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Computed Tomography in the Diagnostic Radiography Curriculum

Computed Tomography in the Diagnostic Radiography Curriculum. My Disclaimer. My position on CT in the Diagnostic Curriculum is that it is more beneficial than harmful. I am not suggesting that students graduate from our Programs as CT techs.

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Computed Tomography in the Diagnostic Radiography Curriculum

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  1. Computed Tomography in the Diagnostic Radiography Curriculum

  2. My Disclaimer • My position on CT in the Diagnostic Curriculum is that it is more beneficial than harmful. • I am not suggesting that students graduate from our Programs as CT techs. • I AM suggesting that they have an understanding of the modality, its basic concepts, and focused clinical opportunities.

  3. The Premise • I look at CT within the curriculum as a two-fold activity from the student perspective. • One, provides students a basic overview of what CT is, how it works, and why its ‘better’ for some diagnoses. • Two, CT provides an excellent means of review for general radiography principles that may be old hat for some, boring for others, or just offers a different perspective than the original explanations.

  4. When to Present CT • CT has to be in the second year or later. There needs to be a foundation of relevance and understanding. • In our Program, CT is officially taught in the Rad T 265 course, first semester second year. • Clinical rotations begin in the middle of the first semester second year. • Unofficially CT is found throughout our second year curriculum.

  5. Radiologic Technology 265 • Principles of Digital Imaging and Computer Applications(2)Prerequisite: Radiologic Technology 165. Introduction to computer aided medical imaging's as used in radiography departments. Applications include computed and digital radiography (CR/DR), CT, MRI, and other modalities. Basic imaging principles are applied, including physics, imaging protocols, and systems electronics. Software and display strategies for varying modalities will be discussed.

  6. Date Lecture Topic Reading Assignment • Aug 28 Orientation/Principles of CT B. Ch 29, M v3 Ch 33 • Sep 4 HOLIDAY • Sep 11 Components of a CT scanner • Sep 18 Data Acquisition technology B. Ch 30 • Sep 25 Spiral CT • Oct 2 Image reconstruction • Oct 9 Image quality • Oct 16 Image manipulation M. Ch 36 • Oct 23 MRI physics and equipment • Oct 30 MRI image acquisition M. v. 3 Ch 36 • Nov 6 Computer literacy and its relevance B. Ch 26 • Nov 13 Basic concepts of digital imaging B. Ch 27, M v3 Ch 34 • Nov 20 Digital fluoroscopy M v 3 Ch 35, B. Ch 28 • Nov 27 Digital fluoroscopy • Dec 4 Ultrasound and Nuc. Med. Applications M v3 Ch 37&38 • Dec 11 FINAL

  7. Why the importance of teaching CT? • Provides a break from the regular routine. • Offers ‘new’ technology or info that may be exciting. • Reviews existing (hopefully) knowledge. • For example, Photon/tissue interactions • Great way to review anatomy and pathology as seen clinically. • Provides an excellent opportunity to experience a modality first hand.

  8. The Clinical Component • We began a clinical affiliation this year with a free-standing imaging center. • Last year, we had an observational agreement that allowed students to visit and only watch. • This year students have clinical expectations based on the time they spend there.

  9. Clinical continued • Students are allowed to pick a three week optional rotation. • We chose this in order to have students doing something that interested them thereby decreasing the possibility of discontent. • Also, students looking for additional education, therapy or nuclear medicine, could get their observational requirements met.

  10. The Proposed CT Curriculum • CT Generations • Components, Operations, and Processes • Radiation Protection Practices

  11. CT Generations • This is really the only area that has limited value in the diagnostic curriculum.

  12. First and Second Generation CT • The first and second generations of CT were very similar. • Both used a scanning technique called translate/rotate in order to move around the patient. • The first generation scanner used a single detector and thin beam. While the second generation scanner use several detectors and a fan beam. • These changes resulted in a significantly faster scanner.

  13. Third Generation • The big change here was that the tube was in constant motion throughout the exposure, no more stops and starts. • The detectors were also moving during the exposure and more detectors were added. • As before, we now have an even faster scanner.

  14. Fourth Generation • It became obvious that moving detectors introduces noise into the image. • Now the detectors are fixed in a ring around the patient and only the tube moves. • Thousands of detectors are now needed to generate an image. • Faster imaging with increased spatial resolution.

  15. Fifth Generation • Electron beam CT • EBCT • Ultrafast

  16. Spiral • Slip-ring technology eliminates power cables. • Constant power to moving tube. • Continuous exposure • Patient moves through the beam during exposure • A stream a data is generated (spiral) as opposed to a series of individual slices.

  17. CT scanner generations have limited value outside of understanding CT. However, it does provide a mechanism to see the development of a modality. • Additionally, the advantages of each generation and its evolution illustrates the thought processes that go into learning and adapting.

  18. Components, Operations, and Processes • Most of these topics have direct correlation to diagnostic radiography. • Data acquisition • Factors controlling image appearance • Anatomical structures • Post-processing

  19. Data Acquisition • Methods • Slice by slice • Contiguous • Volumetric • Spiral/helical

  20. Beam Geometry • Parallel • Fan • The traditional beam geometry, it is opened along the width of the patient. • Spiral • The beam is continuously on allowing for more anatomical coverage in a shorter time.

  21. Data Acquisition system (DAS)Components • Tube • Detectors • Filters • Collimators • ADC

  22. CT Tubes • Much higher heat loading than conventional tubes • 8MHU and up • Generally have two focal spots

  23. Filters • Again CT filtration is similar to diagnostic radiography • All tubes are required to have minimum filtration • Primary purpose is patient protection • Also, in CT the filter is used to harden the beam; thereby, decreasing absoption • Compensating filters • ‘Bow-tie’ • Uniform beam intensity at the detectors • Think ‘wedge ‘ filter in diagnostic radiography.

  24. CT Collimators • CT consists of both pre and post-patient collimation • Pre-patient collimation is analogous to the collimation we already know. • Controls beam coverage or amount of anatomy exposed.

  25. Post-patient Collimation • Controls slice thickness. • Additionally, it serves to define the slice profile which provides a sort of grid effect. • Scatter rejection

  26. Analog-to-Digital Convertor (ADC) • Converts the analog signal from the detectors to a digital signal for processing. • Rated by bits • Most scanners today are 16-bit systems • Produce 4096 data points • The more data points, the better the gray scale (contrast) resolution.

  27. Measurement of the Transmitted Beam • A ray • Basically, the detected value of a single photon • Several rays combine to form a view. • The data from multiple photons hitting the detector during a single translation. • Profile • The electrical signal produced by the detector.

  28. Encoding into Binary Data • The data from the views is converted into attenuation coefficients using the formula: • The attenuation coefficients are then sent to the ADC. 1 ___ lnIo/I = x

  29. Data Transmission to the Computer • Data processing begins • The raw (detector) data is preprocessed to remove bad data sectors. • The reformatted raw data is now sent to the array processors. • The array processors are using filter algorithms to produce the desire image appearance, i.e. soft tissue, bone, high-res.

  30. After the array processors, the data is then subjected to a reconstruction algorithm that produces the cross-sectional image we see. • The most common reconstruction algorithm today is the filtered back projection. • The data is now image data and available for image manipulation.

  31. The CT Image • Any digital image, including CT, is comprised of picture elements (pixels). • The pixels are 2-dimensional elements that represent volume elements (voxels). • Pixels are displayed in a matrix. • The brightness of each pixel is determined by the CT number it represents.

  32. CT Numbers • CT numbers are calculated by comparing the attenuation coefficients of water and tissue. • The formula is: CT # __ . __________ t w K = w

  33. The CT number of water is ‘0’. • Now, if you look at the formula you can see that tissues attenuate more than water will have a positive CT number. • Conversely, tissues less attenuate less have negative CT numbers.

  34. Examples of Tissue Attenuation Coefficients and Their CT Numbers

  35. Factors Affecting Attenuation • Photon energy • Selected kVp • Filtration • Tissue effective atomic number • Tissue mass density

  36. Selectable Scan Factors • Field of View • Scan • Display • Matrix size • Slice thickness • Algorithm • Scan time and rotational arc

  37. Tube output • mAs • Region of Interest (ROI) • Magnification • FSS and Tube geometry

  38. Scan FoV • The total area from which raw data is acquired

  39. Display FoV • Determines how much raw data is used in displaying the acquired image.

  40. Matrix Size • Basically, the number of pixels displayed. • Affects spatial resolution • The bigger the matrix the more pixels. • Given that image size stays the same the pixels have to be smaller; therefore, spatial resolution increases. • Generally, the larger the image matrix the higher the patient dose.

  41. Algorithm • Mathematical formula applied to the raw data in order to produce a specific image outcome.

  42. Scan time and Rotational Arc

  43. Radiographic Tube Output • mAs

  44. ROI • Allows the technologist to select a specific area of interest for image reconstruction. • Uses the raw data for the reconstruction instead of using image data • The result is a better quality image.

  45. Magnification • Defined as a post-processing activity. • Magnification uses image data not raw data, so the final product has less spatial resolution than when using ROI.

  46. FSS and Tube Geometry • FSS • In CT, FSS selection has the same connotations it has in diagnostic radiography. • A smaller FSS has more detail (resolution) than a larger one. However, due to digital imaging issues (monitor and matrices) the effects of a small versus large FSS are not as apparent.

  47. Factors Affecting Image Quality • Spatial resolution • Contrast resolution • Noise • Radiation dose • Artifacts

  48. Spatial Resolution • The degree of blurring within the image • Ability to discriminate objects of varying density a small distance apart. • CT spatial resolution is affected by • Geometric factors • Reconstruction algorithm

  49. Geometric Factors • FSS • Detector aperture width • Slice thickness • SID • SOD – distance to isocenter • Sampling distance • Number of projections

  50. Reconstruction algorithms • Several different types of convolution algorithms are available. • Edge enhancement • Smoothing • Soft tissue • Bone • Matrix size is also going to play a role in spatial resolution

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