CT Notes

Computed Tomography

Objectives

  • 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 ((u<em>t-u</em>w)/u_w)
    • 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.
  • Pre-detector collimation elevates contrast resolution.

Noise

  • CT numbers
    • CT numbers should be specific to tissue type but may vary due to scatter.
    • Collimation and averaging helps this
    • The variation from what it should be to what it is, is called noise.
    • Depends on
      • kVp
      • Filtration
      • Pixel size
      • Slice thickness
      • Detector efficiency
      • Patient dose
    • Appears as graininess on the image
    • Should be evaluated daily with a water bath test object

Linearity

  • Frequent CT calibration ensures water’s CT number remains at 0.
  • Evaluated with a 5-pin performance test created by the American Association of Physicists in Medicine (AAPM).
    • Each pin has a different attenuating pin.
    • Each pin ROI Houndsfield units must be measured and graphed daily.
    • Plots should be a straight line.

Uniformity

  • Uniformity (HU) throughout a uniform test unit should demonstrate minimal deviation.
  • Test with internal software package.
  • Numbers must stay within 2 SD.

Imaging Technique

  • Multidetector arrays have increased up to 320 image slices per rotation.
    • Can image entire body in 1 breath hold.
  • Some systems may have two tubes and two multidetector arrays to complete extremely fast exams.
  • Data acquisition rate: the faster the acquisition, the higher the rate.

CT Quality Control

  • Requires daily, weekly, monthly, and annual QC monitoring.
  • Contains many different sensitive parts, so many things can go wrong.
  • A popular test object is the ACR CT Accreditation Phantom.
  • Noise and Uniformity:
    • 20 cm water bath imaged weekly for a +/-10 HU of 0.
    • Uniformity across the image should be +/- 10 HU from the center to the periphery.

CT Quality Control

  • Linearity:
    • Image the AAPM 5 pin insert
    • Should demonstrate a linear relationship between HU and electron density
    • Must be within 2 standard deviations
    • Completed semiannually
  • Spatial resolution:
    • Image a wire, bar, or hole pattern
    • Completed semiannually
  • Contrast resolution:
    • Test depends on manufacturers recommendations
    • Semiannual test

CT Quality Control

  • Slice Thickness
    • Use specific test tool with ramp, spiral, or step wedge
    • Done semiannually
    • Should operate within 1 mm of the intended thickness
  • Couch Incrementation
    • Monthly evaluation
    • During a scan note couch position at the beginning and the end then verify with a tape measure and a straightedge
    • Should be within +/- 2 mm
  • Laser Localizer
    • Semiannual evaluation
    • Completed like the couch incrementation test

Summary

  • CT is a high dose radiology option that offers superior contrast resolution to Radiography
  • CT equipment has developed significantly over time taking from 5 minutes per image originally to completing a whole body scan in 1 breath hold
  • Because of the many parts (console, computer, gantry), many things can go wrong with CT, requiring careful CT QC to optimize imaging and patient care