Grid Lines

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Gridline Or Grid Line

Grids are placed between the patient and the x-ray film to reduce the scattered radiation reaching the detector (produced mainly by the Compton effect) and thus improve image contrast.

They are made of parallel strips of high attenuating material such as lead with an interspace filled with low attenuating material such as carbon fiber or organic spacer. The strips can be oriented either linear or crossed in their longitudinal axis. As scattered radiation is increased in 'thicker' patients and at larger field sizes, grids are useful in such scenarios to improve image contrast.

The working ability of a grid is described by the grid ratio, which is the ratio of the height of the lead strips (h) to the distance between two strips, i.e. the interspace (D). A grid ratio of 8:1 is generally used for 70-90 kVp technique and 12:1 is used for >90 kVp technique. The strip line density (number of strips per cm) is 1/(D+d), where d is the thickness of the strip. This is typically 20-60 strips per cm.

Types

focused grids (most grids): strips are slightly angled so that they focus in space so must be used at specified focal distances
parallel grid: used for short fields or long distances
moving grids (also known as Potter-Bucky or reciprocating grids): eliminates the fine grid lines that may appear on the image when focused or parallel grids are used; cannot be used for portable films

Uses

Grids are commonly used in radiography, with grid ratios available in even numbers, such as 4:1, 6:1, 8:1, 10:1 or 12:1.

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Generally used where the anatomy is >10 cm:

abdomen
skull
spine (except lateral cervical)
contrast studies
- IVU
- RGU
- MCU
- barium studies (including lateral cervical)
breast (mammography): uses 4:1 grid ratio

Characteristics

The Bucky factoris the ratio of radiation on the grid to the transmitted radiation. It indicates the increase in patient dose due to the use of a grid. It is typically two to six.

The contrast improvement factor is the ratio between the contrast with a grid and without a grid. It is typically two. Image contrast can be improved by increasing the grid ratio by increasing the height of the lead strips or reducing the interspace. However, this leads to increased x-ray tube loading and radiation exposure to the patient.

History and etymology

Anti-scatter grids were developed by Gustav Bucky (1880-1963), a German-American radiologist who patented a stationary grid in 1913. Not long after his original invention, Bucky introduced a moving grid to overcome the problems inherent with a static device.

Two American radiologists independently of Bucky also came up with the idea of a moving grid. These were Eugene W Caldwell (1870-1918), a radiologist and qualified engineer, who received a patent for an automated timing device to move the grid ⁴; and Hollis E Potter, who was the first to present the development at a scientific event, namely the winter meeting of the Central Section of the American Roentgen Ray Society (ARRS) in February 1915 ⁴.

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1. Curry TS, Dowdey JE, Murry RE. Christensen ́s physics of diagnostic radiology 4 Ed. Lippincott Williams & Wilkins. (1990) ISBN:0812113101. Read it at Google Books - Find it at Amazon
2. Walter Huda, Richard M. Slone. Review of Radiologic Physics. (2003) ISBN: 9780781736756
3. Pettet G. A Radiologist's Notes on Physics. (2014) ASIN: B00HTR080O.
4. Ronald L. Eisenberg. Radiology. (1992) ISBN: 9780815130529

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Imaging technology

Grid Lines Definition

imaging physics
x-ray
- x-ray in practice
- x-ray production
  - high voltage generator
  - cathode
  - anode
- filters
- grids
- x-ray film
- image intensifier
- digital radiography
  - direct digital radiography
- digital image
- x-ray artifacts
  - external foreign body artifact
- radiation units
  - absorbed dose
  - equivalent dose
  - effective dose
  - exposure
  - legacy units
- radiation safety
  - stochastic effect
    - radiation-induced carcinogenesis
- radiation detectors
  - dosimeters
fluoroscopy
computed tomography (CT)
- CT technology
  - generations of CT scanners
  - dual energy CT
    - clinical applications of dual energy CT
- CT image reconstruction
- CT image quality
- CT dose
- CT artifacts
  - patient-based artifacts
    - blur
  - physics-based artifacts
  - hardware-based artifacts
    - helical and multichannel artifacts
      - MPR artifact
- CT safety
- history of CT
MRI
- MRI hardware
  - coils
  - magnets
- signal processing
- MRI pulse sequences (basics | abbreviations | parameters)
  - diffusion weighted imaging (DWI)
  - gradient echo sequences
  - inversion recovery sequences
  - perfusion-weighted imaging
    - techniques
    - derived values
  - spin echo sequences
- MR angiography (and venography)
  - non-contrast-enhanced MRA
    - TRICKS
- MR spectroscopy (MRS)
  - 2-hydroxyglutarate peak: resonates at 2.25 ppm
  - alanine peak: resonates at 1.48 ppm
  - choline peak: resonates at 3.2 ppm
  - citrate peak: resonates at 2.6 ppm
  - creatine peak: resonates at 3.0 ppm
  - functional MRI (fMRI)
  - gamma-aminobutyric acid (GABA) peak: resonates at 2.2-2.4 ppm
  - glutamine-glutamate peak: resonates at 2.2-2.4 ppm
  - lactate peak: resonates at 1.3 ppm
  - lipids peak: resonates at 1.3 ppm
  - myoinositol peak: resonates at 3.5 ppm
  - N-acetylaspartate (NAA) peak: resonates at 2.0 ppm
  - propylene glycol peak: resonates at 1.13 ppm
- MRI artifacts
  - MRI hardware and room shielding
  - MRI software
    - slice-overlap artifact a.k.a. cross-talk artifact
  - patient and physiologic motion
  - tissue heterogeneity and foreign bodies
    - magnetic susceptibility artifact
      - blooming artifact
  - Fourier transform and Nyquist sampling theorem
- MRI contrast agents
  - intravascular (blood pool) MRI contrast agents
  - hepatobiliary MRI contrast agents
  - extracellular MRI contrast agents
  - contrast media safety
- MRI safety
ultrasound
- transducers
  - frame averaging (frame persistence)
  - ultrasound image resolution
- imaging modes and display
  - pulse-echo imaging
  - Doppler imaging
    - B flow
    - color box
    - pulse repetition frequency and scale
    - color write priority
    - packet size (dwell time)
  - elastography
  - scanning modes
    - 2D ultrasound
    - 4D ultrasound
    - M-mode
  - ultrasound artifacts
    - reverberation artifact
      - comet tail artifact
    - hardware-related artifacts
    - Doppler artifacts
      - tissue vibration
      - spectral broadening
      - blooming
      - motion (flash) artifact
      - acoustic streaming
  - biological effects of ultrasound
nuclear medicine
- nuclear medicine physics
  - nuclide
    - isomer
    - photopeak
- detectors
  - scintillation detectors (gamma camera)
  - emission tomography
- radiopharmaceuticals
  - fundamentals of radiopharmaceuticals
  - radiopharmaceutical labeling
  - radiopharmaceutical production
    - nuclear reactor produced radionuclides
    - cyclotron produced radionuclides
  - radiation detection
    - dosimetry
  - specific agents
    - carbon-11
    - chromium-51
    - fluorine agents
    - gallium agents
      - Ga-67 citrate
      - Ga-68
    - iodine agents
      - I-123
        I-123 iodide
        I-123 ortho-iodohippurate
      - MIBG scans
        I-123 MIBG
        I-131 MIBG
    - indium agents
      - In-111 Prostascint
    - krypton-81m
    - nitrogen-13
    - oxygen-15
    - phosphorus-32
    - selenium-75
    - technetium agents
      - Tc-99m HMPAO
      - Tc-99m mercaptoacetyltriglycine
      - Tc-99m sulfur colloid (oral)
    - xenon agents
  - in vivo therapeutic agents

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