers.
• A plastic, water-filled phantom is
commonly used to measure the contrast
scale of a CT system.

CONTRAST SCALE ANDTHE MEAN CT
NUMBER OF WATER (DAILY)
• Procedure:
1. Using a specific technique, take a single scan
through the phantom.
2. On the reconstructed image, place ROI 200-300
pixels in center of field and take measurement.
3. From the pixels located within the ROI calculate
the two parameters, the mean CT number and the
standard deviation of the CT numbers.
4. On a monthly basis move the cursor outside of the
phantom on the reconstructed image and perform
the ROI function over air.

NUMBER OF WATER (DAILY)
• Expected results: CT number of water equal to zero,
but range of +/- 3 at center of image is acceptable,
and +/- 5 HU at peripheral locations.
• Cause of failure is usually miscalibration of the
algorithm that generates CT numbers.

NUMBER OFWATER
In this example the ROI measured 0.07 HU with a
standard deviation of 3.33 HU.

HIGH-CONTRAST SPATIAL RESOLUTION
(MONTHLY)
• The parameters that influence the high-contrast spatial resolution of a
CT scanner include:
1. Scanner design (focal spot size, detector size and
spacing, magnification).
2. Image reconstruction (pixel size, reconstruction
algorithm, slice thickness).
3. Sampling (number of rays per projection and
number of projections).
4. image display capabilities (display matrix).

• Procedure:
1. Take a single scan through the test object
(Phantom with equally spaced holes
drilled in the plastic, used to measure
high-contrast spatial resolution).
2. On the resultant image determine which
row has the smallest set of holes in which
all the holes can be clearly identified.
• This is known as the limiting resolution of
the CT scanner.

• The limiting high-contrast spatial resolution of a CT
scanner is measured in line pairs per centimetre.
• The range of 0.45 to 1.5 lp/mm represents the typical
high-contrast resolution of CT scanners used today.
• Even though many modern
scanners have the ability to
resolve holes as small as
0.3 mm, the spatial resolution
of CT scanners is still much
lower than that of conventional
radiography.

To validate this test of the bar pattern is measured using a box ROI
(yellow arrow). The box ROI should be sized until it fits into the pattern.
Measure the standard deviation of the pixels in this ROI to get a
quantitative assessment of changes in system resolution. The standard
deviation measurement should be 40 +/- 4. This should be compared to
the baseline measurement at equipment acceptance for accuracy

LOW-CONTRAST RESOLUTION
• Compared with conventional radiography, CT provides superior low
contrast resolution.
• Typically, contrast resolution is expressed in one of two ways) the
smallest diameter of an object with a specific contrast that can be
detected or the smallest difference in x-ray attenuation that can be
discriminated for an object of a specific diameter.
• A phantom consisting of test objects, such as holes drilled into
plastic, is used for this test.
• The rows of holes should be of varying sizes and filled with a liquid
that has a CT number that differs from the CT number of the plastic
by approximately 0.5% ex(polystyrene membrane).

This low contrast detectability phantom image displays various sized holes
used to determine low contrast. This test measures the scanners ability to
detect an objects density when it is close to background density.

• Low-contrast resolution is determined as the difference in HU of
objects and background.
• To get an accurate measure of low contrast we need to know the CT
number for the polystyrene membrane.
• This is accomplished by taking the CT number for water over an area
that does not include the membrane.
• A second measurement is taken over an area that includes the
membrane superimposed on water (called water plus the
membrane).
• The CT number taken for water is subtracted from the water plus
membrane to get the CT number for the membrane.

• To perform this test, measure using a
box ROI above and below the
membrane in the water section
(labelled A and D in the phantom image
below).
• Take a box ROI in the polystyrene
membrane above the holes (labelled B)
and below the holes (labelled C).
Subtract "A" from "B" and subtract "D"
from "C".
• When the measurements are completed
and recorded adjust the Window Width
to 20 and the Window Level to the CT
number recorded for water.
Contrast measurements
should be within
equipment manufacturer
specifications.

NOISE AND UNIFORMITY (WEEKLY)
• Noise represents the portion of the CT image that
contains no useful information. It is defined as the
random variation of CT numbers about a mean value
when an image of a uniform object is obtained.
• The contrast resolution of a CT system is primarily
determined by the amount of noise in the images.
Noise produces a "salt and-pepper" appearance or
grainy quality in the image.
• The sources of noise in a CT image include quantum
(statistical) noise, electronic noise, object size,
reconstruction algorithms, and artefacts.

NOISE AND UNIFORMITY
• Factors under the influence of the technologist that
affect the amount of noise in an image are pixel size,
slice thickness, and technique factors.
• Methods to minimize noise and help provide uniform
images include tube warmups and daily system
calibrations.
• Because the amount of noise contained in an image is
inversely proportional to the total amount of radiation
absorbed, noise can be measured by obtaining the
mean and standard deviation of the CT numbers within
an ROI.

• Uniformity refers to the ability of the CT scanner to
yield the same CT number regardless of the location of
the ROI within a homogenous object.
• A simple 20-cm water phantom can be used to
measure noise and uniformity in CT
• Procedure:
1. Take a scan through the water phantom, and position
a cursor over the resultant image in 3-6 different
locations.
2. The cursor should be positioned in the center, at the
top, and at the sides of the image.
3. At each cursor location, take an ROI measurement
and record the standard deviation and mean CT
number.

For uniformity measurements, the uniformity (of the CT
number of water) should not vary more than ±2 HU from
the center of the phantom to the periphery.

SLICE THICKNESS
• The slice thickness in CT is determined primarily by the
collimators.
• The position of the-collimators determines the width of the slice
that falls within the view of each detector.
• Another factor affecting slice thickness is focal spot size. The focal
spot size can influence the penumbra, or sharpness of the edge of
the x-ray beam, which can cause the edge of the slice to spread.
• Focal spot size in CT is determined by the technique factors and/or
algorithm selected for the scan parameters.

SLICE THICKNESS
• Measurements of slice thickness are
determined using a phantom that
includes a ramp, spiral, or step-wedge
in the test objects.
• The test objects have known
measurements and provide a
standard to compare with the
scanner.
• Typically, the test objects are aligned
obliquely to the scan plane

SLICE THICKNESS
• Procedure:
1. Perform three separate scans through the test object, each with a
specified thickness (10 mm, 5 mm, 1 mm).
2. Display the images using the manufacturer's recommendations.
3. If a ramp is used, the length of the resultant image gives the slice
width. For a spiral test object, the resultant image will contain an
arc, with the arc length giving the slice width. When a step-wedge
is used, the slice width can be determined by the number of steps
that are imaged

SLICE THICKNESS
• On the figure The quality
assurance phantom is used to
perform several tests including
the slice thickness test. (ramp
pattern).
• For a slice thickness of 5 mm or
greater, the slice thickness
should not vary more than ±1
mm from the intended slice
thickness.
• For a slice thickness of 5 mm or
less, the slice thickness should
not vary more than ±0.5mm.

LINEARITY
• Linearity refers to the relationship between CT
numbers and the linear attenuation values of the
scanned object with a particular kVp value.
• When linearity is present within an image, it is an
indication that subject contrast is constant across the
range of CT numbers within the image.
• A standard phantom containing materials with known
physical and x-ray absorption properties is used for
this test.

LINEARITY
• Procedure:
1. Take a single scan
through the
appropriate phantom.
2. Plot the average CT
numbers as a function
of the attenuation
values corresponding
to the materials within
the phantom.

LINEARITY
• Over time, these values can vary
because of changes in system
components.
• The plotted values should
demonstrate a straight line
between the average CT numbers
and the linear attenuation
coefficients. Any deviation from
the straight line could indicate
that inaccurate CT numbers are
being generated or the scanner is
malfunctioning.

PATIENT DOSE
• It is important that personnel monitor the amount of radiation to
which patients and staff are exposed.
• It is equally important for a CT technologist to realize that the
patient dose can increase with changes in slice thickness, kVp,
and mAs.
• Image quality is not always performed at maximum resolution
because it requires an increase in patient exposure and patient
dose.

PATIENT DOSE
• Specially designed ionization chambers or
thermoluminescent dosimeters (TLD) are used
to measure the radiation dose.
• These specially designed radiation detectors are
capable of providing measurements from which
the dose can be calculated for the exposure
factors used (slice thickness, mAs, and kVp).

PATIENT DOSE
• This is a picture of a pencil ionization
chamber that is inserted into a head or
body phantom for patient dose
calibration testing.
• A head phantom of 16 cm diameter is
used for head dose calculation.
• 32 cm diameter body phantom is used
for calibrating dose.

PATIENT DOSE
The pencil ionization chamber is inserted into the center hole (A) and
peripheral holes that are 1 cm from the surface (A through E) in the 12
o’clock, 3 o’clock, 6 o’clock, and 9 o’clock positions to determine various
simulated patient doses.

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