Week 7 C Chapters 29

1
Week 7 C Chapters 2930 Computed Tomography

• Godfrey Hounsfield of EMI, LTD demonstrated the
  principle for computed tomography in 1970.
• Alan Cormack developed the mathematics used to
  reconstruct the CT images.
• They shared the 1982 Nobel Prize for physics.

2
Computed Tomography

• Computed Tomography is the most significant
  development in radiology in the past 40 years.
• MRI and Ultrasound are also significant
  developments but they do not use x-ray to produce
  the image.
• The x-ray tube spins around the patient.

3
Basic C T Principles

• Instead of film, radiation detectors measure the
  radiation attenuation as the beam passes through
  the body.
• The detectors are connected to a computer that
  uses algorithms to process the data into useful
  images that are then recorded on film and viewed
  on a computer monitor.

4
Basic C T Principles

• Conventional tomography has the image parallel to
  the long axis of the body. This is referred to as
  Axial Tomography.

5
Basic C T Principles

• Computed Tomography has the x-ray tube move
  across the so the image is called a transverse
  image or one perpendicular to the long axis of
  the body.

6
Computed Tomography Development

• Computed tomography has gone through five major
  design advancements since 1970
• Each development improved both scan time and
  resolution or image quality.
• Scan time have been reduced from 5 minutes to 50
  ms.
• First scanner used a very tightly collimated
  pencil beam.

7
First Generation CT Scanner

• Pencil Beam
• Translate-Rotate Design
• 180 one degree images or translations.
• One or two detectors.
• 5 minutes scan time

8
Second Generation CT Scanner

• Translate-Rotate
• Fan beam collimation so there is more scatter
  radiation.
• 5 to 30 detectors
• 10 degrees /translation 18 per scan.
• 30 second scan times
• Faster scan time

9
Third Generation CT Scanner

• Rotate-Rotate
• Fan shaped beam of 30 to 60 for full patient
  coverage.
• Constant Source to detector distance due to
  curvilinear detector array.

10
Third Generation CT Scanner

• If one detector fails, a ring artifact appears.
• 1 second scan times
• Superior reconstruction and resolution.

11
Fourth Generation CT Scanner

• The tube rotates around s stationary ring of
  detectors.
• Fan beam
• Variable slice thickness with pre and post
  patient collimation.

12
Fourth Generation CT Scanner

• As many as 8000 detectors.
• 1 second scan time.
• Auto-detector calibration so no ring artifact.
• High radiation dose compared to earlier scanners.

13
Fifth Generation CT Scanner

• This is the latest generation of CT.
• Allows for continuous rotation of the tube for
  spiral CT.
• 5th Generation also includes two novel designs

14
Fifth Generation CT Scanner

• Toshiba maintains the same SID by wobbling the
  detectors.
• Heartscan by Imatron used an electron beam
  instead of x-ray tube and 50 ms scan times.

15
Fifth Generation CT Scanner

• Spiral CT scanners allow for contiguous or even
  overlapping data acquisition.
• As the tube spins, the table moves.
• On earlier units, the table moved between scans.

16
Spiral C T Scanner

• Spiral CT is made possible by slip-ring
  technology. The tube can continuously rotate 360
  degrees, where it must stop after each rotation
  with conventional CT.

17
Spiral C T Scanner

• The detector array may contain as many as 14,600
  detectors that are 1.25mm wide.
• This allows multiple slice to be made with one
  scan and more tissue volume to be imaged.

18
Benefits of Spiral CT

• Less motion artifacts
• Improve lesion detection because the
  reconstructed image can be at arbitrary
  intervals.
• Reduced partial volume because of overlapping
  reconstruction intervals.
• Reduced scan time.

19
Benefits of Spiral CT

• Advances in computer processing allows for
  multi-planar reconstruction and even 3D
  reconstruction.

20
Basic CT Scanner Components

• Gantry includes the
• Pedestal or table
• Tube, Collimators, Detectors High Voltage
  Generator
• Mechanical Supports
• Operators Console
• Computer

21
Basic CT Scanner Components

• Multi-format laser camera using either dry
  chemical images or conventional laser film.
• Viewing station for radiologist (optional)

22
CT Components

• Table, pedestal or couch holds patient and is
  motor driven to move the patient into the scanner
  at the correct rate and distance.
• X-ray tube with very high heat capacity, measured
  in millions of heat units.

23
Two Collimators in CT

• Prepatient collimator determines slice thickness
• Predetector collimators reduce scatter radiation
  to improve contrast.

24
Large Computer

• A very large and fast computer is needed to
  perform over 250,000 calculations per image.
• Newer scanners use an array processor so the
  calculations are done simultaneously.

25
CT Image Characteristic

• Image matrix Original EMI format was 80 x 80 so
  there were 6400 cells of information called
  pixels.
• Today the format is 512 x 512 resulting in
  262,144 pixels.
• The numerical number in each pixel is a CT number
  or Hounsfeld Number.

26
CT Image Characteristic

• CT number or Hounsfeld Number represents the
  tissue volume in the pixel.
• Field of View (FOV)is the diameter of the
  reconstructed image. As the FOV increases, the
  size of the pixel increases.
• Voxel is the square of the matrix times the
  thickness of the slice.

27
Hounsfeld or CT Number

• The precise CT number is related to the
  attenuation of the tissue contained in the voxel.
• Bone 1000
• Muscle 50
• Lung -200
• Air -1000

28
Image Quality

• Spatial Resolution The motion of CT tends to
  blur the image compared to the actual object.
• The ability of the scanner to reproduce high
  contrast or sharp edges (edge response function)
  is measured as Modulation Transfer Function (MTF).

29
Image Quality

• The best possible resolution is equal to the
  pixel size. In terms of line pairs, 1 would be
  two pixels.
• Items that impact spatial resolution include
  collimation, detector size and concentration and
  the mechanical gantry control. Much like
  conventional radiography.

30
Image Quality

• Contrast Resolution The ability to distinguish
  one soft tissue from another is contrast
  resolution. This is where CT excels.
• The absorption or attentation characteristics
  is affected by the atomic number and the mass
  density of the tissue.

31
Contrast Resolution

• Conventional Radiography has relatively poor
  contrast resolution.
• CT can amplify the tissue characteristics to
  provide superior contrast resolution.

32
Contrast Resolution

• The Contrast Resolution is improved because of
  the predetector collimation.
• The contrast resolution for low contrast tissues
  is limited by the size and uniformity of the
  object and the noise in the system.
• Noise is determined by the number of x-rays used
  by the detector to make the image.

33
Computed Tomography Problems

• CT scans require significantly higher doses of
  radiation compared to conventional radiography.
  Therefore the risks of the radiation and the
  benefits of the information gained by the scan
  must be factored when determining the need for
  Computed Tomography.

34
Computed Tomography Problems

• If a chest x-ray is equal to the amount of
  radiation received in 10 days from our natural
  environment, a CT of the brain is equal to 8
  months exposure and CT abdomen, chest or lumbar
  spine is equal to 3 years each.
• Did they mention this when they advertised total
  body CT scanning?

35
Computed Tomography Problems

• Computed Tomography equipment are expensive and
  have high service costs.
• Computed Tomography is expensive for the patient
  or insurance. As much as 1,000 per exam. HMOs
  require preauthorization

36
Nuclear Medicine

• Conventional Radiology and Computed Tomography
  use x-ray.
• Nuclear Medicine uses radioactive compounds that
  are injected into the patient referred to as
  radionuclides or radiopharmaceuticals.

37
Nuclear Medicine

• The radionuclide is used as a tracer in nuclear
  medicine studies. A tracer is a substance that
  emits radiation and that can be identified when
  placed inside the human body.

38
Nuclear Medicine

• By detecting the tracer, information about the
  structure, function, secretion, excretion and
  volume of the target organ can be obtained.
• The organ imaging involves administration of the
  radionuclide to the patient either orally or
  intravenously.

39
Nuclear Medicine

• Depending upon the radionuclide used, it will
  localize in a specific organ of the body and
  provide a way of identifying the structure and
  function of that organ.
• Scanning instruments detect the radiation
  produced by the radionuclide that is concentrated
  in the organ and produce an image that can be
  recorded on film or paper.

40
Bone Scan

• In a bone scan, the nuclide is concentrated in
  the bone blood flow.
• Hot spots can be the result of fractures or
  metastasis from cancer.

41
Other Nuclear Medicine Exams

• Thallium is used to study the heart and can
  detect injury from a myocardial infarction or
  evaluate the wall motion of the heart.
• Iodine is used to image the thyroid gland and to
  treat thyroid disease.
• In a laboratory, radionuclides can be added to
  specimens for analysis.

42
Other Nuclear Medicine Exams

• With x-ray, the patient is not radioactive after
  the exposure.
• With nuclear medicine in vivo exams the patient
  is radioactive after the injection and until the
  material is either excreted or sufficient half
  lives have passed. Short half life materials are
  generally used.

43
Nuclear Medicine

• SPECT is a form of computed tomography using a
  radionuclide instead of x-ray.
• PET is Positron Emission Tomography uses
  specially formulated nuclides from a linear
  accelerator and shows the metabolic activity of
  the brain or heart. PET scanners are now being
  combined with CT scanners.

44
Nuclear Medicine

• Nuclear Medicine is useful in evaluating function
  and blood flow.
• It has very poor spatial resolution.

45
End of Lecture

• Return to Lecture Index
• Return to Physics Homepage