DE102005020986A1 - Image providing method for e.g. brain diagnostics, involves producing deflection angle of transversal magnetization in range of larger phase gradient, which is smaller than in range of small phase gradient - Google Patents

Abstract

The method involves applying a temporally changing phase gradient (G) to an object e.g. head of a patient, to be imaged, and generating a transversal magnetization. A deflection angle of the transversal magnetization is produced in a range of larger phase gradient, which is smaller than in a range of smaller phase gradient. The deflection angle continues a function which falls from the smaller phase gradient to the larger phase gradient. An independent claim is also included for a magnetic resonance tomography.

Description

The The invention relates to an imaging method according to the preamble of claim 1.

traditional Nuclear magnetic resonance methods are based on the coding of k-space through the application of magnetic field gradients (P. Mansfield, P.G. Morris, NMR imaging in bio-medicine, Academic Press, New York 1982 and P.G. Morris, NMR imaging in biology and medicine, Clarendon Press, Oxford 1986).

typically, becomes a spatial Dimension by data acquisition as a function of time in presence a pulsed magnetic field gradient, while a second spatial Dimension through the phase of the measured signal by application a pulsed magnetic field gradient of a variable amplitude for one fixed, predetermined duration is encoded (phase encode gradient). The acquired signal S (k) is a Fourier transform of the spin density each for a dimension as given in equation (1).

In Equation (1) denotes ρ the Spin density and z the position vector.

Of the reciprocal space vector k is conjugate to the real space variable with k = 1 / (2π) γGt.

The Fourier transform of the experimental signals S (k) reconstructed the experimentally determined image generated by the acquisition time parameters, namely the echo time or repetition time is weighted.

The short observable signals do not allow the use of pulsed Field Gradients for Information Coding in an Echo Experiment (Frequency coding) nor the selection of defined layers of the measuring object with band-limited RF pulses. This turns into a frequency blur or a large dot image function (PSF - point Spread function) in the image space noticeable, the excessive smearing leads. (M. D. Robson, J.C. Gore, R.T. Constable, Measurement of the Point Spread Function in MRI Using Constant Time Imaging, Magn Reson Med. 1997: 38: 733-740)

The SPRITE imaging method solves many of these problems. (BJ Balcom, RP MacGregor, SD Beyea, DP Green, RL Armstrong, TW Bremner, Single-Point Ramped Imaging with _T1 Enhancement (SPRITE), J Magn Reson A Series 1996:. 123: 131-134.)

The technique avoids problems with the resolution that arise when using frequency-encoding readout gradients using exclusively phase-encoding gradients. The finite rise times of the field gradients are bypassed by switching the gradients before RF excitation. Broadband RF pulses of limited length are used to achieve uniform spin deflection across the target. The spatial space is coded in the signal S (k) with the aid of the amplitude change of the applied phase gradient G. A single point of free induction decay (FID) is taken after a fixed coding time t _p after RF excitation. Unlike frequency-coded images, SPRITE images are free of distortions due to B ₀ inhomogeneities, subtle variations, or chemical shift. The resolution is also dependent on the maximum gradient height that can be applied to the DUT, even for nuclei with short relaxation times T ₂ *. The bandwidth of the RF pulse must be greater than the maximum spectral width (gradient height times object length) to ensure a uniform spin deflection.

The logical extensions of the original SPRITE sequence include the step-shaped circuit also of the second phase-encoding gradient. Furthermore, the recording of several data points at different times t _p after each excitation pulse has already been used. After proper field of view scaling, these images can be combined for signal-to-noise enhancement or used point by point for T ₂ * mapping. Magnetic Resonance Imaging (JM, Rioux J, Romanzetti S, Kaffanke J, MacMillan B, Mastikhin I, Shah NJ, Aubanel E, Balcom BJ, Centric scan. 2004 Jul; 169 (1): 102-17)

Especially There is a need in medicine for introducing energy into the patient for the To reduce the intake of images, as the patients are not with too much high-frequency energy input to be charged.

But also outside The medical application, it is of interest, the energy input on a sample and thus the energy consumption of the magnetic resonance tomograph to reduce. It should continue to receive good image qualities become.

It is therefore the object of the invention, an imaging method in particular for the medical application available to provide a lower energy input into the examined Object or lower energy consumption requires, as an imaging method according to the prior art, each according to the same basic procedure work. In the medical application, a particularly gentle Treatment of patients possible be.

outgoing from the preamble of claim 1, the object is achieved with the specified in the characterizing part of claim 1 features.

With the method according to the invention is it now possible Pictures of good quality with lower energy consumption or lower energy input in to pick up the object to be examined.

advantageous Further developments of the invention are specified in the subclaims.

The Figures show the procedure according to the invention as well as comparative experiments by way of example.

It shows:

1 : An embodiment in which the amplitude of the RF excitation pulse is varied with the k-space coordinates.

2 : An embodiment in which the duration of the RF excitation pulse is varied with the k-space coordinates.

3 Comparison between the prior art a) and the method b) according to the invention

4 , Comparison between prior art a) and the inventive method b) when using variable amplitude

In 1 the course of the phase gradient G over time is shown in the lower part of the graph. The upper part of the graph shows the change in the amplitude of the RF pulse with time, which leads to a change in the deflection angle α of the transverse magnetization with α ₁ to α _n .

2 shows in the lower part the course of the phase gradient G with time. The upper part shows the variation in the duration of the RF pulses as a function of time.

3 Figure 4 shows in (a) an imaging measurement result with an associated horizontal profile taken with a prior art SPRITE sequence, and in (b) a corresponding imaging image result with comparable image quality but taken with a variable RF pulse amplitude.

4 shows the function for reducing the RF pulse amplitude, each for imaging in 3 has been used. In (a) no reduction of the pulse amplitude and thus of the deflection angle and the energy input was made according to the prior art. In (b) the pulse amplitude was calculated according to equation 3 with the parameters σ = 0.25; R = 0.5; offset = 0.5 and thus achieved a 41.6% reduction in energy input.

in the The invention will be explained below become.

In imaging NMR tomography, the object to be examined, for example the head of a patient, is placed in the magnetic resonance tomograph to measure the k-space. For this purpose, a Magnetgradietenfeld = phase gradient G) is applied, which causes a coding of the k-space. Thereafter, an RF pulse (= radio frequency pulse = high-frequency pulse = RF pulse) is applied, which causes a transverse magnetization of the spins and is followed by the recording of a measuring point. Along at least one selected axis of the k-space, the phase gradient G is varied at predetermined time intervals corresponding to the duration T _R , so that a new coding of the k-space takes place. This procedure can be carried out for all three spatial directions of k-space. After each change in the phase gradient G, the transverse magnetization is measured following a radio-frequency pulse HF.

According to the invention Deflection angle α of transverse magnetization to the k-space center - ie to low gradient values of the phase gradient G is higher than to the edge regions of the k-space - ie to high phase gradient values down, where the deflection angle α of transversal magnetization decreases.

The is called, In the center of the k-space, the RF energy inputs become larger compared to the peripheral areas of k-space. That means the energy input is smaller Magnetic field gradients largest.

Surprisingly can with the variation of the invention the deflection angle α of the transverse Magnetization of the energy input into the object to be examined for example, be reduced by up to 88%, using as a reference the maximum deflection angle α used which is achieved in the experiment.

The proposed method is based on the fact that outside the central region of k-space, often referred to as "keyhole" (M. Zaitsev, K. Zilles, N.J. Shah. Shared k-space echo planar imaging with keyhole. magn Reson Med. 2001 Jan; 45 (1): 109-17.), There is a low signal to noise ratio which makes a small contribution to the overall intensity of the image.

The inventive method but is not limited to the SPRITE method, but is applicable on all single point imaging procedures.

The Values of this function for the deflection angle α of Transverse magnetization of the RF pulse are from the peripheral areas of k-space growing toward the center, or from the center of k-space falling to the edge areas.

It But it is also conceivable that the function, the deflection angle α of the transversal Magnetization of the RF pulses describes, in the edge regions of the k-space intermediate maxima having.

Preferably it is monotone from the edges of k-space to the center growing, or from the center of the k-space to the edge areas falling monotonously.

The Function may or may not be symmetric.

The Maximum for the deflection angle α of transversal magnetization is located essentially in the Minimum of the phase gradient G preferably in the minimum of the phase gradient G.

The Function may have an offset with respect to the ordinate, the is called, The function may have a constant added to it.

Farther it is advantageous if the function with the deflection angle α of the transverse Magnetization changes, is a continuously differentiable function.

The Function that the change the deflection angle α of the describes, for example, a Gaussian function follow or be linear from the edge of k-space to its center.

By way of example, but not limitation, the following functions may be mentioned: hyperbolic functions, Gaussian functions, parabolic functions or functions with plateaus in the lower region Gradient fields and / or constant offset.

Generally speaking, all elements of the real function space {L ^∞ (IR ⁿ ), || · || _∞ } which is almost everywhere (f.ü.), ie modulo arbitrary zero sets, measurable and f.ü. significantly limited functions than a possible distribution of the variable repetition time T _R are considered.

When Example function can be given Equation 2, which is one possible embodiment characterized the invention.

n

= Index des Phasengradienten

N

= Matrixgröße des Bildes in einer Dimension

R

= Radius des konstanten Plateaus

σ²

= Varianz der Gaussfunktion

F

= analytische Funktion, hier: Gaussfunktion

In it is:

n

= Index of the phase gradient

N

= Matrix size of the image in one dimension

R

= Radius of the constant plateau

σ ²

= Variance of the Gaussian function

F

= analytic function, here: Gaussian function

For the three-dimensional case, Formula 3 can be given by way of example:

In Formula 3 is r a normalized radius.

Of the Deflection angle α of Transverse magnetization and thus the energy input can by variation the duration and or the amplitude and / or the shape of the RF pulse be effected.

The Variation according to the invention the deflection angle α of the transverse magnetization may be in at least one direction of the k-space, that is she can for at least one component of the three possible spatial directions take place. The variation according to the invention preferably takes place for all three directions Application, but it can also be applied in two or one direction become. With regard to more than one spatial direction of the k-space can the function which the deflection angle α of the transverse magnetization determined, preferably be substantially radially symmetrical.

Basically the procedure of the invention be applied to any phase-coded NMR method. So, for example various trajectory tours (Spiral or in parallel lines).

The method can be used for measurements on all nuclei accessible by NMR. In particular, Na ²³ , H ¹ , O ¹⁷ or P ^{31 may} be mentioned.

The method according to the invention is preferably used in the acquisition of T ₂ * data.

The inventive method should be used in particular in medicine, preferably in brain diagnostics be there because it is there especially to reduce energy input arrives.

• 1. Variation des Auslenkungswinkels α abhängig von der Position im k-Raum.
• 2. Der Auslenkungswinkel α sollte in den Randbereichen des k-Raumes niedrig im Zentrum größer gewählt werden.
• 3. Die Variation des Auslenkungswinkels α in jeder gegebenen Dimension sollte eine hinreichend glatte Funktion der k-Raum Position sein.
• 4. Die vorgeschlagene Methode kann in jede SPRITE Variante implementiert werden.
• 5. Die Punktbildfunktion und die räumliche Auflösung in dem resultierenden Bild werden durch die Gewichtung des Auslenkungswinkels α der transversalen Magnetisierung im k-Raum beeinflusst.

The proposed method has the following special features:

• 1. Variation of the deflection angle α depending on the position in k-space.
• 2. The deflection angle α should be chosen to be larger in the edge regions of the k-space low in the center.
• 3. The variation of the deflection angle α in any given dimension should be a sufficiently smooth function of the k-space position.
• 4. The proposed method can be implemented in any SPRITE variant.
• 5. The dot image function and the spatial resolution in the resulting image are affected by the weighting of the deflection angle α of the transverse magnetization in k-space.

It It should be noted that the method is not significantly different from the exact one Function of the variation of the deflection angle α depends.

Advantages of the proposed Method:

• 1. The total energy input can be dramatic be reduced. This is especially important in the case of In vivo imaging of sodium.
• 2. It produces a good picture quality.

From The invention also includes a magnetic resonance tomograph, which the method according to the invention is working.

He contains Control means, the energy input depending on the phase gradient controls. This allows the controller varies the duration and / or the amplitude and / or the shape of the RF pulse analogous to the variation of the deflection angle α after the described procedure.

Effect of the change of amplitude and duration on the energy input:

The power P introduced into a measurement object with radius R and specific conductivity σ is a quadratic function of the radio frequency ω and the radio frequency field strength B ₁ . P = 4/15 πσω 2 B 2 1 R 5

The high-frequency field strength B ₁ is inter alia dependent on the quality of the coil used and the current supplied. With the integral of the high frequency field strength B ₁ over the length of the RF pulse τ, the deflection angle α is given in proportion to the gyromagnetic constant γ of the observed nucleus. α = γB 1 τ

The introduced energy per RF pulse results from the integral of the introduced power over the pulse length. W = Pτ

The specific absorption rate SAR is defined as the average introduced energy with respect to the repetition rate T _R. SAR = W / T R

Thus, the energy input is a quadratic function of the RF pulse amplitude, given with B ₁ , and linearly dependent on the RF pulse length τ.

In order to the variation of the amplitude of the RF pulse is the more efficient method to reduce the energy input.

Claims (29)

Imaging method in which a temporally changing phase gradient G is applied to an object to be imaged and a transverse magnetization is generated whose decay is measured, characterized in that smaller deflection angles α of the transverse magnetization are generated in the regions of the larger phase gradients G than in the regions smaller Phase gradient G. Imaging method according to claim 1, characterized that the deflection angle α of the transverse Magnetization from low to higher phase gradient G out decreases. An imaging method according to claim 1 or 2, characterized characterized in that the deflection angle α of the transverse magnetization follows a function that goes from low to higher phase G decreases monotonically. An imaging method according to any one of claims 1 to 3, characterized in that the maximum of the deflection angle α of the trans versalen Magnetization in the range of the lowest phase gradient G is. Imaging method according to claim 4, characterized that's the maximum for the deflection angle α of transverse magnetization in the minimum of the phase gradient G is. An imaging method according to any one of claims 1 to 5, characterized in that the function, the deflection angle α of the transversal Magnetization describes, has an offset with respect to the ordinate. An imaging method according to any one of claims 1 to 6, characterized in that the function describing the deflection angle α of the transverse magnetization comprises an element of the real function space {L ^∞ (IR ⁿ ), || · || _∞ } is. An imaging method according to any one of claims 1 to 7, characterized in that the function which the deflection angle α of the transversal Magnetization describes a function consisting of the group from hyperbolic Functions, parabolic Functions, Gaussian functions, linear functions and functions with a plateau in the range smaller Phase gradient is. An imaging method according to any one of claims 1 to 8, characterized in that the deflection angle α of the transverse Magnetization of the phase gradient G follows a function, continuous is differentiable. An imaging method according to any one of claims 1 to 9, characterized in that the function of which the deflection angle α of the transverse Magnetization describes, in relation to the minimum of the phase gradient G is symmetrical. An imaging method according to any one of claims 1 to 10, characterized in that the function, the deflection angle α of the transversal Magnetization describes, reaches a maximum value, which over a Range of low phase gradient is constant. An imaging method according to any one of claims 1 to 11, characterized in that it at least one spatial direction of k-space applied becomes. Imaging method according to one of claims 1 to 12, characterized in that it is used for recording measurement data of nuclei of the group Na ²³ , H ¹ , O ¹⁷ or P ³¹ . An imaging method according to any one of claims 1 to 13, characterized in that the change of the deflection angle α of the transversal Magnetization by varying the duration of the RF pulse takes place. An imaging method according to any one of claims 1 to 14, characterized in that the change of the deflection angle α of the transversal Magnetization by variation of the amplitude of the RF pulse takes place. An imaging method according to any one of claims 1 to 15, characterized in that the change of the deflection angle α of the transverse Magnetization by variation of the shape of the RF pulse takes place. An imaging method according to any one of claims 1 to 16, characterized in that it is used in medicine. Magnetic resonance tomograph, characterized in that it comprises a control, which in the areas of the larger phase gradients G generates RF pulses of a smaller duration and / or a smaller amplitude, as in the ranges of small phase gradients G. Magnetic resonance tomograph according to Claim 18, characterized that it comprises a controller which generates RF pulses whose amplitude and / or duration from lower to higher phase G gradients lose weight. Magnetic resonance tomograph according to claim 18 or 19, characterized characterized in that it comprises a controller which RF pulses generated whose duration and / or amplitude follows a function, the from lower to higher Phase gradient G decrease monotonically. Magnetic resonance tomograph according to claims 18 to 20, characterized characterized in that it comprises a controller which RF pulses generated, whose duration and / or amplitude in the range of the lowest Phase gradient G have a maximum. Magnetic resonance tomograph according to claims 18 to 21, characterized characterized in that it comprises a controller which RF pulses generated, whose duration and / or amplitude in the minimum of the phase gradient G have a maximum. Magnetic resonance tomograph according to claims 18 to 22, characterized characterized in that it comprises a controller which RF pulses whose duration and / or amplitude follow a functi on which in terms of of the phase gradient G have an offset with respect to the ordinate. Magnetic resonance tomograph according to claim 18 to 23, characterized in that it comprises a control which generates RF pulses whose duration and / or amplitude with respect to the phase gradient G follows a function which is an element of the real function space {L ^∞ (IR ⁿ ), || · || _∞ } is. Magnetic resonance tomograph according to claims 18 to 24, characterized characterized in that it comprises a controller which RF pulses generated, whose duration and / or amplitude with respect to the phase gradient G is a function of the group consisting of hyperbolic functions, parabolic Functions, Gaussian functions, linear functions and functions with a plateau in the range smaller Phase gradient is. Magnetic resonance tomograph according to claims 18 to 25, characterized characterized in that it comprises a controller which RF pulses generated, whose duration and / or amplitude with respect to the phase gradient G is a function that is continuously differentiable. Magnetic resonance tomograph according to claims 18 to 26, characterized characterized in that it comprises a controller which RF pulses generated, whose duration and / or amplitude with respect to the phase gradient G follows a function which is related to the minimum of the phase gradient G is symmetrical. Magnetic resonance tomograph according to claims 18 to 27, characterized characterized in that it comprises a controller which RF pulses generated, whose duration and / or amplitude for at least one spatial direction of k-space can be varied. Magnetic resonance tomograph according to claims 18 to 28, characterized characterized in that it comprises a controller which RF pulses generated, whose duration and / or amplitude over a range of low Phase gradient G is constant.