JP4435488B2 - Sequence preconditioning for ultrafast magnetic resonance imaging - Google Patents

Description

【００２５】
したがって、シーケンス調整パラメータは、診断パラメータから判断でき、かつ画像化シーケンスを前調整するのに使用でき、したがってどのような後収集方法がデータ補正のために規定されている応用においても、ＥＰＩエコーは最適に位置付けられる。
【００２６】
診断パラメータの測定に使用されるデータは、位相エンコード勾配がオフにされていることを除いて、同じＥＰＩ画像化シーケンスを使用して求めることができる。最初に、全ての偶数のエコーを時間反転する。次に単に最大振幅をサーチするピークサーチアルゴリズムによってか、または三次スプライン補間と組合せて、エコー信号の中心を判断する。
【００２７】
【数１０】

【００２８】
ＥＰＩ画像化シーケンスは、既に測定した診断パラメータから求められるシーケンス調整パラメータにしたがって微調整される。
【００２９】
図６ａおよび６ｂには、スピンエコーのＥＰＩまたはフィールドエコーのＥＰＩを収集するためのＲＦパルスと読み出し勾配パルスとのタイミング図が示されている。前調整プロセスは、シーケンスの他の部分へは影響を与えないので、シーケンスの他の部分は図示されていない。実線は前調整前の読み出し勾配パルスであり、点線は前調整後の読み出し勾配パルスである。画像化シーケンスの前調整に使用されるシーケンス調整パラメータも示されている。とくに、シーケンス調整パラメータΔＭ、ｇ_０、およびｄは、既に記載したように診断パラメータから計算され、パルスシーケンスを前調整するのに使用される。
【００３０】
図７ａは、２つのスピンエコーのＥＰＩのデータの組の中の前調整する前のエコーを示しており、図７ｂは前調整した後のエコーを示している。図７ａは、前調整する前のデータの組を示しており、図７ｂは、上述の診断パラメータから判断されたｇ_０を使用して前調整した後のデータの組を示している。とくに、図７ａは、シーケンスの前調整前の、位相エンコードのないスピンエコーのＥＰＩシーケンスからのエコー信号を示しており、図７ｂは、ｇ_０を使用して前調整した後の同じシーケンスのエコー信号を示している。エコー信号の位置付けは、前調整プロセスによって向上することが明らかに示されている。
【００３１】
本発明の方法の最も重要な長所は、前調整プロセスに必要なパラメータが、同じＥＰＩシーケンスを使用することによって判断されること、およびハードウエアまたは複雑なアルゴリズムを追加せずに直接に前調整プロセスを実行できることである。前調整プロセスのアルゴリズムは実行し易く、引き続く前調整プロセスの診断パラメータから、プロセスの効果を自己検査することができる。
【００３２】
前調整パラメータを記録して、システムの特徴を反映することができ、記録を更新して、新しい前調整プロセスを行うたびに蓄積することができる。記録からのパラメータを使用して、新しい前調整プロセスを初期設定することもできる。
【００３３】
前調整プロセスは、ＥＰＩ走査の事前走査の一部、または品質制御プロセスの一部の何れかとして実行することができる。
【００３４】
本発明は、最も実用的で好ましい実施形態であると現在考えられているものと関連付けて記載されているが、本発明は開示されている実施形態に制限されず、逆に、本発明の特許請求項の意図および範囲内に含まれる種々の変更および同等の構成をカバーすることが意図されていることが分かるであろう。
【図面の簡単な説明】
【図１】 ＭＲＩシステムの模式図。
【図２】 一般的なＥＰＩパルスシーケンスを示す図。
【図３】 図２のＥＰＩパルスシーケンスの一部を拡大して詳細に示した図。
【図４】 理想的なデカルトグリッドを模式的に示す図。
【図５】 ある特定のずれのある非理想的なＥＰＩデータの例を示す図。
【図６ａ】 スピンエコーのＥＰＩを収集するためのＲＦパルスと読み出し勾配パルスとのタイミングを示す図。
【図６ｂ】 フィールドエコーのＥＰＩを収集するためのＲＦと読み出し勾配パルスとのタイミングを示す図。
【図７ａ】 前調整する前のデータの組を示す図。
【図７ｂ】 前調整した後の図７ａと同じデータの組を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention generally relates to magnetic resonance imaging (MRI) techniques. In particular, the present invention relates to a preconditioning method for a pulse sequence in ultrafast MRI, using an echo signal suitable for a post-acquisition method, correcting data defects, and The quality has been improved.
[0002]
[Prior art]
Magnetic Resonance Method (MRI) is a significant amount of nuclei whose nuclei are sensitive to what is also known as nuclear magnetic resonance (NMR), or magnetic resonance (MR) phenomenon. A generally accepted and commercialized technique for obtaining a digitized visual image representing the internal structure of an object (eg, a human body) that contains a population. In MRI, by applying a strong main magnetic field (B ₀ ) to the nuclei, polarity is imparted to the nuclei in the patient's body being imaged. The nuclei are excited by radio frequency (RF) signals of the characteristic NMR (Larmor) frequency. Map these nuclear responses as a function of the spatial location of these local magnetic fields by spatially dispersing the local magnetic fields around the body and analyzing the response of the generated RF from the nuclei. Alternatively, an image is generated and displayed. From the image of the nuclear response, a non-invasive image of the patient's internal organs or other tissue is obtained.
[0003]
As shown in FIG. 1, an MRI system typically includes a magnet 10 for applying a static magnetic field (B ₀ ), three gradient magnetic fields (G _X , G G) spatially distributed along three orthogonal coordinates. A gradient coil 14 for applying _{Y 2} and G _Z ) and RF coils 15 16 for transmitting and receiving RF signals to and from selected nuclei of the body being imaged are included. As the patient 13 lies on the movable patient table 12, the portion of the patient to be imaged is moved in three dimensions, creating an “imaging volume” 11 between the magnet and the coil, and MRI. A field of view (FOV) of the system is defined.
[0004]
To collect MRI data, the MRI system generates magnetic gradient and RF nutation pulses via MRI pulse sequence controllers 17 and 18 under the control of a programmable computer / processor 19. In addition, the processor 19 controls the gradient pulse amplifier 20 and the RF power and amplifier circuits 21,22. An MR signal (RF detector) circuit 22 suitably interfaces with an MR signal RF coil 16 located in the shielded MRI system gantry. The received MR response is digitized by the digitizer 23 and sent to the processor 19, which includes an array processor for image processing or the like and a suitable computer program storage medium (not shown). The computer program storage medium stores the program, selectively uses the program to control the collection and processing of MR signal data, and generates an image display on the CRT of the control terminal 24 To do. The control terminal 24 of the MRI system includes appropriate keyboard switches and the like, and the operator controls the imaging sequence controllers 17 and 18. The image is recorded directly on the film or by the printing device 25 on other suitable media.
[0005]
The diagnostic MRI as described above is affected by the selected imaging mode and imaging parameters. If the MRI is unsatisfactory or the physician wants to view the image from a different perspective, by adjusting the desired mode and / or selected image parameter values and repeating the entire imaging procedure, Generate another MRI. For example, if the contrast between two or more objects of interest shown in the MRI is not optimal, the MRI imaging parameters must be adjusted to obtain the proper contrast. Similarly, if the doctor determines the axial image obtained by MRI or if good diagnostic information cannot be obtained from a certain part of the head, select another MRI from a different viewpoint Must be generated.
[0006]
The operator selects the desired imaging parameters before the MRI is generated. By selecting imaging parameters, the position, slice direction, image quality, viewpoint and other features of the image are determined. It is difficult to optimally select many imaging parameters before the image is generated. Since the selected imaging parameters are not optimal when viewed later, the image generated from the initially selected parameters may be inappropriate. Whether or not some or all of the selected imaging parameters should be reset can only be determined by checking the actual image. However, the process of generating MRI, resetting imaging parameters, and generating another image is too time consuming, especially when using MRI technology in diagnostic mode, which requires a long scan period (eg, A few minutes).
[0007]
Thus, it has long been thought that a high speed imaging system was needed. Echo-planar imaging (EPI) and echo-volume imaging (EVI) are described in the literature by P. Mansfield and ILPykett (“Biological and Medical Imaging by NMR”, J. Magn. Reson. 29, 355-373, 1978), which are methods commonly employed in ultrafast magnetic resonance imaging. In the following, for the sake of brevity, EPI is used collectively for both EPI and EVI. As known to those skilled in the art, a gradient pulse sequence in an EPI scan includes a gradient pulse train with alternating alternating polarity in the readout direction and a short accompanying pulse train in the phase encoding direction. . In an EPI scan, gradient echoes of the corresponding column or series are generated and constitute a continuous MRI signal.
[0008]
Raw MRI data can be properly disclosed using k-space. For example, the literature by DBTwieg (“The k-space trajectory formulation of the NMR imaging process with applications in the analysis and synthesis of imaging methods”, Med. Phys. 10, 610-621, 1983) and the literature by S. Ljunggren (“ A simple graphical representation of Fourier-based imaging methods ”, J. Magn. Reson. 54, 338-343, 1983), encoding spatial information by varying the k-value individually along each spatial dimension. It is described to do. In a conventional imaging sequence, one line in k-space is recorded for each phase encoding stage. Since each phase encoding step is performed at each repetition time (TR) (second), the time required to generate an image is determined by the product of TR and the number of phase encoding steps. In EPI, all lines in k-space are measured in a single TR period.
[0009]
As shown in FIG. 2, a typical echo planar imaging sequence includes a 90 ° slice select RF pulse (RF), which is a slice select gradient pulse. Applied simultaneously with (G _s ). Thereafter, an initial phase encoding gradient pulse (G _Φ ) and an initial frequency encoding gradient pulse (G _f ) are applied to position the spin at the corner of k-space. After that, there is a 180 ° RF pulse (RF), then the phase encode gradient pulse and the frequency encode gradient pulse are repeated across the k-space. A signal is recorded while these pulses are repeated.
[0010]
FIG. 3 shows these repeated pulses in more detail. As shown in FIG. 3, the phase encode gradient pulse (G _Φ ) is followed by a frequency encode gradient pulse (G _f ), and the signal (S) is recorded when the frequency encode gradient pulse. Another phase encode gradient pulse is followed by a reverse polarity frequency encode gradient pulse and the signal is recorded at the opposite polarity frequency encode gradient pulse.
[0011]
Ideally, EPI data is collected in k-space according to an equally spaced Cartesian grid with the origin properly centered along all dimensions. In particular, the k value is incremented evenly along each dimension, and when the k values are all zero, data is collected at a single origin. FIG. 4 schematically shows an ideal Cartesian grid, and ideal EPI data settings can be obtained according to this. In FIG. 4, k-space data is sampled with k values, and the k-space data is evenly distributed in the k-space, and is therefore called an equally spaced Cartesian grid.
[0012]
In conventional EPI methods for collecting MRI data, k-space data is digitized at a constant sampling rate. In such an environment, in order to obtain the above ideal EPI data, the following conditions must be met:
(1) The readout gradient (Gro) must be kept constant while reading each k-space line;
(2) The center of the echo must be positioned with respect to all k-space lines so that the origins of all data lines coincide;
(3) During the collection of each line, the phase-encoding gradient (Gpe) must be turned off and the Gpe region between collecting two consecutive lines must remain constant. .
[0013]
Although hardware performance has improved considerably in recent years, there remains a need for faster image data collection. EPI data is often collected under conditions that are significantly less than the ideal conditions described above, and the most commonly recognized deviations are described below:
(1) The readout gradient (Gro) is actually changing over time during the collection of each k-space line, either because the hardware performance is not ideal, or sometimes for safety reasons . As a result, k-space data spacing is non-uniform along the readout dimension;
(2) The origin of k-space differs between odd and even echoes for a number of possible reasons;
(3) Instead of a blipped gradient pulse, a constant phase encoding gradient is used. As a result, k-space lines are skewed;
(4) The center of the echo drifts in the EPI echo train.
[0014]
FIG. 5 shows an example of such non-ideal EPI data in k-space, which includes such a shift and that all even echoes are time-reversed. Has been. The center of the echo is not at the center of the data acquisition window, but there is a relative shift between the center of the odd echo and the center of the even echo, and during the EPI readout, the center of the echo is There is a gradual drift from the collection window.
[0015]
Many data correction techniques have been developed to handle non-ideal EPI data. For example, Sekihara and Kohno disclosed a reconstruction technique for handling data collected with a constant phase encoding gradient (Kensuke Sekihara, Hideki Kohno, “New Reconstruction Technique for Echo-Planar Imaging to Allow Combined Use of Odd and Even Numbered Echoes ”, Magn. Reson. Med. 5. 485-491, 1987)). Bruder et al. Generated a method for reconstructing images by sampling k-space data at non-uniform intervals (H. Bruder, H. Fischer, HEReinfelder, F. Schmitt, “Image Reconstruction for Echo Planar Imaging with Nonequidistant k-Space Sampling ”, Maga. Reson. Med. 23, 311-323, 1992)). In order to correct for N / 2 artifacts by the difference between odd and even echoes, many techniques have been developed to measure and correct for differences between echo signals, and these techniques include calibration scans. Some have been added, for example, see US Pat. Nos. 5,818,229, 5,621,321, and 5722409.
[0016]
However, in order for post-collection methods, such as those already cited, to be effective, the original k-space data must be “reasonably well positioned”. For example, to correct for the difference between odd and even echoes, the two groups of echoes must be sufficiently separated. More importantly, the echo signal must be positioned within the acquisition window in order to correct with the post-acquisition method. As shown in FIG. 5, echoes outside the acquisition window, such as a delayed echo, can no longer be “corrected”.
[0017]
[Problems to be solved by the invention]
The present invention relates to a method for preconditioning a sequence of EPIs, which “reasonably and appropriately locates” echo signals and effectively uses post-acquisition methods to correct data defects and reduce image quality. It relates to a further improved method. In particular, the present invention improves the quality of magnetic resonance images by preconditioning the MRI pulse sequence used in ultra-high speed MRI systems such as EPI.
[0018]
Initially, several sequence diagnostic parameters are determined from a modified MRI pulse sequence without a phase encoding gradient. Next, a pulse preconditioning parameter is calculated using the determined sequence diagnostic parameters. Using the calculated pre-sequence adjustment parameters, the MRI pulse sequence is modified to obtain a diagnostic image in the EPI system.
[0019]
The diagnostic parameters determined by the preconditioned sequence include the deviation of the echo center from the ideal position Δ, the relative shift of the center of the even echo by δ from the center of the odd echo, and the echo A linear drift of the center of the echo for the amount of σ per hit is included. These diagnostic parameters have a fixed relationship with the pre-sequence adjustment parameters, so the pre-sequence adjustment parameters can be calculated in a straightforward manner.
[0020]
Once the pre-sequence adjustment parameters are calculated, this is used to modify the MRI pulse sequence and the modified MRI pulse sequence is used to obtain a diagnostic image. In particular, the gradient readout pulse sequence is modified according to the calculated sequence adjustment parameters, and a diagnostic image is obtained by using an MRI pulse sequence that includes the modified gradient readout pulse sequence.
[0021]
The objects and advantages sought by the present invention will become apparent upon careful consideration of the following detailed description of the presently preferred embodiments, particularly with reference to the accompanying drawings.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
An improperly adjusted EPI sequence consists of the following three diagnostic parameters: (1) the deviation of the echo center from the ideal position by Δ, (2) the odd echo center of the even echo center. It can be characterized by a relative shift of δ from the center and (3) a linear drift of the center of the echo in the amount of σ per echo. The center of the echo is measured after time-reversing all even echoes. Thus, the center of the EPI data set echo can be represented by the following equation:
C _i = C ₀ + Δ + (i% 2) ^* δ + i ^* σ (i = 0, 1, 2,..., N−1)
i represents an index of the echo EPI, thus odd echoes related to i values of the even, the even echo associated with odd i values, C _i is the (i + 1) th echo center of the echo _C0 is the ideal echo center position, and (i% 2) is 0 for odd echoes and 1 for even echoes.
[0023]
Three sequence adjustment parameters corresponding to the diagnostic parameters defined above: (1) mistuning of the first EPI echo for the amount of ΔM at a gradient moment with units of gauss · ms · cm ⁻¹ , (2) the amount minute delay d of the gradient waveforms, can be defined (3) read g ₀ min offset gradient.
[0024]
Sequence adjustment parameters and diagnostic parameters are related according to the following formula:
[Equation 9]

[0025]
Thus, the sequence adjustment parameters can be determined from the diagnostic parameters and can be used to precondition the imaging sequence, so in any application where any post-acquisition method is specified for data correction, the EPI echo is It is positioned optimally.
[0026]
Data used to measure the diagnostic parameters can be determined using the same EPI imaging sequence, except that the phase encoding gradient is turned off. First, all even echoes are time reversed. The center of the echo signal is then determined either by a peak search algorithm that simply searches for the maximum amplitude, or in combination with cubic spline interpolation.
[0027]
[Expression 10]

[0028]
The EPI imaging sequence is fine-tuned according to sequence adjustment parameters determined from already measured diagnostic parameters.
[0029]
FIGS. 6a and 6b show timing diagrams of RF pulses and readout gradient pulses for collecting spin echo EPI or field echo EPI. Since the preconditioning process does not affect other parts of the sequence, the other parts of the sequence are not shown. A solid line is a read gradient pulse before pre-adjustment, and a dotted line is a read gradient pulse after pre-adjustment. The sequence adjustment parameters used for preconditioning the imaging sequence are also shown. In particular, the sequence adjustment parameters ΔM, g ₀ , and d are calculated from the diagnostic parameters as previously described and are used to precondition the pulse sequence.
[0030]
FIG. 7a shows the echo before preconditioning in the EPI data set of two spin echoes, and FIG. 7b shows the echo after preconditioning. Figure 7a shows a set of data where preconditioning, Figure 7b shows a set of data after the pre-adjusted using g ₀ that is determined from the diagnostic parameters described above. In particular, Figure 7a, before adjustment previous sequence shows the echo signal from the EPI sequence without phase encoding the spin echo, Figure 7b, echo the same sequence after adjusting before using g ₀ The signal is shown. It is clearly shown that the positioning of the echo signal is improved by the preconditioning process.
[0031]
The most important advantages of the method of the present invention are that the parameters required for the preconditioning process are determined by using the same EPI sequence, and the preconditioning process directly without adding hardware or complex algorithms Can be executed. The algorithm of the preconditioning process is easy to implement and the effect of the process can be self-checked from the diagnostic parameters of the subsequent preconditioning process.
[0032]
Precondition parameters can be recorded to reflect the characteristics of the system, and the records can be updated and accumulated with each new precondition process. New preconditioning processes can also be initialized using parameters from the recording.
[0033]
The preconditioning process can be performed either as part of the EPI scan pre-scan or as part of the quality control process.
[0034]
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, the invention is not limited to the disclosed embodiments and, conversely, patents of the invention It will be appreciated that various modifications and equivalent arrangements included within the spirit and scope of the claims are intended to be covered.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of an MRI system.
FIG. 2 is a diagram showing a general EPI pulse sequence.
FIG. 3 is an enlarged view showing a part of the EPI pulse sequence of FIG. 2 in detail.
FIG. 4 is a diagram schematically showing an ideal Cartesian grid.
FIG. 5 is a diagram showing an example of non-ideal EPI data having a certain deviation.
FIG. 6A is a diagram showing timings of an RF pulse and a readout gradient pulse for collecting EPI of a spin echo.
FIG. 6b is a diagram showing the timing of RF and readout gradient pulses for collecting EPI of field echoes.
FIG. 7A is a diagram showing a data set before pre-adjustment.
7b shows the same set of data as FIG. 7a after pre-adjustment.

Claims (18)

In a method for improving the quality of a magnetic resonance image by preconditioning an EPI pulse sequence used in an ultrafast MRI system and canceling out gradient magnetic field anomalies, wherein the system is controlled by a control means,
(I) executing an altered EPI pulse sequence with a phase encoding gradient switched off in an ultra-fast MRI system, thereby determining a co-sequence diagnostic parameter, the co-sequence diagnostic parameter comprising: Echo center joint shift by Δ minutes from the center of the echo acquisition window, joint shift of even echo centers by δ minutes from the center of odd echoes, and echo center by the amount of σ per echo Including at least one of the joint linear drifts of
(Ii) calculating a pre-sequence adjustment parameter of an EPI sequence from the determined joint sequence diagnostic parameters;
(Iii) When the next EPI pulse sequence is executed , the next executed EPI pulse sequence is shifted by shifting the readout gradient pulse sequence of the EPI pulse sequence in the time domain according to the calculated pre-adjustment parameter. Modifying and thereby obtaining an improved magnetic resonance image when the next EPI pulse sequence is performed. The joint sequence diagnostic parameter determines the center of each echo signal;

The method of claim 1, wherein the method is determined by calculating at least one of the co-diagnostic parameters. The calculation process is

According The method of claim 2 further comprising at least one of a and calculating the offset amount fraction readout gradient of g _0.   The method of claim 1, wherein the EPI pulse sequence is used in an echo planar imaging process.   The method of claim 1, wherein the EPI pulse sequence is used in an echo volume imaging process. An apparatus for improving the quality of magnetic resonance images by preconditioning an EPI pulse sequence in an ultrafast MRI system and canceling out gradient magnetic field anomalies,
Sequence diagnostic means for determining a modified EPI pulse sequence with a phase encoding gradient switched off in an ultrafast MRI system, thereby determining a co-sequence diagnostic parameter, the co-sequence diagnostic parameter comprising: Echo center joint shift by Δ minutes from the center of the echo acquisition window, joint shift of even echo centers by δ minutes from the center of odd echoes, and echo center by the amount of σ per echo A sequence diagnostic means comprising at least one of the joint linear drifts of:
Calculating means for calculating a sequence preconditioning parameter of an EPI sequence from the determined joint sequence diagnostic parameters;
When the next EPI pulse sequence is executed , the EPI pulse sequence is changed by shifting the read gradient pulse sequence of the EPI pulse sequence in the time domain according to the calculated preconditioning parameter, thereby changing the next EPI pulse sequence. An imaging means for obtaining an enhanced magnetic resonance image when a pulse sequence is performed. The joint sequence diagnostic parameter determines the center of each echo signal;

7. The apparatus of claim 6 , wherein the apparatus is determined by calculating at least one of the co-diagnostic parameters. The calculating means is

8. The apparatus of claim 7 , wherein at least one of calculating an readout slope offset by an amount of g ₀ is calculated according to. The apparatus of claim 6 , wherein the apparatus is used in an echo planar imaging system. The apparatus of claim 6 , wherein the apparatus is used in an echo volume imaging system. In a method for improving the quality of a magnetic resonance image by preconditioning an EPI pulse sequence used in an ultra-fast MRI system and canceling out gradient magnetic field anomalies, wherein the system is controlled by a control means,
(I) executing an altered EPI pulse sequence with a phase encoding gradient switched off in an ultra-fast MRI system, thereby determining a co-sequence diagnostic parameter, the co-sequence diagnostic parameter comprising: Determine the center of each echo signal,

To determine by calculating at least one of said co-diagnostic parameters,
(Ii) calculating a pre-sequence adjustment parameter of an EPI sequence from the determined sequence diagnostic parameter;
(Iii) When the next EPI pulse sequence is executed, the EPI pulse sequence to be executed next is shifted by shifting the readout gradient pulse sequence of the EPI pulse sequence in the time domain according to the calculated preconditioning parameter Modifying and thereby obtaining an improved magnetic resonance image when the next EPI pulse sequence is performed. The calculation process is

According The method of claim 11 further comprising at least one of a and calculating the offset amount fraction readout gradient of g _0. The method of claim 11 , wherein the EPI pulse sequence is used in an echo planar imaging process. The method of claim 11 , wherein the EPI pulse sequence is used in an echo volume imaging process. An apparatus for improving the quality of magnetic resonance images by preconditioning an EPI pulse sequence in an ultrafast MRI system and canceling out gradient magnetic field anomalies,
Sequence diagnostic means for determining a modified EPI pulse sequence with a phase encoding gradient switched off in an ultrafast MRI system, thereby determining a co-sequence diagnostic parameter, the co-sequence diagnostic parameter comprising: Determine the center of each echo signal,

A sequence diagnostic means determined by calculating at least one of said joint diagnostic parameters according to
Calculating means for calculating a sequence pre-adjustment parameter of an EPI sequence from the determined joint sequence diagnostic parameters;
When the next EPI pulse sequence is executed, the EPI pulse sequence is changed by shifting the readout gradient pulse sequence of the EPI pulse sequence in the time domain according to the calculated preconditioning parameter, thereby changing the next EPI pulse sequence. An imaging means for obtaining an enhanced magnetic resonance image when a pulse sequence is performed. The calculating means is

According The apparatus of claim 15, wherein calculating at least one of the calculating the offset amount fraction readout gradient of g _0. The apparatus of claim 15 , wherein the apparatus is used in an echo planar imaging system. The apparatus of claim 15 , wherein the apparatus is used in an echo volume imaging system.

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