| 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 |
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