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 birdcage can be loaded/excited to model a highpass or lowpass coil.

Creation

Syntax

bc = birdcage

bc = birdcage(Name,Value)

Description

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

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

NumRungs — Number of rungs

16 (default) | scalar integer

Number of rungs, specified as a scalar.

Example: 'NumRungs',20

Example: bc.NumRungs = 20

Data Types: int8

CoilRadius — Coil radius

0.4000 (default) | scalar

Coil radius, specified as a scalar in meters.

Example: 'CoilRadius',0.2

Example: bc.CoilRadius = 0.2

Data Types:

CoilHeight — Coil height

0.0400 (default) | scalar

Coil height, specified as a scalar in meters.

Example: 'CoilHeight',0.089

Example: bc.CoilHeight = 0.089

Data Types: double

RungHeight — Height of rungs

0.4600 (default) | scalar

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

Example: 'RungHeight',0.56

Example: bc.RungHeight = 0.56

Data Types: double

ShieldRadius — Shield radius

0 (default) | scalar

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

ShieldHeight — Shield height

0 (default) | scalar

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

Phantom — Dielectric mesh to load birdcage

structure

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

Points — Points in custom dielectric mesh

N-by-3 matrix

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 — Tetrahedra in custom dielectric mesh

M-by-4 integer matrix

Tetrahedra in custom dielectric mesh, specified as an M-by-4 integer matrix. M is the number of tetrahedra.

Data Types: double

EpsilonR — Relative permittivity of dielectric material

scalar

Relative permittivity of dielectric material, specified as a scalar.

Data Types: double

LossTangent — Loss in dielectric material

scalar

Loss in dielectric material, specified as a scalar.

Data Types: double

Data Types: struct

FeedLocations — Location of feeds in Cartesian coordinates

0 (default) | N-by-3 matrix

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

FeedVoltage — Magnitude of voltage

1 (default) | scalar | 1-by-N vector

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

FeedPhase — Phase shift to the voltage

0 (default) | scalar | 1-by-M vector

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

Load — Lumped elements

[1x1 lumpedElement] (default) | lumped element object handle

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 — Tilt angle of antenna

0 (default) | scalar | vector

Tilt angle of the antenna, specified as a scalar or vector with each element unit in degrees. For more information, see Rotate Antennas and Arrays.

Example: 'Tilt',90

Example: 'Tilt',[90 90],'TiltAxis',[0 1 0;0 1 1] tilts the antenna at 90 degree about two axes, defined by vectors.

Data Types: double

TiltAxis — Tilt axis of antenna

[1 0 0] (default) | three-element vector of Cartesian coordinates | two three-element vectors of Cartesian coordinates | 'X' | 'Y' | 'Z'

Tilt axis of the antenna, specified as:

• Three-element vectors 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-axes.

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

• A string input describing simple rotations around one of the principal axes, 'X', 'Y', or 'Z'.

For more information, see Rotate Antennas and Arrays.

Example: 'TiltAxis',[0 1 0]

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

Example: ant.TiltAxis = 'Z'

Object Functions

getLowPassLocs	Feeding location to operate birdcage as lowpass coil
getHighPassLocs	Feeding location to operate birdcage as highpass coil
show	Display antenna or array structure; Display shape as filled patch
axialRatio	Axial ratio of antenna
beamwidth	Beamwidth of antenna
charge	Charge distribution on metal or dielectric antenna or array surface
current	Current distribution on metal or dielectric antenna or array surface
design	Design prototype antenna or arrays for resonance at specified frequency
EHfields	Electric and magnetic fields of antennas; Embedded electric and magnetic fields of antenna element in arrays
impedance	Input impedance of antenna; scan impedance of array
mesh	Mesh properties of metal or dielectric antenna or array structure
meshconfig	Change mesh mode of antenna structure
pattern	Radiation pattern and phase of antenna or array; Embedded pattern of antenna element in array
patternAzimuth	Azimuth pattern of antenna or array
patternElevation	Elevation pattern of antenna or array
returnLoss	Return loss of antenna; scan return loss of array
sparameters	S-parameter object
vswr	Voltage standing wave ratio of antenna

Examples

Birdcage Antenna

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: [2x3 double]
      FeedVoltage: 1
        FeedPhase: 0
             Tilt: 0
         TiltAxis: [1 0 0]
             Load: [1x1 lumpedElement]

show(bc);

Plot the radiation pattern at 128 MHz.

pattern(bc,128e6)

Human Head Model Inside BirdCage

Antenna Toolbox™ provides two .mat files to load a phantom human head model into a birdcage antenna. The humanheadcoarse.mat contains a coarse dielectric mesh of the human head model and the humanheadfine.mat provides the user with a finer dielectric mesh. Load the coarse human head model.

Load human head model file. Extract the values of Points and Tetrahedra. Add a relative permittivity (EpsilonR) of 10 and a dielectric loss (LossTangent) of 0.002. Scale the dielectric mesh to fit in the birdcage antenna. In this case, the mesh points are multiplied by 0.003.

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

humanhead = struct with fields:
         Points: [584x3 double]
     Tetrahedra: [2818x4 double]
       EpsilonR: 10
    LossTangent: 0.0020

Add and view the human head mesh inside the birdcage.

b = birdcage('Phantom',humanhead)

b = 
  birdcage with properties:

         NumRungs: 16
       CoilRadius: 0.4000
       CoilHeight: 0.0400
       RungHeight: 0.4600
     ShieldRadius: 0
     ShieldHeight: 0
          Phantom: [1x1 struct]
    FeedLocations: [2x3 double]
      FeedVoltage: 1
        FeedPhase: 0
             Tilt: 0
         TiltAxis: [1 0 0]
             Load: [1x1 lumpedElement]

show(b)

Birdcage In High-Pass Operation

Create a birdcage antenna.

b = birdcage;
show(b);

Use the birdcage as a high-pass coil.

b.FeedLocations = getHighPassLocs(b)

b = 
  birdcage with properties:

         NumRungs: 16
       CoilRadius: 0.4000
       CoilHeight: 0.0400
       RungHeight: 0.4600
     ShieldRadius: 0
     ShieldHeight: 0
          Phantom: []
    FeedLocations: [32x3 double]
      FeedVoltage: 1
        FeedPhase: 0
             Tilt: 0
         TiltAxis: [1 0 0]
             Load: [1x1 lumpedElement]

show(b);

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

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