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birdcage

Creates birdcage (MRI coil)

Description

The birdcage object creates to create a birdcage MRI coil. This antenna is most commonly used in clinical MRI. The antenna structure consists of two circular coils connected by conductive elements called rungs. The number of rungs depends on the size of the coil and is generally an even number.

The coil is operated at 64 MHz or 128 MHz. The bircage can be loaded/excited to model a highpass or lowpass coil.

Creation

Syntax

bc = birdcage
bc = birdcage(Name,Value)

Description

example

bc = birdcage creates a birdcage antenna to model an MRI coil.

example

bc = birdcage(Name,Value) sets properties using one or more name-value pairs. For example, bc = birdcage('NumRungs',8) creates a birdcage with eight rungs. Enclose each property name in quotes.

Properties

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Number of rungs, specified as a scalar.

Example: 'NumRungs',20

Example: bc.NumRungs = 20

Data Types: int8

Coil radius, specified as a scalar in meters.

Example: 'CoilRadius',0.2

Example: bc.CoilRadius = 0.2

Data Types:

Coil height, specified as a scalar in meters.

Example: 'CoilHeight',0.089

Example: bc.CoilHeight = 0.089

Data Types: double

Height of rungs, specified as a scalar in meters. Distance is measured from the middle the of upper coil to the middle of the lower coil.

Example: 'RungHeight',0.56

Example: bc.RungHeight = 0.56

Data Types: double

Shield radius, specified as a scalar in meters. A value of zero indicates that the shield is absent.

Example: 'ShieldRadius',0.2

Example: bc.ShieldRadius = 0.2

Data Types: double

Shield height, specified as a scalar in meters. A value of zero indicates that the shield is absent.

Example: 'ShieldHeight',0.089

Example: bc.ShieldHeight = 0.089

Data Types: double

Dielectric mesh to load birdcage, specified as a structure having the following fields:

Points in custom dielectric mesh, specified as an N-by-3 matrix in meters. N is the number of points.

You can use the phantom property to insert a dielectric mesh in the shape of a human head into the bird cage antenna. This dielectric cylinder has a permeability of 80. You can upload this mesh in the form of a mat file.

Data Types: double

Tetrahedra in custom dielectric mesh, specified as an M-by-3 integer matrix. M is the number of tetrahedra.

Data Types: double

Relative permittivity of dielectric material, specified as a scalar.

Data Types: double

Loss in dielectric material, specified as a scalar.

Data Types: double

Data Types: struct

Location of feeds in Cartesian coordinates, specified as an N-by-3 matrix. You can also use the getLowPassLocs and getHighPassLocs functions to determine the feed locations in low-pass or high-pass mode.

Example: 'FeedLocations'= [0.3981 0.0392 -0.2300;0.3528 0.1886 -0.2300]

Example: b.FeedLocations = getLowPassLocs(b)

Data Types: double

Magnitude of voltage applied to each feed, specified as a scalar or 1-by-N vector with each element unit in volts.

Example: 'FeedVoltage',2

Example: bc.FeedVoltage = 2

Data Types: double

Phase shift to the excitation voltage at each feed, specified as a scalar or 1-by-M vector with each element unit in degrees.

Example: 'FeedPhase',45

Example: bc.FeedPhase = 45

Data Types: double

Lumped elements added to the antenna feed, specified as a lumped element object handle. You can add a load anywhere on the surface of the antenna. By default, it is at the origin. For more information, see lumpedElement.

Example: 'Load',lumpedelement. lumpedelement is the object handle for the load created using lumpedElement.

Example: bc.Load = lumpedElement('Impedance',75)

Tilt angle of antenna, specified as a scalar or vector in degrees.

Example: 'Tilt',90

Example: bc.Tilt = [90 90 0]

Data Types: double

Tilt axis of the antenna, specified as:

  • A three-element vector of Cartesian coordinates in meters. In this case, each vector starts at the origin and lies along the specified points on the X, Y, and Z axis.

  • Two points in space as three-element vectors of Cartesian coordinates. In this case, the antenna rotates along the line joining the two points.

  • A string input for simple rotations around the principal planes, X, Y, or Z.

For more information see, Rotate Antenna and Arrays

Example: 'TiltAxis',[0 1 0]

Example: 'TiltAxis',[0 0 0;0 1 0]

Example: bc.TiltAxis = Z

Data Types: double

Object Functions

getLowPassLocsFeeding location to operate birdcage as lowpass coil
getHighPassLocsFeeding location to operate birdcage as highpass coil
showDisplay antenna or array structure; Display shape as filled patch
axialRatioAxial ratio of antenna
beamwidthBeamwidth of antenna
chargeCharge distribution on metal or dielectric antenna or array surface
currentCurrent distribution on metal or dielectric antenna or array surface
designDesign prototype antenna for resonance at specified frequency
EHfieldsElectric and magnetic fields of antennas; Embedded electric and magnetic fields of antenna element in arrays
impedanceInput impedance of antenna; scan impedance of array
meshMesh properties of metal or dielectric antenna or array structure
meshconfigChange mesh mode of antenna structure
patternRadiation pattern of antenna or array; Embedded pattern of antenna element in array
patternAzimuthAzimuth pattern of antenna or array
patternElevationElevation pattern of antenna or array
returnLossReturn loss of antenna; scan return loss of array
sparametersS-parameter object
vswrVoltage standing wave ratio of antenna

Examples

expand all

Create and view a default birdcage antenna.

bc = birdcage
bc = 
  birdcage with properties:

         NumRungs: 16
       CoilRadius: 0.4000
       CoilHeight: 0.0400
       RungHeight: 0.4600
     ShieldRadius: 0
     ShieldHeight: 0
          Phantom: []
    FeedLocations: [2×3 double]
      FeedVoltage: 1
        FeedPhase: 0
             Tilt: 0
         TiltAxis: [1 0 0]
             Load: [1×1 lumpedElement]

show(bc);

Plot the radiation pattern at 128 MHz.

pattern(bc,128e6)

Antenna Toolbox provides two .mat files to load a phantom human head model into a birdcage antenna. The human head model is a dielectric mesh.

Load human head model file. Extract the values of "Points" and "Tetrahedra". Add a relativite permittivity (EpsilonR) of 10 and a dielectric loss (LossTangent) of 0.002. Scale the dielectric mesh to fit in the birdcage antenna. Here a value of 0.003 is multiplied to the mesh points.

load humanheadcoarse.mat
humanhead = struct('Points',0.003*P,'Tetrahedra',T,'EpsilonR',10,...
                  'LossTangent',0.002)
humanhead = 

  struct with fields:

         Points: [584×3 double]
     Tetrahedra: [2818×4 double]
       EpsilonR: 10
    LossTangent: 0.0020

Add and view the human head mesh inside the birdcage.

b = birdcage('Phantom',humanhead)
show(b)
b = 

  birdcage with properties:

         NumRungs: 16
       CoilRadius: 0.4000
       CoilHeight: 0.0400
       RungHeight: 0.4600
     ShieldRadius: 0
     ShieldHeight: 0
          Phantom: [1×1 struct]
    FeedLocations: [2×3 double]
      FeedVoltage: 1
        FeedPhase: 0
             Tilt: 0
         TiltAxis: [1 0 0]
             Load: [1×1 lumpedElement]

Create a birdcage antenna.

b = birdcage;
show(b);

Use the birdcage as a high-pass coil.

b.FeedLocations = getHighPassLocs(b)
show(b);
b = 

  birdcage with properties:

         NumRungs: 16
       CoilRadius: 0.4000
       CoilHeight: 0.0400
       RungHeight: 0.4600
     ShieldRadius: 0
     ShieldHeight: 0
          Phantom: []
    FeedLocations: [32×3 double]
      FeedVoltage: 1
        FeedPhase: 0
             Tilt: 0
         TiltAxis: [1 0 0]
             Load: [1×1 lumpedElement]

Shield the antenna to ensure that radiation does not leak out.

b.ShieldRadius = 0.5;
b.ShieldHeight = 0.5;
show(b) ;

Introduced in R2017b

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