Nuclear magnetic resonance tomography apparatus operable with a pulse sequence according to the echo planar method

• US 5,304,929 A
• Filed: 11/27/1992
• Issued: 04/19/1994
• Est. Priority Date: 11/29/1991
• Status: Expired due to Term

• Current Assignee: Siemens AG
• Original Assignee: Siemens AG
• Inventors: Stehling, Michael, Schmitt, Franz, Ladebeck, Ralf, Fang, Ming
• Primary Examiner(s): Tokar, Michael J.
• Application Number:
  US07/982,444
• Time in Patent Office: 508 Days
• Field of Search: 324/300, 324/307, 324/309, 324/310, 324/311, 324/312, 324/313, 324/314, 128/653.2
• US Class Current: 324/309
• CPC Class Codes: G01R 33/5615 Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
• CPC Class Codes: G01R 33/5616 using gradient refocusing, e.g. EPI

Abstract

In nuclear magnetic resonance tomography apparatus operable with a pulse sequence according to the echo-planar method, only a part of the k-space is scanned in the phase-coding direction per data acquisition, i.e., per radio-frequency excitation pulse. A phase-coding gradient is used such that regions of the k-space which are interleaved relative to each other are scanned in successive data acquisitions in the phase-coding direction. The number of echoes employed for the raw data matrix, and thus the resolution in phase-coding direction, or the length of the individual pulses of the read-out gradient, and thus the resolution in read-out direction, can thereby be enhanced.

Claims

1. A method for operating a nuclear magnetic resonance tomography apparatus with a pulse sequence according to a modified echo-planar imaging method, comprising the steps of:

generating a radio frequency excitation pulse which causes nuclear magnetic resonance signals to arise in an examination subject;

after each radio frequency pulse, generating a read-out gradient consisting of a plurality of sub-pulses having alternating polarity and generating a phase-coding gradient;

acquiring said nuclear magnetic resonance signals in the presence of said read-out gradient and said phase-coding gradient and digitizing the acquired nuclear magnetic resonance signals;

entering the acquired nuclear magnetic signals in respective rows of a raw data matrix in the k-space per pulse of said read-out gradient and ordering said rows according to phase factors defined by said phase-coding gradient; and

scanning only a portion of said k-space in the phase-coding direction per signal acquisition by selecting said phase-coding gradient so that regions of said k-space interleaved relative to each other are scanned in the phase-coding direction in successive signal acquisitions.

2. A method as claimed in claim 1 comprising the additional step of:

generating respectively different setting gradients in the phase-coding direction from signal acquisition-to-signal acquisition before generating said read-out gradient, for scanning different regions of said k-space.

3. A method as claimed in claim 2 comprising the additional step of inserting a pre-phasing pulse in the phase-coding direction between the excitation pulse and the read-out gradient wherein the step of inserting different setting gradients is further defined by inserting a setting gradient in a time span between said pre-phasing pulse and a first of said pulses of said read-out gradient, and maintaining said time span constant for all signal acquisitions.

4. A method as claimed in claim 1 comprising the additional step of:

scaling signal amplitudes acquired in different signal acquisitions for compensating any signal amplitude differences between the said signal acquisitions.

5. A method as claimed in claim 1 comprising the additional step of:

selecting respective flip angles for the excitation pulse for different signal acquisitions so that a transversal magnetization associated with said flip angle remains the same from signal acquisition-to-signal acquisition.