CTA Physics and Dosimetry
Nausheen Akhter, MD
Division of Cardiology
University of Illinois at Chicago
December 21, 2007
Contents
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•
•
•
Review of X-ray Production
CT Components
CT Scanner Generations
CT Image Formation and Reconstruction
– Retrospective vs. Prospective
• Digitalization
– Hounsfield Unit
• Radiation Dosimetery
The CT Scan Defined
• Computed tomography is an imaging modality that uses
x-rays to image cross-sectional slices through the body.
• X-rays are directed into a localized section of the patient.
• Attenuated x-rays completely penetrate the patient and
are detected.
• Detected signals are then constructed into an image.
X-ray Production
• Three paths of an
electron (e-):
(1) e- may directly collide
with nucleus (unlikely)
(2) e- may enter vicinity
of nucleus, but not
collide
(3) e- collides with inner
shell e- and outer shell
e- shifts position
X-ray Production
• Paths (1) and (2) cause e- to lose much energy
which is converted into x-ray photons.
• Bremsstrahlung phenomena (aka. braking
radiation): x-ray photons are created by paths
(1) and (2)
• Characteristic radiation: x-ray photons created
by path (3)
– 10-12% of x-ray beam emitted from CT x-ray tube
X-ray Production
CT Components
• The gantry:
– Aperture
– Contains a rotating
frame assembly (x-ray
tube, detector array,
data acquisition
system)
– Contains slip-ring and
high voltage generator
CT Components
• The patient table:
– X: Left to Right
(saggital)
– Y: Anterior to Posterior
(coronal)
– Z: Head to Feet (axial)
CT Components
• The x-ray tube
– Generates and
focuses the x-ray
beam to strike a target
material (Tungsten)
and produce photons.
– Two electrodes:
rotating anode and
cathode
– High voltage generator
Tube Current (mA)
• When tube voltage (kV) is applied across the cathode and anode at
a specific current (mA), the e- beam flows from the cathode and
collides with the target material on the anode.
• The cathode contains a heating filament wire. The temperature of
this filament controls the current of e-.
• Temperature of the filament is controlled by the tube current (mA).
• The tube current (mA) determines the number of x-ray photons
produced.
• Higher mA  hotter filament  greater e- escape and create
photons  greater x-ray dose to the patient.
Tube Voltage (kV)
• Tube voltage (kV) controls the x-ray
photon energy level.
• X-ray photon energy level determines how
easily the x-ray penetrates the patient’s
body and image quality.
• 120 kV are higher energy and more
penetrating than 100 kV.
CT Components
• Collimation
– Prevents ‘scatter
radiation’ and
minimizes radiation
dose
– Restrict the path of xrays and define slice
thickness
– Two types: pre-patient
and post-patient
CT Components
• Detectors
– In order for an x-ray photon to generate a
signal, four steps must occur:
(1) X-ray must enter a detector (ie. ‘capture’)
(2) X-ray must collide with a detector atom
(3) The collision must result in an electromagnetic
conversion suitable for measurement (eg. Light)
(4) This event must be amplified and conducted to a
data acquisition system.
CT Components
• Detectors
– Two types: gas
(xenon) and solid-state
– Solid-state (aka.
scintillation detector) is
made of a solid
crystalline substance
which is more
sensitive to x-rays at
various angles.
Linear Attenuation Coefficient
• X-ray photons are attenuated via two processes:
absorption and scatter
• Amount of attenuation is dependent on:
– Atomic number of the tissue, density of electrons in
the tissue, thickness of the tissue, and energy of the
x-ray photons
• Lower energy photons are more easily
attenuated than higher energy photons.
Linear Attenuation Coefficient
• Exponential decay
– The number of photons detected is reduced in
an exponential fashion as the thickness of the
tissue traversed is increased.
CT Components
• Array processor
– Primary function is the reconstruction of the
projected attenuation raw data into a CT
image.
CT Scanner Generations
• First Generation
– X-ray tube generated a thin
‘pencil beam’.
– Single detector
– Tube and detector
translated across gantry
while patient stationary
– Rotation angle 1 degree
– Scan time: 5 minutes
CT Scanner Generations
• Second-generation
– X-ray ‘fan beam
geometry’
– Detector array (30)
– Rotation angle >5
degrees
– Scan time: 20 sec
CT Scanner Generations
• Third generation
– Fan-beam x-rays
– Curved arc detector array
(improved reproducibility of
data, >750 detectors)
– Tube/detector array rotates
around patient with a
‘continuous slip ring’
– Scan time: < 1 sec
• Fourth generation
CT Image Formation
• Scanning methods:
– The scout film
– Conventional/Axial/
Sequential CT
– Helical/Spiral CT
CT Image Formation
• Conventional/Axial/Sequential CT
– The x-ray tube rotates around a stationary patient.
The patient table must be incremented after each
image is acquired.
– For a third generation scanners, a ‘projection’ is made
up of all the x-rays emanating from the tube in one
position.
– Data measured at each projection is called the raw
data.
CT Image Formation
• Helical/Spiral CT
– Continuous data collection.
– Tube rotates around the
patient as the patient table
moves at a fixed speed
through the gantry.
– Need slip-ring scanner and
high heat capacity tube
– The slice thickness of a
helical scan is defined as
the table increment/pitch
Advantages vs. Disadvantages
• Advantages of Helical Scanning
–
–
–
–
–
–
More coverage in a breath-hold
No misregistration of slices
Less contrast injection
Gapless slices
Images may be constructed at any arbitrary position
Multiplanar reconstruction and 3D
• Disadvantage of Helical Scanning
– Continuous radiation
Image Reconstruction
• Most CT systems use a ‘filtered back projection
algorithm’ for reconstruction.
• Back projection is a summation technique of all of the
projections.
• A correction filter (aka. reconstruction filter or kernel) is
applied to accentuate the edges and allow for exact
representation of the original object.
• ‘Convolution’ is the process of applying filtration.
Filtered Back Projection
Retrospective Reconstruction
• Simultaneous recording of ECG with continuous x-ray
data acquisition.
• The ECG is retrospectively used to assign images to the
respective phases of the cardiac cycle.
• Retrospective, phase-specific segments of the R-R
intervals are combined to reconstruct an axial slice.
• Strength: oversampling allows for gap-less and motionreduced imaging
• Drawback: higher radiation
Retrospective Reconstruction
Prospective Reconstruction
• “Step and shoot modality”
– Images obtained at a pre-determined offset of the
ECG-detected R wave
• Only 1 image per detector per cardiac cycle
• Strength: lower radiation
• Drawbacks: inability to perform overlapping
images, longer acquisition time
Multi-planar Reconstruction and 3D
• Multi-planar reconstruction is a post-processing
technique that requires only the image data to
reconstruct images in planes other than the
plane of the original scan.
• 3D surface rendering is another post-processing
technique that reconstructs images of the
surfaces of anatomical structures.
Contrast Media Injection
• Scan delay is determined by the contrast transit
time
• Bolus tracking technique
– The coronary CT imaging sequence is initiated when
the contrast enhancement in the ascending aorta
reaches 100 Hu.
• Timing bolus technique
– Images acquired at level of carina every 2s starting
10s after injection of constrast (10-20 mL), followed
by saline flush (30-50 mL).
Timing Bolus Technique, Scan delay 20s
Digitalization
• The first step in generating a digital image is to divide the
object into a grid of small regions.
• Each region of the grid is called a ‘pixel’.
• A grid of pixels is the image matrix.
• Assign shades of gray to the pixels.
• Gray-scale values for pixels within the reconstructed
tomogram are defined as CT numbers or “Hounsfield
units” (HU).
Hounsfield Unit (HU)
• CT uses X-ray absorption to create images
which differ in brightness depending on their
physical density (atomic composition).
• HU are defined with a reference to the value for
water.
• Higher densities will be brighter, lower densities
darker.
Hounsfield Unit (HU)
Air
-1000 HU
Water
0 HU
Atherosclerosis
130-600 HU
Bone
+1000 HU
Metal
>1000 HU
CT Parameters
• Spatial Resolution
– Measure of the size of the smallest object that can be visualized
in an image
– Directly related to the slice thickness and reconstruction matrix
– Best cardiac CT spatial resolution (x, y, and z-axes) is 0.6 x 0.6 x
3mm (nearly cubic/isentropic voxel)
• Temporal Resolution
– Precision of a measurement with respect to time
– High temporal resolution needed to minimize coronary motion
artifact
– Determined by pitch, gantry rotation time, and the patient’s heart
rate
Radiation Dosimetery
• 50% of a person’s lifetime radiation exposure is due to
medical testing.
• Three primary factors that go into radiation dosimetry: xray energy (kV), tube current (mA), exposure time.
• ‘Effective dose’ is an estimate of the uniform, whole-body
equivalent dose that would produce the same level of
risk for adverse effects that results from the non-uniform
partial body irradiation.
• The unit for the effective dose is the milliSievert (mSv).
Radiation Dosimetery
PA and lateral CXR
0.04-0.06 mSv
Calcium Scan
1-1.5 mSv
Ave. annual background
radiation in the U.S.
Cardiac cath
3.6 mSv
Stress MIBI
6 mSv
Abdominal/Pelvic CT
8-11 mSv
MDCT (dose modulated)
8-13 mSv (5-8 mSv)
2.5-10 mSv
Radiation Dosimetery
• There are various choices with MDCT that can
dramatically change the patient radiation dose.
• Dose modulation was a large advancement.
– Tube current (mA) is reduced during systole by 80%,
then the full dose is used in diastole (40-80% R-R
interval)
– May reduce radiation exposure from 30-50%
– Slower heart rates, less systole, reduction is less.
Radiation Dosimetery
• Other methods to decrease radiation:
– Current (mA) can be increased/decreased,
most often based on body habitus.
– Increase the pitch (table speed/collimator
width)
• Lower pitch = more radiation
• Slice thickness can be increased (faster scan).
– Prospective reconstruction
Articles to Review
• Morin RL et al. “Physics and dosimetry in computed
tomography” Cardiol Clin 21 (2003), pp. 515-520
• Budoff et al. Cardiac CT Imaging: Diagnosis of
Cardiovascular Disease
– Chapters 1 and 2
• CT Registry Review Program
– Chapters 1, 2 and 3
• Budoff et al. “ACCF/AHA Clinical Competence Statement
on Cardiac CT and MR” JACC Vol. 46, No. 2, 2005