L56 - kVP, mAS, Image Storage

What are the Three Major Factors of a High-Quality Radiographic Image?

1. Appropriate exposure

2. Appropriate contrast/latitude

3. Low scatter radiation

What is Radiographic Exposure?

The amount of ionization in air produced by radiation

• Associated with overall image blackness and opacity

What is the Difference Between an Overexposed and Underexposed Radiograph?

• Overexposed: Too dark/black

• Underexposed: Too light/white

What Three Factors determine Overall Radiographic Exposure?

1. mAs (milliampere-seconds)

2. kVp (peak kilovoltage)

3. Focal spot-film distance (FFD)

What does mAs stand for, and How is It Calculated?

mAs = strength of electric current within given time

• mAs = milliamperes (mA) × exposure time (seconds)

What is the Primary Function of mAs in Radiography?

Controls the quantity (number) of X-ray photons produced

How does Increasing mAs affect X-ray Production and Image Exposure?

Increasing mAs produces more X-ray photons → increasing detector exposure and image blackness

What is the Relationship Between mAs and Radiation Exposure?

A linear relationship: doubling mAs doubles X-ray photon quantity and radiation exposure

How does Increasing mAs affect Motion Artifacts?

If exposure time increases, motion artifacts increase because the patient has more time to move during the exposure

Why does Increasing mA Without Changing Exposure Time NOT Increase Motion Artifacts?

Because exposure duration remains unchanged, even though more X-ray photons are produced

How should mAs be Adjusted for Underexposed vs. Overexposed Images?

• Underexposed → Increase mAs

• Overexposed → Decrease mAs

Does Changing mAs Directly affect Radiographic Contrast?

No

• mAs primarily controls X-ray quantity and overall exposure, not contrast

What does kVp stand for, and What does It represent?

Peak kilovoltage

• The voltage difference between the cathode and anode of the X-ray tube

What is the Primary Function of kVp in Radiography?

Controls X-ray photon energy, penetration, and radiographic contrast

How does Increasing kVp affect X-ray Energy and Penetration?

Higher kVp → Faster electrons → Higher-energy X-rays → Greater tissue penetration

How does Increasing kVp affect Overall Image Exposure?

Greater penetration allows more X-rays to reach the detector → increasing image blackness

How should kVp be Adjusted for Underexposed vs. Overexposed Images?

• Underexposed → Increase kVp

• Overexposed → Decrease kVp

What is the Major Difference Between kVp and mAs?

• kVp: Controls X-ray energy and contrast

• mAs: Controls X-ray quantity and exposure time

What is Focal Spot-Film Distance (FFD)?

The distance between the X-ray focal spot (anode) and the film/detector

What does the Inverse Square Law state?

Radiation intensity decreases with the square of the distance from the source.

I ∝ 1/d².

What happens to X-ray Intensity When FFD is Doubled or Tripled?

• Double distance → ¼ of original intensity

• Triple distance → ⅑ of original intensity

Why is the Inverse Square Law Important for Radiation Safety?

Increasing distance reduces radiation exposure, supporting ALARA (As Low As Reasonably Achievable)

What is Radiographic Contrast?

The difference in radiographic opacity between adjacent structures

• Black and White

What is Radiographic Latitude?

The range of tissue densities displayed as useful shades of gray

• Shades of Grey

What is the Difference Between High Contrast and High Latitude?

• High contrast: Strong black-and-white differences with fewer shades of gray

• High latitude: More shades of gray, allowing subtle tissue differences to be visualized

Which Machine Setting Primarily Controls Contrast and Latitude?

kVp

How does Low kVp affect Contrast and Latitude?

Low kVp → High contrast → Low latitude

How does High kVp affect Contrast and Latitude?

High kVp → Low contrast → High latitude

Why does Low kVp produce High Contrast?

Lower-energy X-rays favor photoelectric absorption, which depends strongly on tissue atomic number and produces greater differences in tissue absorption

Why does High kVp produce High Latitude?

Higher-energy X-rays penetrate tissues more readily, reducing differences in absorption and producing more shades of gray.

Why can Lowering kVp Alone cause an Underexposed Image?

Lower kVp reduces X-ray energy and penetration → so fewer photons reach the detector

How can Contrast be Increased While Maintaining Overall Exposure?

Decrease kVp and increase mAs to compensate for reduced penetration

How can Latitude be Increased While Maintaining Overall Exposure?

Increase kVp and decrease mAs to compensate for increased penetration

What is the 15% Rule in Radiography?

A 15% change in kVp produces approximately the same change in detector exposure as doubling or halving mAs

What happens When kVp is Increased by 15%?

Detector exposure approximately doubles

What happens When kVp is Decreased by 15%?

Detector exposure approximately halves

How can kVp and mAs be Adjusted to Increase Contrast Without Changing Exposure?

Decrease kVp by approximately 15% and double mAs

How can kVp and mAs be Adjusted to Increase Latitude Without Changing Exposure?

Increase kVp by approximately 15% and halve mAs

A Radiograph is Underexposed at 85 kVp and 1.5 mAs. How can Exposure be Corrected Using mAs?

Double mAs from 1.5 to 3 mAs while maintaining 85 kVp

A Radiograph is Overexposed at 85 kVp and 6 mAs. How can Exposure be Corrected Using mAs?

Halve mAs from 6 to 3 mAs while maintaining 85 kVp

What kVp and mAs Technique is Preferred for Thoracic Radiographs?

High kVp + Low mAs

Why is High kVp Preferred for Thoracic Radiographs?

The thorax already has natural contrast between air, soft tissue, and bone

• Higher kVp increases latitude and improves visualization of subtle pulmonary structures

Why is Low mAs Preferred for Thoracic Radiographs?

Lower mAs allows shorter exposure times, reducing motion artifacts from respiration

At What Respiratory Phase should Thoracic Radiographs be Taken?

End inspiration → when the lungs are maximally inflated

Why can Excessively Low kVp be Problematic for Thoracic Radiographs?

Excessively high contrast can obscure peripheral pulmonary vessels and small nodules

Why can Excessively High kVp be Problematic for Thoracic Radiographs?

Excessively low contrast can reduce bone detail and visibility of rib margins

What kVp and mAs Technique is Preferred for Abdominal Radiographs?

Low kVp + Higher mAs

Why is Low kVp Preferred for Abdominal Radiographs?

It increases contrast between abdominal organs and surrounding fat, improving visualization of organ margins

Why is Higher mAs Often Needed for Abdominal Radiographs?

To maintain adequate detector exposure when using lower kVp

At What Respiratory Phase should Abdominal Radiographs be Taken?

Near end expiration → when the diaphragm moves cranially and provides more space for abdominal organs

What is Scattered Radiation, and How does It affect Image Quality?

X-rays that change direction after interacting with the patient, primarily through Compton scattering. Scatter reduces image contrast and visibility of structures.

What Three Patient or Imaging Factors Increase Scattered Radiation?

1. Greater tissue density

2. Greater tissue thickness

3. Larger X-ray field size

4. Higher X-ray energy also increases scatter contribution

What is the Most Important Controllable Factor for Reducing Scatter Radiation?

Field size, controlled through collimation

What is Collimation, and Why is It Important?

Restricting the X-ray beam to the anatomical region of interest

• It reduces scatter, improves contrast, and decreases patient radiation dose.

How does a Bladder Paddle Shot Improve Radiographic Image Quality?

• Compression displaces overlapping intestinal loops

• Decreases tissue thickness

• Reduces scatter

• Improves visualization of urinary bladder stones

What is the Function of an Anti-Scatter Grid?

Absorbs scattered X-rays traveling at different angles while allowing more primary radiation to pass toward the detector, improving image contrast

Why must mAs be Increased When Using an Anti-Scatter Grid?

The grid absorbs some useful primary X-rays, so mAs must be increased to maintain adequate detector exposure without unnecessarily changing contrast.

Why may Anti-Scatter Grids be Unnecessary for Very Small Patients?

Small patients, such as kittens, puppies, and birds, produce less scatter because of their smaller tissue thickness

What are the Three Major Radiographic Image Recording Systems?

1. Screen-film radiography

2. Computed radiography (CR)

3. Digital radiography (DR)

How does Screen-Film Radiography Record and Display Images?

• Uses photosensitive film inside a cassette

• Intensifying screens convert X-rays into light, exposing silver halide film that is chemically developed and viewed on a light box.

What are the Major Advantages and Disadvantages of Screen-Film Radiography?

Advantages:

• Inexpensive equipment

• Permanent physical images

• High resolution

Disadvantages:

• Requires chemical development

• Darkroom/storage space

• Has low tolerance for exposure errors

How does Computed Radiography (CR) Record Images?

1. X-rays

2. Storage phosphor plate

3. Stored X-ray information

4. Laser scanning in CR reader

5. Visible light

6. Digital image

What is the Major Advantage of Computed Radiography (CR)?

Reusable storage phosphor cassettes that can be erased with intense light after image processing.

What is the Difference Between Direct and Indirect Digital Radiography (DR)?

• Indirect DR: X-rays → Visible light → Electronic signal → Digital image

• Direct DR: X-rays → Electronic signal → Digital image

What are the Major Advantages and Limitations of Digital Radiography (DR)?

Advantages:

• Rapid image acquisition

• Post-processing

• Higher tolerance for minor exposure errors

• Easy image sharing

Limitations:

• Expensive equipment

• Severely incorrect exposures cannot always be corrected through post-processing

kVp vs. mAs SUMMARY

Feature	kVp	mAs
Primary function	X-ray energy	X-ray quantity
↑ Setting	↑ Energy and penetration	↑ Number of photons
↓ Setting	↓ Energy and penetration	↓ Number of photons
↑ Image exposure	Yes	Yes
↑ Contrast	Decrease kVp	No direct effect
↑ Latitude	Increase kVp	No direct effect
Motion artifacts	No direct effect	Increase if exposure time increases
Radiation exposure	Affects patient dose	Directly proportional to mAs

Remember: 

• kVp = ENERGY + CONTRAST

• mAs = QUANTITY + EXPOSURE TIME

High vs. Low kVp — Contrast and Latitude SUMMARY

Feature	Low kVp	High kVp
X-ray energy	Low	High
Penetration	Low	High
Contrast	HIGH	LOW
Latitude	LOW	HIGH
Image appearance	More black and white	More shades of gray
Interaction emphasized	Photoelectric absorption	Greater penetration
Best clinical application	Abdomen	Thorax
Reason	Organs have similar opacities	Already has natural contrast

Remember:

• Low kVp → High contrast → Low latitude

• High kVp → Low contrast → High latitude

The 15% Rule — How to Adjust kVp and mAs SUMMARY

Goal	kVp Adjustment	mAs Adjustment
Double exposure using kVp	↑ 15%	No change
Halve exposure using kVp	↓ 15%	No change
Double exposure using mAs	No change	Double
Halve exposure using mAs	No change	Halve
Increase contrast, maintain exposure	↓ 15%	Double
Increase latitude, maintain exposure	↑ 15%	Halve

Important: When changing contrast without changing overall exposure, adjust BOTH kVp and mAs.

Increasing Contrast Without Changing Exposure SUMMARY

1. Start with a properly exposed image.

2. Decrease kVp by approximately 15%.

3. This increases contrast but decreases exposure

4. Double mAs to compensate

5. Result: Higher contrast with approximately the same exposure

Increasing Latitude Without Changing Exposure SUMMARY

1. Start with a properly exposed image.

2. Increase kVp by approximately 15%.

3. This increases latitude but also increases exposure

4. Halve mAs to compensate.

5. Result: Higher latitude with approximately the same exposure

Thorax vs. Abdomen — Clinical Application SUMMARY

Feature	Thorax	Abdomen
Preferred kVp	HIGH	LOW
Preferred mAs	LOW	HIGHER
Primary priority	Latitude + minimize motion	Contrast
Natural contrast	High (air, soft tissue, bone)	Low (similar soft-tissue opacities)
Reason for kVp	Visualize subtle lung structures	Differentiate organs from fat
Exposure time	Short	Longer tolerated
Respiratory phase	End inspiration	Near end expiration
If kVp is too high	Loss of bone detail	Loss of organ contrast
If kVp is too low	Peripheral lung detail may be obscured	Inadequate penetration if too low

Remember:

• Thorax = HIGH kVp + LOW mAs + INSPIRATION

• Abdomen = LOW kVp + HIGHER mAs + EXPIRATION

Scatter Radiation — Factors and Methods of Reduction SUMMARY

Factor/Method	Effect on Scatter	Clinical Importance
↑ Tissue density	↑ Scatter	Patient-dependent
↑ Tissue thickness	↑ Scatter	More interactions
↑ Field size	↑ Scatter	Most important controllable factor
Collimation	↓ Scatter	Improves contrast and reduces patient dose
Compression	↓ Scatter	Bladder paddle shot
Anti-scatter grid	↓ Scatter reaching detector	Improves contrast but requires higher mAs

Remember: More scatter = lower image contrast

How an Anti-Scatter Grid Works SUMMARY

1. X-rays pass through the patient.

2. Some X-rays undergo Compton scattering.

3. Scattered X-rays travel in different directions.

4. Metal strips in the grid absorb many scattered photons.

5. More primary radiation relative to scatter reaches the detector.

6. Image contrast improves.

7. Increase mAs as needed to compensate for radiation absorbed by the grid.

Screen-Film vs. CR vs. DR — Image Recording SUMMARY

Feature	Screen-Film	Computed Radiography (CR)	Digital Radiography (DR)
Recording material	Photosensitive film	Storage phosphor plate	Electronic detector
Cassette	Yes	Yes	No separate cassette-reading step
Image viewing	Light box	Computer	Computer
Processing	Chemical	Laser reader + digitization	Digital
Reusable	Film: No	Plate: Yes	Detector: Yes
Image availability	After development	After reader processing	Rapid
Main advantage	High resolution, inexpensive	Reusable cassettes	Fast, post-processing
Main disadvantage	Darkroom, storage, artifacts	Separate reader required

Image Recording Pathways SUMMARY

1. Screen-Film Radiography

X-rays → Intensifying screen → Visible light → Silver halide film → Chemical development → Image viewed on light box

2. Computed Radiography (CR)

X-rays → Storage phosphor plate → Stored information → Laser scanning → Visible light → Digital image

3. Digital Radiography (DR)

• Indirect DR: X-rays → Visible light → Electronic signal → Digital image

• Direct DR: X-rays → Electronic signal → Digital image