ct chapters 7-10

Card Set Information

ct chapters 7-10
2013-02-19 15:05:52
ct cbc

CT ch 7-14
Show Answers:

  1. Aperture of CT is usually....
  2. 3 major systems of CT
    imaging system, computer system & image display, recording, storage & communication system
  3. Imaging system consists of
    Gantry-xray tube, generators, collimators, filter, detectors & detector electronics (DAS)--all located in scan room
  4. Computer system performs what on images?
    image manipulation & image processing techniques
  5. Computer systems include
    input/output devices, interface devices, back projection processors, storage devices & communication hardware
  6. Gantry houses?
    xray tube, detectors, slip rings, high voltage generator, collimators, and data acquisition system
  7. Anode is how big usually?
    5 inches in diameter
  8. Tilting range of gantry is?
    =/- 12 to 30 degrees at 0.5 degree increments
  9. 2 most important characteristics of the computer system...
    • large storage capacity
    • & fast & efficient processing of various kinds of data
  10. Array Processor is what?
    it is capable of the high-speed calculations needed in CT. It may have multiple dedicated processors & image storage 
  11. Operating systems are?
    programs that control the hardware components & overall computer operations.
  12. Ct software (part of computer system) consists of?
    • reconstruction
    • pre-processing
    • image post processing
  13. Reconstruction builds image from
    raw data collected by detectors
  14. Pre-Processing does what
    performs corrections on data before it is reconstructed (example is beam hardening)
  15. image post processing does what?
    operates on reconstructed data/images for viewing (MPR, 3D)
  16. low ct # (-1000) is white

    true or false
    false...low ct numbers are black
  17. High ct numbers are white
     true or false
    true--bone (+1000)
  18. Image post processing uses various techniques that modify what
    the reconstructed images displayed for viewing through image processing software & algoritms
  19. Info content in processed images is always equal to the original image

    true or false
    false alway less than or equal...never more info
  20. techniques of image post processing consist of
    linear & nonlinear
  21. Linear is what
    it is part of image post processing--image smoothing & image enhancement
  22. Nonlinear is what
    • part of image post processing
    • concerned w/gray level mapping, contrast enhancement, contrast stretching, histogram modification, histogram stretching & windowing
  23. Image post processing uses what image data to produce new 2D & 3D images
    axial image data set stored in the computer
  24. What is the most common image processing technique?
  25. Windowing affects what?
    ct number (brightness & contrast)
  26. Range of CT numbers in the image is referred to as
    window width
  27. window width is the _______ number of shades of gray that can be displayed.
  28. a large ww indicates ____ contrast
  29. a short gray scale indicates ___ ww
  30. fewer shades of gray indicate a ______ ww
  31. Center or midpoint of range of ct numbers is:
    window level
  32. ct number for air
    -1000 (black)
  33. Bone ct number is
    +1000 (white)
  34. abdomen ww & wl
    • ww=300-400
    • wl=40
  35. head ww & wl
    • ww= 100-200
    • wl= 35-40
  36. lung ww & wl
    • ww=-1500 to -2000
    • wl= -700 to -400
  37. bone ww & wl
    • ww=2000-2500
    • wl=350-400
  38. liver ww & wl
    • ww=120-160
    • wl=40-70
  39. soft tissue ww & wl
    • ww=300-400
    • wl=0-50
  40. a wide window width should be used to
    encompass tissues of greatly differing attenuation within the image
  41. narrow window width should be used to display
    soft tissues within structures that contain different tissues of similar densities
  42. window widths should be centered near what
    the average of attenuation of the tissues of interest
  43. a large ww indicates __________ contrast
    less (decrease)
  44. a narrow ww indicates_______contrast
    more (increase)
  45. as ww increases, contrast _____
    decreases (more shades of gray)
  46. as ww _________, contrast increases and has less shades of gray.
  47. MPR
    • multiplanar reconstruction/reformation (aka image reformation)
    • -computer program that can create coronal, sagittal & paraxial images from the original transverse axial images
  48. sagittal
    passes through the body from anterior to posterior & divides the body into right and left
  49. coronal
    passes through the body from right to left & divides the body into anterior and posterior
  50. paraxial
    cuts through the sagittal and coronal planes in longitudinal direction
  51. disadvantages of MPR
    image detail is decreased ( the image is never better than the original transverse axial slice)
  52. High contrast spatial resolution is the scanners ability to
    resolve closely placed objects that are significantly different from their background
  53. in-plane spatial resolution is measured how
    in line pairs per cm or mm
  54. MTF
    • modulation transfer function-
    • -it is used to compare the performance of different ct systems
  55. term for--compares the performance of different ct systems
    modulation transfer function
  56. factors that affect high contrast resolution
    • focal spot size & shape
    • detector cell size
    • scanner geometry
    • sampling frequency
    • & reconstruction algorithms
  57. focal spot affects resolution how
    • small spot = higher spatial resolution
    • large spot = lower spatial resolution
  58. a large focal spot size does what to resolution
    lowers it
  59. sampling frequency affects resolution how?
    smaller the slice thickness the better the resolution
  60. convolution algorithms affect resolution how
    • specific to each body part and emphasizes the body part being imaged
    • ex. bone kernal produces a sharper image, increases spatial resolution (but also increases noise)
  61. how does field of view affect spatial resolution?
    reconstructed fov (sampling intercal has to be sufficiently small to support the reconstruction of small objects 

    take smaller samples to get better resolution
  62. bigger the matrix means ______ pixel
  63. pixel size is equal to
    fov divided by matrix size
  64. cross plane spatial resolution
    uses MPR & MIP to reconstruct 3D images
  65. resolution of new CT scanners is
  66. low contrast resolution aka
    sensitivity of the systems
  67. low contrast spatial resolution is the ability oc the ct system to
    show the different tissue denisities
  68. ct can show tissue density differences of
  69. factors that affect low contrast spatial resolution
    • low contrast detectability LCD
    • size, contrast, noise, kV, slice thickness
  70. an increase in tube current will _____ noise
  71. a thicker slice will produce __________ noise
    less noise (however greater dose)
  72. Temporal resolution is
    indication of CT systems ability to freeze motion of the scanned object
  73. to have better temporal resolution we need to increase?
    scan speed
  74. what type of imaging is used for cardiac imaging to help minimize motion?
    • gating--uses ECG to match diastole with scan time 
    • does not actually improve temporal resolution, but makes it more appealing to the eye
  75. linearity refers to the relationship of
    ct #'s to the linear attenuation coefficient of the object to be imaged
  76. linearity can be checked how often?
    daily calibration
  77. Uniformity is what?
    • dictates that for a uniform phantom, the ct measurements should not change with the location of the selected regions of interest or phantom position
    • -should be maintained within a reasonable range (2HU)
  78. exposure is the concentration of
    • -radiation at a particular point on a patient
    • -measure of the amount of ionization (amount of radiation to which the pt is exposed) 
    • -produced in a specific mass of air by xrays or gamma radiation
  79. exposure is measured by
    ionization chamber
  80. Unit of EXPOSURE is
    Roentgen (R)
  81. Absorbed dose is what
    • aka radiation dose
    • amount of energy absorbed per unit of mass of material (pt)
  82. Unit of ABSORBED DOSE is
    rad (r) & grays (Gy)
  83. Any risk associated with radiation is related to the amount of
    energy absorbed
  84. 1r= __ Gy
  85. _____rad=1Gy
  86. Effective dose is
    • -aka quantity effective dose 'E'
    • -used to quantify the risk from the partial-body exposure to that from an equivalent whole body dose
    • -basically a weighted average of organ dose
  87. unit of measurement of EFFECTIVE DOSE is
    rem or sieverts
  88. 1rem = ____ sv
  89. 1rem=0.01sv
  90. ___sv=100rem
  91. types of radiation bio effects
    stochstic & deterministic
  92. stochastic effects is
    the probability (rather than severity) of the effect occurring....it is dependent on dose
  93. probability of stochastic effect _______ with dose
  94. stochastic or deterministic have threshold dose?
  95. linear no threshold dose model response states
    • (stochastic effect)
    • radiation risk increases as the dose increases and that there is no threshold dose
  96. Which radiation bio effects occur years after exposure
  97. examples of stochastic effects are
    cancer, leukemia & hereditary effects
  98. stochastic effects are considered to be _____ effects
  99. Deterministic effects aka
    nonstochastic effects
  100. deterministic effects is
    where the severity of the effect (rather than the probability) increases with the increasing of the dose
  101. Threshold dose
    relatively high doses that can kill cells and cause degenerative changes in tissue that have been exposed to radiation
  102. examples of deterministic effects are
    skim erythema, epilation, pericarditis & cataracts
  103. threshold dose for cataracts is
    2Gy or (200rad)
  104. CT beam geometry refers to
    size and shape of xray beam emanating from xray tube and passing through the pt to strike a set of detectors that collect radiation attenuation data
  105. types of dosimetry concepts
    film, tld & ionization chamber
  106. Ionization chamber is what
    • type of dosimetry
    • instrument used to accurately quantify radiation exposure
  107. phantoms are used to
    standarize the measurement of the dose and provide a clinically realistic geometry
  108. MSAD
    • multiple-scan average dose
    • first ct dose descriptor
  109. CTDI
    • CT dose index
    • next dose descriptor after MSAD
    • developed by the FDA
  110. what is the CTDI
    it represents the mean absorbed dose in the scanned object volume and therefore the unit of CTDI is the Gy
  111. dose length product (DLP) is
    dose descriptor used in CT dose studies & reported in the literature & on CT scanners
  112. DLP provides what
    a measurement of the total amount of exposure for a series of scans
  113. DLP is ______ proportional to scan length
  114. 2 types of factors that effect dose in CT
    indirect & direct
  115. Indirect factors that affect dose in ct
    have direct influence on image quality, but NO direct effect on radiation dose

    ex. reconstruction filter
  116. Direct factors that affect dose in ct
    increase or decrease dose to the patient and are under the direct control of the technologist

    ex. exposure factors, collimation, pitch, pt centering, # of detectors & overanging
  117. dose is ______ proportional to mAs

    if mAs is doubled, dose will be doubled (not takind AEC into account)
  118. Effective mAs (used in MSCT)
    • to keep the effective dose constant, as pitch increases, the true mAs must also increase.  
    • ex. increase pitch from 1 to 2, increases teh mAs from 100 to 200 while the CTDI remains unchanged
  119. effective mAs=
    true mAs / pitch
  120. peak kV determines
    power of the photons coming from xray tube
  121. higher kV indicates the photons have
    higher energies & can penetrate thicker objects
  122. Radiation dose is proportional to the ______ of the kVp
  123. quantity of photons is _____ by the square of the kVp
  124. Collimation is used to define
    • beam width for the exam
    • it reflects the efficient use of xray beam at the detector
  125. Collimation effects dose how
    narrow collimation, increases dose
  126. Pitch definition
    ratio of the distance the table travels per rotation to the total collimated beam width
  127. If pitch increases by 2, the dose will
    be reduced by 1/2
  128. increasing pitch will ___ image quality
  129. decreasing pitch will _______ image quality
  130. The measured radiation dose is _____ proportional to the # of detector rows
  131. As the number of detector rows increase, the dose will
  132. ATCM
    • automatic tube current modulation
    • (ct's form for AEC)
  133. ATMC is used to
    optimize the dose to the pt while maintaining constant image quality, regardless of the pt in the z-axis
  134. 2 types of automatic tube current modulation
    angular & longitudinal
  135. angular modulation
    part of ATMC-xy axis
  136. longitudinal modulation
    part of ATMC-z axis
  137. Use of angular & longitudinal modulation (ATMC) can reduce dose by ______, compared with use of only angular modulation.
  138. decrease noise will _____ dose
  139. to reduce Noise in an image by a factor of 2, requires an increase in dose by a factor of
  140. to improve spatial resolution (pixel size) by a factor of 2 it requires an increase in dose by
  141. to decrease slice thickness and pixel size by a factor of 2 requires an increase in dose by a factor of
  142. if double mAs you increase dose by a factor of
  143. if increase square of kVp you will _____ the dose
  144. single slice spiral/helical ct invented by
    Dr. Kalender & Peter Vock
  145. 4 steps of scanning sequence in single slice ct
    • 1. tube must be accerated to constant speed of motion 
    • 2. tube produces xrays that are transmitted through pt to fall on the detectors, pt holds breath & data is collected for axial slice
    • 3. tube & detectors slow down & come to a stop, pt resumes breathing
    • 4. cables unwind, & then pt & table are indexed to next scan position
  146. Interscan delay is
    ISD--steps 1, 3 & 4
  147. Scan rate is equal to
    • number of scans per group divided by
    • sum of time to collect slices +ISD
  148. Limitations of slice by slice sequential ct scanners
    • 1 long exam times
    • 2 certain portions of anatomy are omitted due to pt breathing
    • 3 innactuarte generation of 3D images & MPRs (steplike artifact)
    • 4 Only a few slices are scanned during max contrast enhancement
  149. ISD was eliminated why
    by slip/ring technology
  150. 2 interpolation reconstruction algorithms for single slice spiral ct
    350 deg & 180 deg Linear Interpolation algorithm