Imaging I and II Final Study Guide (Registry)
Differential Absorption
"Process of creating the image on the IR" How the primary beam changes as it passes through matter. Some of the beam is absorbed by the body and some passes through the body.
Added Filtration vs. the X-ray Emission Spectrum
*Intensity* went down because more of the lower intensity x-rays were filtered out. *RE* went down because more low energy x-rays were removed with filtration.
How would you change the following technique if you wanted to use kVp to help reduce motion but maintain the original RE? 80kVp 100mA 200ms
15% rule is used. Raise the kVp 15% and reduce time 50%. 92kVp 100mA 100ms
Grid Construction
Consists of alternating lead strips and interspace material The greater the angle of the incident scatter, the more likely that the scatter radiation will be absorbed
Collimation vs. Contrast (no collimation)
Contrast goes down because more scatter comes back and hits the IR. This creates noise and increases gray in the image.
Grid Ratio
GR=h/D -With all other grid construction factors constant, the higher the GR, the greater the scatter clean-up -Higher GRs also require more accuracy in their use and result in a higher patient dose -Grid Ratio does not take into account frequency.
Why is time not the primary controlling factor for RE?
It will increase the likelihood of motion which can cause a blurry image.
How will reducing the kVp by 15% and doubling the mAs affect the patient dose?
It will increase the patient dose by 1/3.
Exit or Image Forming Beam (remnant beam)
X-Ray beam which exits the patient and hits the IR. Inclusive of the Primary Beam Transmission aka transfer x-ray, Scatter Radiation and Secondary Radiation
Primary Beam
X-Ray beams which pass between the X-Ray tube and the patient
X-Ray Absorption
X-Rays that are absorbed by the body and show up clear/white on the image.
Secondary Radiation
X-Rays that are absorbed by the body with new x-rays being created by the body and leaving in an incorrect direction. It creates noise and degrades the image.
Scatter Radiation
X-Rays that bounce off the body and go off in an incorrect direction. It creates noise and degrades the image.
Will more x-rays be produced at the anode?
Yes
Signal difference
comparing the magnitude of one x-ray to another.
How are x-ray beams emitted from the anode?
isotropically - in every direction
What is responsible for the primary measurement of beam quality and to a lesser extent quantity (intensity)?
kVp
What is the primary controlling factor for subject and image contrast?
kVp (this is the basis for optimum kVp)
What is the starting point for image contrast?
kVp - all other factors that impact image contrast will cause it to increase or decrease from here.
What is the primary factor for controlling receptor exposure (RE) or beam quantity?
mA
mAs
mA-milliamperage X s-seconds - quantity of x-rays.
Magnitude
strength of the x-rays
Radiographic Quality
Affected by Photographic and Geometric properties
Short scale
Aka high contrast Major differences between shades of gray.
Long scale
Aka low contrast Slight differences between shades of gray.
Image contrast is determined by?
Algorithms that are applied to raw data collected during image capture.
What kind of relationship does mA have on RE.
It has a direct and proportional relationship mA RE 100 15 mR 200 30 mR 400 60 mR
15% Rule
-To maintain the RE, increase the kVp by 15% and reduce the mAs by 50% - this will reduce patient dose by approximately 1/3. The 15% Rule vs. RE and Patient Dose kVp mAs RE Patient Dose 115 5 40 mR 50 mR 100 10 40 mR 60 mR 85 20 40 mR 70 mR
Casts
-Wet plaster: double the mAs and increase the kVp by 15% -Dry plaster: double the mAs or increase the kVp by 15% *-Fiberglass: increase mAs by 30% or increase kVp by 5%* (most common) -Plaster/fiberglass: increase mAs by 50% or increase kVp by 7.5%
Additive Diseases
-When the body part is thicker than it should be. -Require a 50% increase in mAs or a 7.5% increase in kVp to maintain RE -It is preferable to go up in kVp to penetrate the thick part of the body.
Image Contrast categories
1. Grayscale 2. Short Scale - High Contrast 3. Long Scale - Low Contrast
Added filtration vs. X-ray Emission Spectrum
1. The average energy of the primary beam increases with added filtration. 2. Contrast will decrease when you add filtration because the average energy has increased which has the same effect as raising kVp although kVp does not increase. (long scale - high kVp - low contrast)
Grayscale
1. The number of brightness levels or shades of gray visible on an image. 2. Linked to the bit depth of the system which is a reference to the total number of shades of gray available.
Primary Beam Transmission - transfer x-ray
X-Ray beam that goes straight through the patient and shows up black on the image.
Inherent Filtration
An example of inherent filtration is the pyrex that encompasses the anode and cathode.
Reciprocity Law
Any combination of mA and time that gives you the same mAs. - When used properly is allows for a shorter time which reduces the likelihood for the patient to move without increase the dose.
Added Filtration
Any filtration beyond that which is inherent
Geometric Properties
Are inclusive of Distortion and Spatial Resolution. Shape and Size are affected by Distortion. Foreshortening and Elongation are affected by Shape. Magnification is affected by Size.
Photographic Properties
Are inclusive of Receptor Exposure and Contrast
Additive Diseases (Examples)
Ascites: Abdomen is full of fluid. Paget's: Bone destruction which initiates massive bone growth to repair. Pleural Effusion: Fluid in the pleural cavity.
Destructive Diseases (Examples)
COPD - chronic obstructive pulmonary disease. Osteoporosis - weakening of the bones to loss of calcium. Pneumothorax - air in the pleural cavity which causes collapse of the lung.
Size (Radiographic Quality)
Can be affected by the OID
Tissue Opacity
Determined by the atomic number of the cells that comprise the anatomy of interest
Contrast
Difference between the dark and light pixels
What kind of relationship does mAs have on RE?
Has a direct and proportional relationship on RE
What kind of relationship does time have on RE.
Has a direct and proportional relationship on RE time RE 50 ms (0.05 s) 15 mR 100 ms (0.1 s) 30 mR 200 ms (0.2 s) 60 mR
Body Habitus
Hypersthenic - Large Sthenic - Normal Hyposthenic - Lean and Athletic Asthenic - Very thin, Anorexic
Filament
Is heated to create electrons Small Filament = high detail associated with small focal spot/high resolution Large Filament = lower detail associated with large focal spot/low resolution
Why is kVp not the primary controlling factor for receptor exposure?
It does not have as much affect on dose as mAs.
Example of Inverse Square Law
If your original beam intensity was 5.0 at a 30" SID, what would be your new beam intensity if the SID was changed to 60"? New mR = 5.0(30"/60")^2 New mR = 1.25 mR
What function does the mA setting perform?
Increasing or decreasing electron production
Radiographic Grids
Invented by Gustav Bucky in 1913 Refined by Hollis Potter in 1920 by adding a side-to-side motion to blur the grid lines.
kVp
Kilovoltage peak - voltage which attracts the electrons to the anode.
Collimation Bottom Line
Lack of collimation adds unwanted scatter (noise) to the IR.
Collimation affecting
Lack of collimation adds unwanted scatter (noise) to the IR.
In general, how accurate are the mA settings?
Least reliable and accurate
Contrast Scale 2
Left: Short scale = low kVp = high contrast (at the top of the image there is not enough kVp to penetrate the body) Right: Long scale - high kVp = low contrast
Dose
Measured in Milliroentgens. How much radiation comes out of the x-ray tube and patient. They differ as some of the radiation is absorbed by the patient.
mA
Milliamperage - mA heats the filament to produce electrons
mR
Milliroentgen - unit to describe radiation. mR are used to measure tube output.
Cathode
Negative Electrode. Contains the filament.
If you increase the kVp, will more electrons be produced at the filament?
No
What amount of mAs can compsentate for a lack of kVp?
None
Distortion
Perversion of the image
Anode
Positive Electrode. Electrons with a small mass are attracted to the anode which creates x-rays and heat.
Added Filtration Function
Primary function is to decrease patient skin dose
Focal Range
Range of SIDs you can use with a particular grid.
RE
Receptor Exposure - Dose of radiation that comes out of the patient and exposes the IR.
Destructive Diseases
Require a 30% decrease in mAs or a 5% decrease in kVp to maintain RE
What function does the time setting perform?
Sets the amount of time to produce electrons
Spatial Resolution (Radiographic Quality)
Smallest detail that can be detected on the image. Affected by the OID and focal spot
What techniques would be optimal for a small child?
Techniques with low time stations.
How will reducing the kVp by 15% and doubling the mAs affect the RE?
The RE will stay the same.
Subject Contrast (definition)
The difference in the thickness and atomic numbers of the structures that comprise the body part of interest.
Added Filtration - how it works
The filter will stop the low energy x-rays which allows only the high energy x-rays to pass through the aluminum.
SID & the "Direct" Square Law
The following versions of this formula can be employed to *maintain beam intensity* at a new distance *new mAs = old mAs (new SID/old SID)^2*
How would noise affect grayscale?
The greater the noise the make the image appear more grainy and washed out.
SID & the Inverse Square Law:
The intensity (mR) of the beam is inversely proportional to the square of the distance between the source and the image receptor. This formula is employed to determine *changes in beam intensity* at a new distance. *new mR = old mR (old SID/new SID)^2*
The Anode Heel Effect
The intensity of the beam decreases as you travel closer to the anode side of the tube. (tube output is greater at the cathode side)
Collimation vs. RE: Magnified
The less you collimate the higher the RE because of the scatter radiation bouncing back and hitting the IR on the reverse side. Plus more electrons are being emitted. The inverse is true for more collimation.
Collimation vs RE
The less you collimate the higher the RE. The inverse is true for more collimation.
Optimum kVp
The minimum level of kVp required to penetrate each body part. Note: If you use too much kVp it will create too much noise on the image.
How accurate is the timer?
The most accurate out of mA, s and kVp.
Grid Frequency
The number of lead strips you have per inch. Grid frequency will tell you about the quality of grid.
Primary Beam Attenuation
The reduction in the primary beam as it passes through the body. As the primary beam enters the body it is either absorbed, passes completely through the body, produces scattered radiation or secondary radiation.
Image Contrast (definition)
The visible difference between any two selected brightness levels within a displayed image.
Cathode/Anode Relationship
They must be close together so the electrons do not scatter to where they don't need to be.
Added Filtration Composition
Thin sheets of aluminum (Al) are added to the port of the x-ray tube
ms
Time - length of the exposure. Length of time used to create the electrons. It also produces more heat.
What happens to the wavelength and penetrating ability as you increase kVp?
Wavelength decreases and penetrating ability increases.
Image Contrast - what is examined?
We are examining the contrast on the image as opposed to the contrast in the patient.
Grid Cut-Off
When the grid removes part of the exit beam that you want. (This can happen when you use the wrong grid.)
The XRay Tube
Where the x-rays are produced. Contains the anode which is positive and cathode is negative
Thermionic emission
the process of producing electrons at the filament by heating it.
