COMPUTED TOMOGRAPHY
1. History of Computed Tomography (CT).
A. Inventor Hounsfield
1. The components necessary to construct a CT scanner were available several
years prior to its invention in 1970.
2. Godfrey Hounsfield put all of these components together and demonstrated its
remarkable use.
a. Hounsfield was an engineer with EMI, Ltd. in Great Britain.
B. Basic Concept of Operation
1. A thin cross-section of the head (tomographic slice) is examined form multiple
angles, originally with a pencil-like X-Ray beam.
2. The transmitted radiation (remnant) is counted by a scintillation detector.
This information is fed into a computer for mathematical analysis.
4. The computer reconstructs the tomographic image.
C. Basic Principle Behind CT
1. The internal structure of an object can be reconstructed from multiple
projections of the object.
D. Nomenclature
1. CT has had many names, each referring to at least one aspect of the technique.
a. CAT = Computerized Axial Tomography.
b. CTAT = Computerized Trans Axial Tomography.
c. CRT = Computerized Reconstruction Tomography.
d. DAT = Digital Axial Tomography.
2. Computed Tomography abbreviated CT is now the widely accepted name.
II. Generations of CT Scanners.
A. First Generation (Original EMI Scanner)
1. Translate-rotate mechanism.
a. Linear translation through the patient called a scan.
b. Each scan is followed by a 1O rotation before beginning
another scan.
c. This continued until the tube had moved a total of 180 O.
2. Pencil-thin X-Ray beam.
3. A single paired detector.
4. Scan time is up to 5 minutes per slice.
a. Total exam time of approximately 25 minutes.
b. Primary disadvantage of lst generation scanners.
B. Second Generation
1. Major objective was to increase speed.
2. Multiple detectors.
a. Incorporated the natural extension of the single detector to
multiple detectors.
b. 5-30 detectors now used.
3. Fan-shaped beam to be used ,with the multiple detectors.
4. Also a translate-rotate mechanism.
a. Fewer linear scans are required because 30 detectors gather
more data per scan.
b. This allows greater rotary steps. Up to 30 o rotation between
scans instead of 1o.
5. Fewer linear scans result in faster scan times.
a. Scan times down to 20 seconds.
C. Third Generation
1. Rotate-rotate mechanism.
a. X-Ray tube and detector are rotated concentrically about the
patient.
b. No linear translations.
2. Larger fan-shaped X-Ray beam, 30o-60o.
a. Pre-patient collimator keeps the fan-shaped pattern very thin.
b. This pre-patient collimation determines the thickness of the
fan-beam and therefore determines the slice thickness.
c. Beam views entire patient at all times.
3. Larger number of detectors used to read the larger fan-shaped beam.
a. Between 288-700 detectors.
4. Decreases scan time to as low as 1second.
5. Faulty detectors produce ring artifacts.
D. Fourth Generation
1. Rotate-fixed mechanism.
a. Only the tube rotates. The detectors are fixed.
2. Beam is fan-shaped as the 3rd generation.
3. Radiation detection is through a fixed circular array of detectors.
a. Up to 2400 elements in the detector array.
4. Capable of 1 second scan times.
5. Eliminates ring artifacts but also increases patient dose compared to
other generations.
E. Superiority
1. No clear advantage between 3rd and 4th generation scanners.
a. Much of the final image quality is dependent on the
mathematics of image reconstruction.
F. Helical Scanners
1. Newest CT technology, available since 1990.
2. Slip-ring technology allows continuous rotation of the tube.
a. Allows simultaneous translation and data acquisition
b. Continuous data acquisition can be achieved in a single
breath hold. (360o rotation in 1 second or 1 rev/sec)
c. Results in a dramatic increase in speed and consequently
throughput. (20-40 seconds for HCT scan vs. 4-6 minutes with
conventional CT)
3. Made possible through advances in technology.
a. More robust tubes. (Increased tube currents for prolonged
periods yet light enough to be mounted in a slip-ring gantry)
b. Development of slip-rings replace the cumbersome cables
which would never have allowed continuous gantry rotation.
c. New generators.
d. New software algorithms allowed the new technology to be
clinically implemented. (Estimation of missing data).
4. Increased performance.
a. Eliminates mis-registration artifacts
b. Minimization of motion artifacts. (Respiration artifacts
eliminated & peristaltic artifacts suppressed)
c. Production of overlapping images without additional
radiation exposure.
5. Increased 3D longitudinal resolution, optimizing image quality
because of the above advantages.
6. Retrospective slice selection.
a. Position and spacing of axial slices can be chosen after data
acquisition.
b. Very small slices can be achieved.
c. Can get highly overlapping images. (Hundreds of images
from single 30 second scan)
3. Comparable images and radiation doses.
a. Radiation dose decreased slightly because of decreased repeats
due to motion artifacts.
4. Limitations.
a. Demand placed on the X-Ray tube.
b. Image reconstruction takes time and can't be viewed
immediately. (Patient can be removed after collection of the
raw data)
111. CT Numbers.
A. Image matrix
1. A CT scan image consists of many cells, each assigned a number and
displayed as a brightness level on the video monitor.
2. Each cell is called a pixel.
3. The numerical information stored in each pixel is a CT number or
Hounsfield unit (HU).
4. The pixel is a 2-dimensional representation of a corresponding
tissue volume.
5. The tissue volume is called a voxel and is determined by the pixel
size X the slice thickness (1-10mm).
B. Gray Scale
1. CT numbers range from -1000 to +1000 for each pixel.
a. Water is defined as 0.
b. -1000 corresponds to air while +1000 corresponds to dense
bone.
c. Soft tissues range from 0-100.
d. Fat is -100.
2. Enormous waste of information.
a. Actual range of image is 2000 HU's or shades of gray.
b. A video screen can only display 256 shades of gray.
C. Image Display
1. Windowing.
a. The technologist or radiologist selects a CT number that is
about the average CT number of the body tissue being
examined.
b. The computer is then instructed to assign 1 shade of gray to
each of the 128 CT numbers below and each of the 128 CT
numbers above.
c. The center CT number is called the window level.
d. The range of CT numbers above and below the window level
is called the window width.
2. A bone window would have a window level of about +200.
3. A soft tissue window would be between +20 to +40.
4. The window width may be set at any level with a maximum of 2000.
IV. Three Major Components
A. Gantry
1. Houses the X-Ray tube and radiation detectors.
2. Can be angled from 15o-30o.
a. This is useful in imaging the skull and IVD'S.
3. Table automatically positions the patient in exact increments.
B. Computer
1. Technology is continually improving, making better images.
C. Operating Console.
1. The technologist enters information such as kVp, mAs, slice
thickness, number of slices, etc.
2. The radiologist or technologist can also control the contrast to an
optimum level by changing the brightness on the screen.
V. When to use CT
A. Clinical settings
1. Exquisite bony detail.
2. Routinely for chest studies assessing for CA
3. Problem solving more complex lesions.
4. Evaluation of GI/GU disorders with contrast use.
5. Skull, pelvis, and scapula problems.
a. 3D reconstructs.
6. Trauma patients. (CT vs. MRI)
a. Faster study time.
b. Lower costs.
c. Greater availability. (Some CT now in Emergency Depts.)
REFERENCES
I. Bushon-, Stewart. Radiologic Science for Technologists. Chapter 24.
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2. Curry, T; Dowdey, R; Murry, R. Christensen's Physics of Diagnostic Radiology.
Chapter 19.
3. Guebert, G; Othel, P; Yochum T. Essentials of Diagnostic Imaging. Chapter 8.