Module D Computed Tomography Physics, Instrumentation, and Imaging

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CT Imaging Systems  High Voltage Generator  3 Phase system - more efficient production of x-rays are possible  The high voltage cables were eliminated with helical technology.

CT Imaging Systems  Generators today are high-frequency and tri- phasic  In contrast to earlier generators today's’ are:  Small and compact  More efficient  Take low voltage 5 – 50 Hz  Flow frequency current is converted into high voltage, high frequency of between 500 – 25,000 Hz.  Power ratings for CT generators range between kilowatts (KW)

Power ratings for CT generators range between kilowatts (KW) allowing for a wide range of exposure techniques kVp selections kVp selections Milliamp selections Milliamp selections

CT Imaging Systems  Generators are mounted  On the orbital scan frame with the tube  In the corner of the gantry (stationary)  Slip ring technology make this possible.

Slip Ring Technology  Electromechanical devices  Transmit energy across a rotating surface  Rings and brushes  Made spiral or volume scanning possible  Two slip ring designs  Disks  Cylinders  PAGES 81 and 82 in Seeram

CT Imaging Systems  Wire brushes are used to transmit power to the CT components –Brushes glide in contact grooves along the slip rings –Two types of brushes –Composite –Wire

Low voltage slip ring scanners  480 AC current  Slip-ring provides power to high- voltage transformer then to radiographic tube  X-ray generator and tube are positioned on the orbital scan frame

High-Voltage Slip-Ring Scanners  AC delivers power to the high-voltage generator  High-voltage generator then supplies high- voltage to the slip-rings  High-voltage from the slip ring is transferred to the x-ray tube  High-voltage generator does not rotate with the x-ray tube

CT Imaging Systems  Main difference is the way in which the images are gathered and how the reconstruction algorithms are used.  Conventional CT –One row of detectors –SDCT  Spiral CT –164 rows of detectors –MDCT

Slice by Slice Scanning  Also called “step and shoot”  Tube rotates around the gantry  Attenuated radiation from the patient is captured by detectors  Tube stops and the couch moves  Process repeated until the entire exam is completed

Z-Axis  CT is often referred to as Axial CT  This is incorrect  CT is an axial scanning TECHNIQUE only  CT images are acquired along the Z-axis of the patient’s body  The Z-axis is along the transverse or axial plane

Volume CT  Tube rotates continuously while making an exposure as the patient moves through the gantry  The continuous rotation of the x-ray tube and the couch top movement equal the spiral geometry acquisition  THUS- the name - Spiral CT

Original spiral CT scanners  Had only 1 row of detectors  (SDCT) single detector computer tomography  Used different algorithms than what are used today with 164 rows of detectors

CT computer systems  Processes information for the DAS  windowing  Image enhancement  Image magnification  ROI  Quantitative measurements  MPR or multiplanar reconstruction  3-D imaging  MIP maximum intensity projections (vascular imaging)  Volume rendering  Surface rendering  Tasks surrounding image manipulation

CT Computer System  Minicomputer  Capable of performing complex computations while receiving high-level input and out put  Used in MRI also  Computer processing architecture  Pipeline  Parallel  distributed

Array Processor  ?most important part?  Dedicated electronic circuitry  Rapid calculations  Receives information from the detectors  Measurements from hundreds of projections are used to piece information back together

Array Processor  Microprocessors assist  Speeds of 1 nanosecond  Number of array processors determines the speed of reconstruction  Retrospective  Post-processing

Scanning Process  Voltage causes electrons to “boil off”  Electrons are accelerated striking the cathode  X-ray production  Beam travels through tube and the “bow tie” filter  Filter shapes and  Defines the beam  After filtration comes collimation  (Filtration occurs twice)- prior to entering the patient and again after it is attenuated by the patient.

Attenuation Depends on: 1. electrons per gram 2. affective atomic density of the absorber 3. atomic number of the absorber 4. energy of the transmitted photons What does the “Z” number of the absorber mean?

Lambert-Beer Law  -is “an exponential relationship that explains what happens to x-ray photons as they travel through body tissue”.  Lambert-Beer Law incorporates the principles of: –Photoelectric effect –Compton Scattering

Lambert-Beer Law I in = I out e -μx  I out = the transmitted intensity  I in = the original intensity  e = Euler’s Constant (2.718) (Base of the Natural Logarithm)  Φ = Linear Attenuation Coefficient  x = thickness of the object

Continue….. Lambert-Beer Law equation Values are known for the transmitted and original beam intensities and for the thickness of the object. Because these values are known, the linear attenuation can be derived.

Linear attenuation coefficient - Linear attenuation coefficient represents the rate that x-rays are attenuated (diminished) as they travel through the body.

Image Display  CRT (Cathode Ray Tube)  Display matrix  Pixel size determinant  Bit depth  CT number value scale  Window width and level  Primarily a factor related to the DFOV (display field of view) (DFOV has the GREATEST IMPACT on image resolution and image noise)  Protocol  Technologist controls or Selectable scan factors

Monitor Matrix  Resolution is based on the size of the matrix –512 x 512 –1024 x 1024 –2048 x 2048 Newer scanners allow for the selection of these larger maxtrices during scanning.