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Method To Improve Image Uniformity At High Field MRI

IP.com Disclosure Number: IPCOM000129735D
Publication Date: 2005-Oct-07
Document File: 8 page(s) / 765K

Publishing Venue

The IP.com Prior Art Database

Abstract

In an embodiment, a method to improve image uniformity at high field MRI is developed which improve the diagnostic quality of all 3T and 7T MR images by minimizing unwanted intensity artifacts. This is achieved by acquiring 3 sets of Images 2 of which are acquired using surface coil to be used whereas the third image is acquired with 35° flip angle. The 1st and 2nd sets are used to calculate the receive field distribution, whereas the 2nd and 3rd set of images are used to calculate the flip angle distribution. Given the flip angle prescribed and the sequence type, it is possible to approximate the intensity distortion due to the dielectric. Combining these two pieces of information, it is possible to correct for both receive sensitivity- and transmit field-induced intensity distortions in the clinical images.

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Method To Improve Image Uniformity At High Field MRI

FIELD OF THE INVENTION

[0001]               This invention relates generally, to Magnetic Resonance as used in Medical Imaging field and specifically in the area of Dielectric resonance induced transmit field in-homogeneity in MRI System.

BACKGROUND OF THE INVENTION

[0002]                  Dielectric resonance induced transmit field inhomogeneity has always been a nuisance in high field MR imaging, especially at 3T and above [1]. The root cause of dielectric resonance is the fact that at high field the wavelength in the body becomes comparable to human body dimensions. The resulting inhomogeneous B1 field (flip angle distribution) leads to

a.       Intensity non-uniformity, and

b.      Contrast non-uniformity depending on the pulse sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]                 Figure 1: The flowchart of the image intensity correction process for high field systems (3T and above).

[0004]               Figure 2: Modified FGRE (RF and read-out only) to desensitize the B1 map to T1 variation in the tissue

[0005]               Figure 3:  Flip-Angle contrast was measured between WM (T1=1000ms) and GM (T1=600ms) phantoms using the proposed method and the traditional dual-pulse method. The results are shown on the images (A) and (B), respectively.

[0006]               Figure 4: An example in-vivo B1 map (128x128). The right and left breast intensity difference is due to dielectric effect at 3T (fat signal also looks leveled with the breast tissue).

[0007]               Figure 5: An example axial brain slice corrected using this method (with Matlab Script), before (A) and after (B) the correction. [This is a GRE scan without any contrast adjustment].

[0008]               Figure 6: A Sample liver+spleen image: (a) original, (b) rx-field corrected only, and (c) rx-tx-field corrected. The high intensity at the bottom of the spleen and the bright pixels are due to the cal-scan mismatch and the flow, respectively (not a result of the correction method)

DETAILED DESCRIPTION OF THE INVENTION

[0009]    Various embodiments of this invention provide a Method To Improve Image Uniformity At High Field MRI.

  The overall flowchart of the proposed correction method can be summarized as shown in Fig 1. During the calibration scan three sets of images are acquired with breath hold. First two image sets are acquired using the surface coil (intended to be used) for the clinical scan, and the body coil, respectively (with 70º flip). The last set is acquired with the body coil transmit/receive with 35° flip angles. The 1st and 2nd sets are used to calculate the receive field distribution, whereas the 2nd and 3rd set of images are used to calculate the flip angle distribution. Given the flip angle prescribed and the sequence type, we can approximately calculate the intensity distortion due to the dielectric. Combining these two pieces of information, we can correct for both receive sensitivity- and transmit field-induced intensity distortions in the clinical images.

In the acquisition of transmit flip-angle dist...