Method and apparatus for correcting distortion in mr images caused by metallic implants

The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to a method of correcting distortion in an MR image caused by an implant.

When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B₀), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B₁) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, M_z, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M_t. A signal is emitted by the excited spins after the excitation signal B₁ is terminated and this signal may be received and processed to fill a data acquisition or k-space matrix. The data stored in the k-space matrix may then be processed to form an image.

When utilizing these signals to produce images, magnetic field gradients (G_x, G_y, and G_z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.

It is well known that, in the presence of a foreign object, the magnetic fields used in MR imaging often induce magnetic fields about the foreign object that can cause noticeable distortion in the resulting MR image. All materials have some form of magnetism, which is measured by its respective magnetic susceptibility, X. Magnetic susceptibility is a measure on how a material reacts to external magnetic fields. This “reaction” of materials to magnetic fields is fundamentally manifested in the magnetic field they induce in response to the external field. Higher magnitudes of magnetic susceptibility induce more severe magnetic fields. In magnetic resonance, a very large magnetic field of relative spatial homogeneity is used to polarize nuclear (or electronic) spins. When a material is placed in this magnetic field, its magnetic susceptibility distribution causes an induced magnetic field. It is this induced magnetic field that can cause distortion in MR images. On its own, the human body induces such fields. However, the magnetic susceptibilities of organic tissue and air have magnitudes roughly 10-100 times less than the relative magnetic susceptibilities of metallic implant components and their surrounding tissue, depending on the type and shape of the metal used in the implant. Therefore, the magnetic fields induced by metallic implants are far more severe and troublesome than the induced fields typically dealt with in biological magnetic resonance applications.

Techniques have been developed to correct distortion caused by a substance\'s induced magnetic field. One such technique is the “line-integral” technique. The basic principle of this technique is described hereinafter and assumes distortion in a single dimension.

First, two images [I₁(x₁) and I₂(x₂)] are acquired with equal and opposing imaging gradients in the desired direction of desired distortion correction (i.e. the phase-encode direction in EPI images or the readout direction of spin-echo images near metal implants). The image distortion then occurs in opposite directions, but with different character due to the added superposition of imaging gradient fields with opposite sign. For each line in the distorted direction, a boundary is found at one of the images and then the distance between this boundary and the corresponding point in the other image is estimated by integrating each image in the distorted direction. Where the integrals match, is where the two points correspond to one another. The midpoint of the two points is then the point where signal would lie in a non-distorted image [I₀(x)]. This process is repeated until a full mapping of points in each image is uncovered:

x→x₁ and x→x₂  [1]

This is effectively the same as knowing the magnetic field map at each point in the image, since:

x₁=x+αB₀(x), and x₂=x−αB₀(x);  [2]

where α is a known constant, and:

x=(x₁+x₂)/2.  [3]

Equation [3] is the mathematical basis of the line-integral method previously described. Knowing this mapping, the intensity of the non-distorted image is also uncovered as:

Continue reading about Method and apparatus for correcting distortion in mr images caused by metallic implants...
Full patent description for Method and apparatus for correcting distortion in mr images caused by metallic implants

Brief Patent Description - Full Patent Description - Patent Application Claims

Click on the above for other options relating to this Method and apparatus for correcting distortion in mr images caused by metallic implants patent application.

Patent Applications in related categories:

20090289632 - Apparatus for application of a magnetic field to a sample - Disclosed is an apparatus for application of a magnetic field to a sample, in particular an elongated sample. At least in places, the apparatus has magnetic bodies which are arranged such that they can rotate with respect to one another and have, and leave open, a common access opening for ...

20090289631 - Magnetic resonance imaging with multiple contrast - A magnetic resonance imaging system comprises an RF-excitation module to generate one of several RF-excitations and a gradient module to generate one of several magnetic gradient pulses, a control unit controls the RF-excitation module and the gradient module and performs an acquisition sequence containing a succession of RF-excitations and gradient ...


###


How KEYWORD MONITOR works... a FREE service from FreshPatents
1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored.
3. Each week you receive an email with patent applications related to your keywords.  
Start now! - Receive info on patent apps like Method and apparatus for correcting distortion in mr images caused by metallic implants or other areas of interest.
###


Previous Patent Application:
Magnetic resonance imaging and radio frequency impedance mapping methods and apparatus
Next Patent Application:
System for seismic detection and analysis
Industry Class:
Electricity: measuring and testing

###