Question | Answer |
X-rays are produced when | high energy electrons interact in matter |
An xray tube includes | an electron source, an evacuated path to allow for electron acceleration, and a target electrode |
We don't want electrons to interact with | air molecules |
The X-ray system includes | The tube housing, collimators, generator |
Collimators do... | direct and limit the X-ray field |
Generator does.. | supplies the energy for electron acceleration |
Generator allows for | user control of X-ray output through selection of voltage, current, and exposure time. |
What can be controlled to produce X-ray output | voltage, current, and exposure time |
Voltage = | energy of xray |
current = | how many xrays |
cathode emits | electrons |
Electrons fire to anode causing | Bremsstahlung producing xrays |
kVp stands for | peak Kilovoltage |
kVp detemines | the KE of the electrons from cathode; the X-ray energy range |
Increasing kVp increases | xray energy |
Increasing kVp allows for | greater degree of penetration and higher contrast |
mA stands for | milliamperes |
mA is the | tube current or number of electrons released at the cathode |
mA determines | number of X-ray photons created |
1 mA = | 1.602x10^16 electrons/sec |
mA causes | patients exposure |
kVp determines how | powerful the X-ray will be |
kVp is equal to nm | energy |
mA is equal to nm | mCi/dose |
| memorize |
Question | Answer |
If you need more photons, | adjust the mA |
When you increase mA, | you increase patent exposure |
Low end photons are | filtered out |
Aluminum filter placed | before the pre-collimator |
Aluminum filter removes | low energy X-rays that will not pass completely through the patient |
Filtering helps to | keep the energy distribution uniform & reduce scatter radiation |
Collimation allows the user to define | size and shape of the X-ray beam |
Collimation allows us to choose which | photons we want |
Collimation defines | slice thickness |
Slice is usually | 1-10 mm |
Thinner slice = | more radiation (more cycles) |
Who presented the first CT image | Hounsfield and Ambrose |
Nobel prize winners for math reconstruction | Hounsfield and Cormack |
First CT was of | brain |
Detectors measure | amount of radiation transmitted through patient |
xray tube generates | fan beam of radiation through the patient in FOV |
CT # AKA | Hounsfield unit |
CT number Water | 0 |
CT number Bone | +1000 |
CT number Air | -1000 |
Air shows | black |
Bone shows | white |
Tissue shows | shades of grey |
Windowing used to display | desired values |
Window width (W) | density range represented within grey scale |
Window center or level (C) | center of the density range |
Technical scanning parameters | mA, kVp, Pitch, Rotation Time |
mAs | tube current as a product of scan time |
Soft tissue requires | higher mA to discriminate |
Soft tissue comparison is considered | low contrast resolution |
In soft tissue, CT values of surrounding tissue are | very close |
Higher mAs keeps | noise to a minimum |
Examples of high contrast structures | bone to lung |
High contrast structure require | lower mAs |
Higher mA = ___ exposure | higher |
___ and ___ determine dose | tube current (mA) and scan time |
The higher the dose, the ___ | lower the noise |
kV = | tube voltage in kilovolts |
higher kV needed in areas of | high attenuation (shoulders, pelvis) |
Increase kV, | increase photon energy/penetrating power |
Slip ring technology aka | Helical scanning or Spiral CT |
Slip ring allows for | continuous rotation, continuous radiation, and continuous table feed |
Spiral CT is a ___ X-ray source | fanbeam |
What does fan beam X-ray source mean | detector 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 the | table feed distance per 360* divided by slice thickness |
Higher pitch | faster scan but less area/data |
Pitch definition | defines 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 pitch | less |
Multi-slice spiral CT advantages | WB 30-90 secs, decreased motion artifacts, narrow collimator, reduced metal artifacts, thinner slices, multiple views |
Rotation time | time it takes the tube and detector assembly to rotate 360 degrees |
Typical rotation times | 1.5 secs, 1.0 sec, 0.8 sec, 0.5 sec |
MIP stands for | multiple image projections |
High contrast resolution examples | lung and head |
High contrast resolution | ability to define objects from their surroundings when they have great attenuation differences |
Low contrast resolution example | liver or brain |
Low contrast resolution | ability to define objects from their surroundings when they have little attenuation differences |
More noise results in | grainy scan |
fMore noise if | fewer counts, lower mA |
Less noise if | higher mA, more exposure |
mAs = ___ of photons | number |
kV = ___ of photons | energy |
How do you fix beam hardening? | increase kVp, cut out low E photons |
Attenuation factor for patient thickness | 2 for each 4 cm |
Beam hardening is created by | high density material, large bones |
beam hardening occurs when | low E photons create an increase in x-ray E at center of an object |
Scan time ratification | motion artifacts, breathing, etc |
slice width artifact | partial volume |
operating mode artifact | beam hardening |
patient artifact | metal, motion |
system defects artifact | ring |
metal absorbs xray producing | radiation shadows |
Coronal | front to back |
Sagittal | side to side |
Axial | head to toe |
| memorize |