II

Image Intensifier (I.I)

Characteristics of Image Intensifier

Brightness Gain
  - Definition: Product of minification gain and flux gain.
  - Purpose: Produce a bright enough image to be visible in daylight.
  - Brightness Gain Range: Usually between 5000 and 30,000.
Multifield Image Intensification (magnification mode)
  - Function: Allows choice between multiple field sizes to produce magnified images.
  - Examples:
    - Tri-field Intensifier (field sizes: 12 cm, 17 cm, 25 cm)
    - Dual field Intensifier (field sizes: 12 cm and 25 cm)
Automatic Brightness Control (ABC) System
- Function: Maintains constant image brightness during fluoroscopy examinations.
Fluoroscopic Technique
- Consideration: Operation of the fluoroscopy system should optimize for lower patient radiation doses.
- Recommended kVp and conditions stated in the subsequent sections.
Fluoroscopic Image Quality
- Multiple factors (contrast resolution, spatial resolution, distortion, quantum mottle) impact image quality.

Brightness Gain

Purpose

Brightness gain means higher illumination of the image.
Traditionally, images had to adapt to darkness; now, they are produced bright enough for daylight.

Total Brightness Gain

Formula:
$ext{Total Brightness Gain} = ext{Minification Gain} imes ext{Flux Gain}$
Typical range for total brightness gain is 5000 to 30,000.

Minification Gain

Definition and Process

Minification Gain: The process in image intensifiers that increases image brightness by concentrating electrons onto a smaller output area.
Input phosphor size can typically range from 15-60 cm, with the output phosphor size being about 2.5 cm.
Formula for Minification Gain:
$ext{Minification Gain} = \frac{ ext{Diameter of Input}^2}{ ext{Diameter of Output}^2}$

Example Calculation

Given: Input phosphor = 25 cm, Output phosphor = 2.5 cm
$ext{Minification Gain} = \frac{25^2}{2.5^2} = \frac{625}{6.25} = 100$
Conclusion: Minification gain is 100.

Flux Gain

Definition

Flux Gain: Related to the rate of transfer of particles (electrons) resulting in increased number of light photons emitted from the output phosphor.
Formula for Flux Gain:
$ext{Flux Gain} = \frac{ ext{Number of Light Output Photons}}{ ext{Number of X-ray Input Photons}}$
Typical range for flux gain is between 50 and 150.

Example Calculation

If 50 light photons are produced for every x-ray photon:
- Calculation:
$ext{Flux Gain} = \frac{50}{1} = 50$

Total Brightness Gain Example

Example Calculation

When minification gain is 100 and flux gain is 60:
$ext{Total Brightness Gain} = 100 imes 60 = 6000$

Conversion Factor

Definition: Another measure of brightness gain focused on the efficiency of conversion from x-ray exposure to visible light.
Formula:
$ext{Conversion Factor} = \frac{ ext{Luminance of Output Phosphor (Candelas/m}^2 ext{)}}{ ext{Exposure Rate of Input Phosphor (mR/s)}}$

Aging and Performance of Image Intensifiers

Average loss of brightness gain over time: 10% per year.
Consistent checks are necessary to maintain effective dosages to the patient.
Typical lifespan of I.I.: 3-5 years.

Multifield Image Intensification

Magnification Mode Description

The diameter of input phosphor defines magnification.
Smaller field sizes (e.g., switching from 25 cm to 17 cm) is executed electronically.
Effects seen include:
- Increased voltage on electrostatic lenses.
- Focal points for electrons shift, impacting the minification gain accordingly.

Effects of Magnification Mode

Minification Gain Reduction
- When moving from 25 cm to 12 cm, the minification gain decreases significantly.
Magnification Factor:
$ext{Magnification Factor} = \frac{ ext{Input Screen Diameter}}{ ext{Input Screen Diameter during Magnification}}$
Magnification factor for going from 25 cm to 12 cm:
$ext{Mag Factor} = \frac{25}{12} = 2.1$
Patient Dose Increase: Estimation based on ratios of the input phosphor areas.
$ext{Dose Increase with Magnification} = \frac{ ext{Original Diameter}^2}{ ext{New Diameter}^2}$

Technical Considerations with Magnification Mode

Ensure effective collimation to maintain visibility of the anatomy under exposure.

Vignetting and Image Artifacts

Definition

Vignetting: Darkening of image edges when using I.I.
Only the central portion contributes to the image during I.I. operation.

Effects of Magnification Mode

Contrast Resolution: Magnifying improves ability to discern tissue differences.
Spatial Resolution: Improved resolution when using a smaller FOV; clearer details are depicted.
Increased Patient Dose: Higher doses linked to trade-offs in improving image quality.

Automatic Brightness Control (ABC)

Definition and Methods

ABC equalizes brightness across diverse anatomical densities and thicknesses.
Methods Used:
1. Automatic Dose Rate Control (ADC): Adjusts procedure parameters to maintain brightness.
2. Automatic Gain Control (AGC): Adjusts signal amplification for brightness enhancement post-image formation.

Gain Control Explained

Gain is likened to adjusting volume; higher gain results in bright images, but may increase electronic noise.

Key Differences between ADC and AGC

Feature	ADC	AGC
Full Name	Automatic Dose Rate Control	Automatic Gain Control
Adjusts	Radiation output	Electronic signal
Affects patient dose?	Yes	No
Primary Role	Control exposure	Image signal amplification
Where applied?	X-ray tube settings	Imaging Intensifier / Camera settings

Fluoroscopic Technique

Operation and Settings

Usage of higher kVp leads to decreased dose with increased transmission through the patient.
Recommended kVp ranges articulated per the imaging type (e.g., 110-120 kVp for Barium Enemas).

Fluoroscopic Image Quality Factors

Key Aspects

Quality is influenced by:
  - Contrast Resolution (affected by vignetting and systems age)
  - Spatial Resolution (based on receptors used)
  - Distortions (pin-cushion and vignetting).

Distortion Effects

Pincushion Effect: Distortion from imperfect electron focusing causing the image edges to pull inward.
- Mitigation: Can be minimized in mag mode operations.
Vignetting: Results in unequal brightness across the image, countered by using mag modes efficiently.

Quantum Mottle

Explanation of the grainy appearance correlates to insufficient radiation exposure; remedy involves increasing mA.

Conclusion on Multifield Image Intensification

Recommendations to utilize the largest Field of View (FOV) for the body part of interest to manage patient dose efficiently without compromising quality.

Recommendations for kVp in Digital Fluoroscopy

Different ranges specified for various procedures to maintain optimal image quality and minimize patient exposure:
  - Barium Enemas (solid-column): 110-120 kVp
  - Small Bowel: 110-120 kVp
  - Upper GI: 110-120 kVp
  - Air Contrast Barium Enema: 90-100 kVp
  - Abdomen and Pelvis: 80-90 kVp
  - Iodine Procedures in Abdomen: 75-85 kVp
  - Myelogram: 75-85 kVp
  - Chest and Mediastinum: 75-85 kVp
  - Joint Girdles: 70-80 kVp
  - Extremities: 65-75 kVp