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CT Physics

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alkathryn's version from 2016-06-28 04:33

Section

Question Answer
X-rays are produced whenhigh energy electrons interact in matter
An xray tube includesan electron source, an evacuated path to allow for electron acceleration, and a target electrode
We don't want electrons to interact withair molecules
The X-ray system includesThe tube housing, collimators, generator
Collimators do...direct and limit the X-ray field
Generator does..supplies the energy for electron acceleration
Generator allows foruser control of X-ray output through selection of voltage, current, and exposure time.
What can be controlled to produce X-ray outputvoltage, current, and exposure time
Voltage = energy of xray
current =how many xrays
cathode emitselectrons
Electrons fire to anode causingBremsstahlung producing xrays
kVp stands forpeak Kilovoltage
kVp deteminesthe KE of the electrons from cathode; the X-ray energy range
Increasing kVp increasesxray energy
Increasing kVp allows forgreater degree of penetration and higher contrast
mA stands formilliamperes
mA is thetube current or number of electrons released at the cathode
mA determinesnumber of X-ray photons created
1 mA =1.602x10^16 electrons/sec
mA causespatients exposure
kVp determines howpowerful the X-ray will be
kVp is equal to nmenergy
mA is equal to nmmCi/dose
memorize
mA = current variation
Question Answer
If you need more photons,adjust the mA
When you increase mA,you increase patent exposure
Low end photons arefiltered out
Aluminum filter placedbefore the pre-collimator
Aluminum filter removeslow energy X-rays that will not pass completely through the patient
Filtering helps tokeep the energy distribution uniform & reduce scatter radiation
Collimation allows the user to definesize and shape of the X-ray beam
Collimation allows us to choose whichphotons we want
Collimation definesslice thickness
Slice is usually1-10 mm
Thinner slice =more radiation (more cycles)
Who presented the first CT imageHounsfield and Ambrose
Nobel prize winners for math reconstructionHounsfield and Cormack
First CT was ofbrain
Detectors measureamount of radiation transmitted through patient
xray tube generatesfan beam of radiation through the patient in FOV
CT # AKAHounsfield unit
CT number Water0
CT number Bone+1000
CT number Air-1000
Air showsblack
Bone showswhite
Tissue showsshades of grey
Windowing used to displaydesired values
Window width (W)density range represented within grey scale
Window center or level (C)center of the density range
Technical scanning parametersmA, kVp, Pitch, Rotation Time
mAstube current as a product of scan time
Soft tissue requireshigher mA to discriminate
Soft tissue comparison is consideredlow contrast resolution
In soft tissue, CT values of surrounding tissue arevery close
Higher mAs keepsnoise to a minimum
Examples of high contrast structuresbone to lung
High contrast structure requirelower mAs
Higher mA = ___ exposurehigher
___ and ___ determine dosetube current (mA) and scan time
The higher the dose, the ___lower the noise
kV =tube voltage in kilovolts
higher kV needed in areas ofhigh attenuation (shoulders, pelvis)
Increase kV,increase photon energy/penetrating power
Slip ring technology akaHelical scanning or Spiral CT
Slip ring allows forcontinuous rotation, continuous radiation, and continuous table feed
Spiral CT is a ___ X-ray sourcefanbeam
What does fan beam X-ray source meandetector array is long enough so that the fan angle encompasses the entire width of the patient
With pitch, the closer together,the better your data
Pitch decreased =exposure increased, dose increased, information increased
Pitch is thetable feed distance per 360* divided by slice thickness
Higher pitchfaster scan but less area/data
Pitch definitiondefines the desired image spacing by coordinating the table speed and gantry rotation speed
Lower pitch =more data, more exposure
Higher pitch =shorter time, less information
More detecters, ____ impact with pitchless
Multi-slice spiral CT advantagesWB 30-90 secs, decreased motion artifacts, narrow collimator, reduced metal artifacts, thinner slices, multiple views
Rotation timetime it takes the tube and detector assembly to rotate 360 degrees
Typical rotation times1.5 secs, 1.0 sec, 0.8 sec, 0.5 sec
MIP stands formultiple image projections
High contrast resolution exampleslung and head
High contrast resolutionability to define objects from their surroundings when they have great attenuation differences
Low contrast resolution exampleliver or brain
Low contrast resolutionability to define objects from their surroundings when they have little attenuation differences
More noise results ingrainy scan
fMore noise iffewer counts, lower mA
Less noise ifhigher mA, more exposure
mAs = ___ of photonsnumber
kV = ___ of photonsenergy
How do you fix beam hardening?increase kVp, cut out low E photons
Attenuation factor for patient thickness2 for each 4 cm
Beam hardening is created byhigh density material, large bones
beam hardening occurs whenlow E photons create an increase in x-ray E at center of an object
Scan time ratificationmotion artifacts, breathing, etc
slice width artifactpartial volume
operating mode artifactbeam hardening
patient artifactmetal, motion
system defects artifactring
metal absorbs xray producingradiation shadows
Coronalfront to back
Sagittalside to side
Axialhead to toe
memorize

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