Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-06-20
2004-10-12
Shaw, Shawna J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S420000, C324S306000, C324S309000
Reexamination Certificate
active
06804546
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the magnetic resonance arts. It finds particular application in medical diagnostic imaging and will be described with reference thereto. However, it will be appreciated that the invention will also find application in other types of imaging, spectroscopy, and the like.
A typical magnetic resonance (MR) imaging sequence includes an RF excitation pulse, e.g. a 90° pulse, with a corresponding slice- or slab-selective magnetic gradient pulse, followed by a series of spatial encoding and readout magnetic field gradient pulses. In some sequences a second 180° refocusing pulse is applied between the initial excitation pulse and the spatial encoding/readout region. The 180° pulse effectively reverses the dephasing effect of small spatial variations in the MR frequency due to spatial variations in the applied magnetic field, and refocuses the magnetization to form a spin-echo. MR imaging is performed using various imaging modes which usually vary with respect to the method and timing of the spatial encoding and data readout sequences.
The choice of spatial encoding and data readout scheme has significant consequences on the imaging contrast, resolution, and scanning speed. Two imaging parameters are the time-to-echo, T
E
, and the repeat time between RF excitations, T
R
. Sampling the induced resonance nearer to the excitation emphasizes proton density weighting or T
1
weighting in which the contrast strongly reflects the regrowth rate of the M
Z
component of the net magnetization. Sampling the magnetization later emphasizes T
2
weighting in which the contrast strongly reflects the decay rate of the M
XY
, component of the net magnetization.
Proton density (&rgr;) weighting is obtained when the T
E
delay is short and the magnetic resonance has minimal time to decay, so that the density of resonant hydrogen protons is measured. T
2
* weighting is obtained using a longer T
E
delay so that the fastest (T
2
*) magnetic resonance decay is a factor. The T
2
* decay differs from T
2
in that T
2
* includes inhomogeneous dephasing due to static magnetic field inhomogeneities. To measure the “pure” T
2
corresponding to dephasing due to molecular interactions (excluding inhomogeneous dephasing), a 180° RF refocusing pulse is applied to induce a spin-echo during the sampling interval. Other types of pre-pulses can also be applied to provide fat suppression, MTC, et cetera.
Prior spatial encoding and readout schemes have been configured to provide a variety of &rgr;, T
2
, or T
2
* weightings. The choice of spatial encoding scheme strongly affects the scan speed and resolution. A popular MR imaging mode is echo-planar imaging (EPI). In the EPI imaging mode, an oscillating read gradient generates a series of gradient echoes. Phase encoding pulses between echoes step the sampling through k-space in a back-and-forth rastering fashion. The speed of EPI is preferably sufficient that the k-space data for an entire planar (slice) image is obtained from a single RF magnetic resonance excitation, i.e. “single-shot” EPI, or SS-EPI. The rapidly switched gradients along with a rastered readout timing sequence of SS-EPI produce complete slice scans in as little as a few hundred milliseconds or less. This speed makes SS-EPI an ideal method for clinical imaging when short scan times are important. Reduced scan times translate to reduced image blurring due to patient movements, respiration, cardiac action, and the like.
The EPI technique encompasses a number of variants, including several techniques collectively known to the art as partial parallel imaging (PPI). In the PPI techniques, a phased array receive coil simultaneously measures the MR response using a plurality of phased receive coils and combines the data from the array to acquire a plurality of k-space samples in parallel.
Enhancement in MR imaging can also be obtained through the use of multiple image techniques. In these methods, the spatial encoding scheme is designed so that multiple images, typically using more than one image contrast mode, are obtained from the echo train following a single RF excitation pulse. For example, a T
2
* weighted image and a T
2
weighted image can be obtained.
Another type of MR imaging is contrast-enhanced imaging. In this type of MR imaging, a magnetic contrast agent, such as a gadolinium chelate, is administered to the patient, such as by a bolus injection. The magnetic contrast agent provides enhanced MR contrast versus intrinsic imaging. In some studies, the preferential concentrating of the contrast agent in particular organs or tissues is imaged. In vascular imaging, the distribution of an administered contrast agent is monitored over time to study the performance of major blood vessels. Similarly, the perfusion of the contrast agent through tissues or organs enables study of the capillary performance in the targeted areas.
In order to quantitatively analyze perfusion by contrast-enhanced MR imaging, it is useful to quantify the concentration of the contrast agent in the imaged area based upon the MR image. In the exemplary case, the gadolinium chelate strongly reduces the T
2
weighted signal and T
2
* weighted signal. In a T
2
weighted MR image, the areas of high gadolinium chelate concentration appear darker than the surrounding areas. In principle, therefore, the contrast agent concentration can be extracted from the percentage darkening or from similar quantitative image analysis. Unfortunately, competing effects, such as brightening due to T
1
shortening, can counteract the T
2
darkening effect of the gadolinium chelate and produce errors in the quantitative analysis.
The prior art also discloses taking a reference image prior to administration of the contrast agent. This approach has the disadvantage that the image of the contrast agent usually needs to be registered spatially with the reference image to correct for patient movement or other spatial shifting.
An effective method is needed for correcting these errors in quantitative contrast-enhanced perfusion imaging. Such correction would preferably utilize additional non-T
2
weighted images to account for extraneous, non-T
2
contrast mechanisms. However, the collection of these additional images is limited by the time constraints imposed by the dynamic perfusion process. The present invention contemplates a new imaging method which overcomes these limitations and others.
SUMMARY OF THE INVENTION
According to one aspect of the invention, a method of magnetic resonance imaging is disclosed. A magnetic resonance contrast agent is administered to a subject, which contrast agent alters T
2
and T
2
* magnetic resonance characteristics. A magnetic resonance is excited in a region of interest of the subject which receives the contrast agent. A first echo planar image readout waveform is applied which generates first image data. After the first echo planar image readout waveform, a second echo planar image readout waveform is applied and a T
2
or T
2
* weighted image data is generated. The image data is reconstructed to generate a proton density weighted or a T
1
weighted image representation and a T
2
or T
2
* weighted image representation. The T
2
or T
2
* weighted image representation is corrected with the first image representation.
Preferably, the method includes applying an RF inversion pulse between the first and second echo planar image readout waveforms.
The method preferably includes applying a third echo planar image readout waveform and generating the other of T
2
and T
2
* weighted image data. Optionally, an RF inversion pulse is applied between the second and third echo planar image readout waveforms, such that the second echo planar image readout waveform generates T
2
* weighted data and the third image readout waveform generates T
2
weighted data. The T
2
weighted data is preferably reconstructed into a T
2
weighted image representation, and the T
2
weighted image representation is preferably modified with the first image representation.
The method preferably inc
Anand Christopher K.
Dannels Wayne R.
Thompson Michael R.
Wu Dee H.
Fay Sharpe Fagan Minnich & McKee LLP
Koninklijke Philips Electronics , N.V.
Shaw Shawna J.
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