IR - Midterms pt. 2

• Image matrix / Pixel count

• Size of the image intensifier / Physical size

In DF, spatial resolution is determined by:

pixel size

Spatial resolution is limited by

Charge-Coupled Device

Major change from conventional fluoroscopy → use of a —— instead of a TV camera tube

Charge-Coupled Device

• IMAGE RECEPTOR

• Developed in the 1970s

• Initially used for military applications (night vision scopes)

• Digital cameras

• Commercial television

• Security surveillance

• Astronomy

CCD is Currently used in

crystalline silicon

• Sensitive component of CCD

• When illuminated:

  • Electrical charge is generated

  • Sampled pixel by pixel

  • Manipulated to produce a digital image

• Each section of a CCD corresponds to one pixel on the monitor

~10 lp/mm

1024 matrix

pincushion or barrel distortion

TV camera tubes produce?

Charge-Coupled Device

• No geometric distortion

• Greater detective quantum efficiency

• Lower electronic noise

• Higher signal-to-noise ratio (SNR)

• Better contrast resolution

SIGNAL-TO-NOISE RATIO

• All analog electronic devices are inherently noisy

• Produces background electronic noise

• Similar to radiographic fog:

  • Conveys no information

  • Reduces image contrast

• Heated filaments

• Voltage differences

Noise is caused by

200 : 1

Conventional TV camera tubes

• Not sufficient for DF

5 : 1

minimally visible signal

Low SNR

• Lower signals are lost in noise

• Especially problematic with subtraction techniques

• —— → severely degraded contrast resolution

<256

200 : 1

1024

1000 : 1

Higher SNR

• Provide more useful information

• Are more compatible with computer-assisted image enhancement

ANALOG-TO-DIGITAL CONVERTER

• Output signal from image receptor sent to ADC

• ADC:

  • Converts analog signal → digital

• FPIR signal is already digital

• ADC must match DF system dynamic range

• Digital image stored in matrix form in memory

8-bit ADC

values 0–255

10-bit ADC

values 0–1023

TELEVISION MONITOR

• Video signal is amplified and sent to the monitor

• Transformed into a visible image

• Closed-circuit television system:

  • One end: CCD or TV camera tube

  • Other end: TV monitor

• Differences from Home TV

  • No audio

  • No channel selection

  • Operator controls:

    • Brightness

    • Contrast

• TEMPORAL SUBTRACTION

• ENERGY SUBTRACTION

• HYBRID SUBTRACTION

IMAGE FORMATION IN DF

• Image subtraction techniques

• Enhanced visualization of vasculature

• Better contrast resolution through postprocessing

TEMPORAL SUBTRACTION

• Image obtained at one time is subtracted from a later image

• If contrast is introduced:

  • Resulting image shows only contrast-filled vessels

• Mask Mode

• Time-Interval Difference (TID) Mode

TEMPORAL SUBTRACTION Methods

MASK MODE

• Produces successive subtraction images

• Requires:

  • X-ray tube current increased 20–100×

  • Pulsed image acquisition

• Mask image

• Subsequent images

• Principal result: Improved image contrast

• Digital subtraction removes static anatomy (e.g. skull)

• Allows better visualization of distal arteries

• Each image:

  • 33-ms x-ray pulse

  • One video frame = 33 ms

Mask image

• Stored in primary memory

• Displayed on Monitor A

Subsequent images

• Mask subtracted from each

• Displayed on Monitor B

IMAGE INTEGRATION

• Summation of several video frames (usually 4–8)

• Purpose:

  • Improve contrast resolution

• Disadvantage:

  • Increases patient dose

• Imaging sequence:

  • Manual or preprogrammed

REMASKING

• Use of later images as a new mask

• Used when original mask is inadequate due to:

  • Noise

  • Patient motion

  • Improper technique

• Composite masks can be created by integrating multiple frames

TIME-INTERVAL DIFFERENCE (TID) MODE

• Subtraction between progressive masks and frames

• Example (cardiac study):

  • Begins 5 s after injection

  • 15 images/sec for 4 s

  • Total = 60 images

Subtraction Pattern

• Frame 1 – Frame 5

• Frame 2 – Frame 6

• Frame 3 – Frame 7

• And so on

Characteristics

• Displays dynamic flow of contrast

• Less motion artifact

• Less contrast than mask mode

• Primarily used for cardiac evaluation

MISREGISTRATION ARTIFACT

• Caused by patient motion

• Same anatomy not aligned in same pixel

• Can be corrected by:

  • Reregistration

  • Shifting mask by one or more pixels

ENERGY SUBTRACTION

• Uses two different x-ray energies

• Based on differences in photoelectric absorption

• Relies on K-edge of iodine

K-Edge Concepts

• Photoelectric absorption decreases with increasing energy

• At 33 keV:

  • Abrupt increase in iodine absorption

• Known as the K absorption edge

• Monoenergetic beams at 32 & 34 keV would produce very high contrast

Limitations

• Requires alternating x-ray emission spectra

• Methods include:

  • Alternating 70 kVp / 90 kVp

  • Alternating metal filters

• Less commonly used due to generator limitations

HYBRID SUBTRACTION

• Combines:

  • Temporal subtraction

  • Energy subtraction

• Mask and subsequent images formed using energy subtraction

• If motion is controlled:

• Produces highest-quality DF images

PATIENT DOSE CONSIDERATIONS

• DF may reduce patient dose

• X-ray beams are pulsed

• Fluoroscopic dose rate:

  • Lower than continuous analog fluoroscopy

• Static DF images:

  • Lower dose than 100-mm spot-film camera

• CCD and TV camera tubes:

  • More sensitive than spot film