List and describe the various generations of Computed Tomography (CT).
Identify the function of CT system components.
Describe reconstruction via interpolation, back projection, and iteration.
Describe technique selection in CT.
Explain helical imaging relationships among pitch, index, and dose.
Discuss image quality as it relates to spatial resolution, contrast, noise, linearity, and uniformity.
The Origins
Godfrey Hounsfield (physicist and engineer) presented the use of CT in 1970.
In 1979, Hounsfield shared the Nobel Prize in Physiology or Medicine with Alan Cormack, who developed the math to reconstruct CT images.
Principles of Operation
X-ray images are degraded by:
Low contrast
Superimposition of anatomy
Techniques to overcome this issue:
Nephrotomography
Digital tomosynthesis (Axial tomography)
CT is transaxial tomography.
Information is collected transverse to the axis of the body.
CT Operation Base Knowledge
A small collimated x-ray beam irradiates a single detector.
As it sweeps across the patient, the body attenuates the photons.
The remnant photons create an intensity profile.
Readings run through a computer processor to create a 3-dimensional profile or image for the 1 translation (sweep across the patient), forming a single projection.
The patient is moved in a small increment, and a new translation is taken.
Projections are “stacked,” making a 3D volume of information that can be “sliced” in axial, coronal, or sagittal planes.
Generations of CT
1st Generation Scanners
Operated as described in the simplified example.
1 image could take 5 minutes.
Rotations completed at 1-degree separation.
Used mostly to show potential in this imaging approach.
2nd Generation Scanners
Started using a fan-shaped beam and multiple detectors (5-30).
Increased radiation at the periphery of images, so a bowtie filter was added.
Decreased time.
Rotations completed at 5-degree separation or more.
Imaging times of 20 seconds or longer per projection.
3rd Generation Scanners
Use curvilinear detector array and fan beam (number of detectors and width of fan beam larger).
30-60 degree detectors and beam – View entire patient at all times
Allows better collimation and decreased scatter
Produces image in <100ms.
Disadvantage of ring artifacts if a detector goes out.
4th Generation Scanners
X-ray tube rotates, but the detector does not (fixed circular array).
May have up to 4000 detectors.
CT Today
Typically, mostly 3rd generation with multislice helical is used.
The tube rotates constantly while the couch moves the patient through the beam.
Interpolation algorithms:
Computer program that participates in the reconstruction of any image is possible because of a mathematical process called interpolation and extrapolation
Interpolation - estimate of unknown value between two unknown values
Extrapolation - estimate of an unknown value beyond a known value
In helical CT, the plane of each image does not contain enough information for data reconstruction, so it must use interpolation/extraction.
CT Today - Pitch
Relationship between couch movement and x-ray beam width.
Expressed as a ratio
0. 5:1 expresses overlap
2:1 expresses a gap
Greater pitch = greater amount of tissue scanned at a time.
Multislice CT
Multidetector array intercepts the collimated beam in all rows of activated detectors.
Ex. 16 slice detector, each detector has thickness of 0.5mm
Each rotation will collect 8mm of information
If table moves 8 mm a second and the gantry rotates 1 x per second, the pitch is 1.
Pitch for multislice CT is usually 1:1.
Except for CTA which pitch is usually smaller.
If pitch exceeds 1:1, resolution will decrease.
Sensitivity Profile
Section sensitivity profile: fancy name for slice thickness.
CT System Components
The CT System:
Console
Computer system
Gantry
CT System Components - Operating Console
May consist of 3 different subunits:
1 operates the imaging system
1 allows post-processing
1 allows radiologist post-processing
Can adjust technical factors
kVp generally 120
Lower mAs constantly adjusting
Tissue slice thickness (adjusts collimator and detectors employed)
Usually 0. 5-5mm
Automatic movement of the table
Radiologists can control contrast, brightness, and ROI viewing
CT System Components - Radiologist’s Console
Allows retrieving and reformatting previously obtained images.
Can adjust window, level, ROI, etc.
Can complete subtraction, volumetrics, etc.
Usually remote from the CT suite.
CT System Components - Computer
Subsystem of the CT system.
Calculates many equations (up to 250k) simultaneously to produce an image.
Utilizes a microprocessor and primary memory or an array processor.
Quantum computing is expected to push these other processing systems out.
These collect and analyze incoming data to reconstruct into a volume of data.
The time this takes is reconstruction time.
CT System Components - Gantry
X-ray tube - much larger than general x-ray
High thermal demand producing xrays up to 60 sec. continually
High anode heat capacities (8M HU)
Larger diameter and thicker anode with greater rpms
Focal spots are still limited in size due to spatial resolution and sometimes use focal spot cooling algorithms to predict the thermal state and adjust mA accordingly.
Detector array
Multidetector array consists of 10s of thousands
Scintillation, solid state (Cadmium tungstate)
The concentration of detectors determines resolution
90% detection efficiency
Generator- high frequency
Attached to the gantry – small
Table (Couch)
Comfortable with low atomic number
Must have accurate movements, or tissue locations will be misregistered
Receives commands from the console and sends information to the computer
Collimation
Reduces dose
Improves contrast resolution
Prepatient collimator
Pre-detector collimator
Defines slice thickness, AKA sensitivity profile
Slip Ring Technology
The electrical technology that allows CT to perpetuate continuous gantry rotation without interruption.
Slip rings are electromechanical devices that conduct electricity and signal through rings and brushes from a rotating surface to a fixed surface.
Allow brushes to transmit power without ever having to stop and reset.
Image Characteristics
Created from data received.
X-rays form a stored electronic image displayed as a matrix.
Matrices are usually 512 x 512 pixels.
Numerical value in each pixel is a Hounsfield Unit (HU).
Field of View
Diameter of image reconstruction
Each increase in FOV will increase the pixel size
Voxel: tissue volume
Pixel size x slice thickness
CT Numbers
Each pixel has a level of brightness.
Levels of brightness correspond to a range of CT numbers from -1000 to 3000 for each pixel.
-1000 air, 3000 dense bone, 0 water
CT number of each pixel is related attenuation coefficient
CT number is calculated =k((u<em>t−u</em>w)/uw)
K is the constant determined by the scale factor.
If k is 1000 nits are called Hounsfield units.
Windows
Bone Window: WL 300, WW 1600
Soft Tissue Window: WL 40, WW 400
Lung Window: WL -600, WW 200
Reconstructions
Images from each projection during the scan are stored in computer memory.
Images can be reconstructed from this data in a process called filtered back projection.
Filter here refers to a mathematical function.
Requires calculation of >250,000 equations at once to ensure pixels correlate to anatomy.
Iterative reconstruction is replacing filtered back projection but requires greater computer capacity.
Lowers dose and increases contrast resolution.
Multiplanar Reformation
Multiplanar Reformation
Axial images are rendered and stacked to create a 3D data set that can be rendered in different ways.
Used in:
Maximum Intensity Projection (MIP) - Selects highest value pixels and exhibits only those pixels
Used in CTA - high contrast 3D image (may lack depth)
Shaded Surface Display (SSD) - Identifies a narrow range of intensities belonging to an object and only reformats pixels within that range, producing an organ surface.
Called volume rendering
Used in CT colonoscopy
Image Quality
Spatial resolution:
Determined by:
Pixel size
Slice thickness (limits partial volume averaging)
Collimators effect contrast making resolution harder/easier to see
Edge Response Function (ERF)
Mathematically describes the system’s ability to depict edge sharpness.
Modulation Transfer Function
Graphic representation that expresses the resolution of a system derived from a complicated formula.
MTF closer to 1 is perfect; closer to 0 is less accurate.
Curves that go farther right have better spatial resolution, those going higher at lower spatial frequency have higher contrast resolution.
Spatial frequency is described in lp/cm.
Contrast Resolution
Contrast resolution: the ability to distinguish one soft tissue from another.
CT excels above radiography in contrast resolution.
Amplifies differences in subject contrast to make it visible.