Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-01-13
2002-06-04
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06400151
ABSTRACT:
The present invention relates to the locally resolved investigation of objects by means of magnetic resonance (MR) and particularly concerns a method of and an apparatus for the acquisition of data for an image representation which shows the spatial distribution of the MR behavior of an object within a selected localised region, according to the preamble of claim
1
and of claim
18
respectively.
In the conventional MR imaging methods, the object region to be investigated, i.e. the “probe”, is arranged in a stationary magnetic field B
0
and a succession of at least one electromagnetic high frequency (HF) pulse of selected frequency and following pulses of magnetic field gradients are applied in different spatial directions, such that, as a consequence of the high frequency excitation, echoes appear which are detected as NMR signals and which give information as to the condition of the probe. In this connection, besides the density of the spin which can be influenced by the high frequency pulses, there are various characteristic relaxation time constants of the spin magnetisation, among others the spin-grid relaxation time T
1
, the spin-spin relaxation time T
2
and the effective spin-spin relaxation time T
2
*. Mention should also be made of the time constant designated as T
1
, which describes the relaxation of the magnetisation in the direction of an effective magnetic field which is composed of static and a high frequency magnetic field. In other words, T
1
describes the relaxation in a rotating coordinate system.
The energy content of the high frequency pulses determines the amount of the excited spin capable of emitting an MR signal (transversal magnetisation) in proportion to the spin present in the equilibrium condition (longitudinal magnetisation). The inverse tangent of this ratio is designated as the flip angle of the high frequency pulse.
The resonance frequency of the spin and consequently the frequency both of an excitable high frequency pulse and also of the measurable MR signals is determined by the localised magnetic field strength. For the localised resolution, therefore for all imaging methods, during the signal detection, a so-called read gradient is imposed in a chosen spatial direction, in order to associate different local regions along this direction with different frequencies in the signal (frequency coding). By a Fourier transformation, the different frequencies and consequently the contributions of different local regions can be separated. In this way, a localised resolution is possible in the relevant spatial direction, which is designated also as the “frequency axis”.
In order to achieve localised resolution in a second spatial direction which is orthogonal to the read direction, it is conventional, before the appearance of the signal to be detected, to impose transiently a gradient in this direction, which has the effect of dephasing the oscillations (spins) excited in the probe along the relevant spatial direction. By stepped changing of the time integral of this “phase gradient” from echo to echo, the phase of the signal contribution originating from one local place changes from echo to echo. The signal contributions of the different places along this direction can be separated from one another by a Fourier transformation with reference to the current number of the echo. Since frequency and phase are separately dependent on the position along orthogonal spatial coordinates, a two-dimensional image of the object can be reconstructed.
A local selection in a third spatial direction is effected by applying a gradient in this direction during the exciting frequency-selective high frequency pulses. By this “slice gradient” a slice is selected in the object for the excitation.
The most common MR imaging methods work with the combined frequency and phase coding described above. For the representation for example of a two-dimensional N-line image, N echoes are produced one after another, each with a different phase coding and with each echo of this N echo sequence being. frequency coded in the same way by the read gradient and scanned as an MR signal. From the scanned values of the detected signals, a two-dimensional matrix of data is formed, the so-called K-space, each row or “line” of which has a different frequency coded echo associated therewith and contains scanned values of the relevant echo. The line direction is also designated as the frequency axis of the K-space. The axis of the K-space which is orthogonal to this is scaled as phase coordinates, i.e. the position of a row along this axis is defined by the integral of the phase gradients. The data matrix which is thus organised is then subjected to a two-dimensional Fourier transformation (
2
D-FT) in order to obtain the pixel values of the image.
Also, other less usual MR imaging methods (projection reconstruction imaging, spiral imaging) can be used to scan the
2
D-K-space, where the strict separation between phase coding direction and read gradient direction is abolished in these methods. In general, with these methods, the K-space is scanned not equidistantly in non-rectangular trajectories. Therefore, for these methods, other image reconstruction methods must be used.
In the MR signals one must differentiate between three different types. The so-called “spin echo signal” arises from refocusing of the magnetic field inhomogeneity effects by means of an additional high frequency pulse which is applied for a certain time after the first high frequency excitation pulse. The so-called “gradient echo signal” is produced by polarity reversal of a magnetic field gradient (usually the read gradient), as a result of which there is a refocusing of the de-phasing brought about by the previous effect of this gradient. So-called “stimulated echo signals” and echo signals of higher order arise after a succession of at least three high frequency pulses with flip angles which are not equal to 180°.
The total echo sequence (“N echo sequence”) required for the receipt of an N-line image can be produced by the most varied of MR sequences. Each MR sequence is composed of a single sequence or by multiple repetition of the same sequence of high frequency pulses and magnetic field gradient shifts.
The required N echo sequence can be produced by sequences consisting of an N-fold repetition of the same sequence, wherein each sequence consists of a single high frequency excitation pulse and a single echo, so-called 1-echo sequence, developed from a read gradient reversal (gradient echo) or a refocusing high frequency pulse in combination with suitable read gradient shifts (spin echo). Alternatively however, after a high frequency excitation pulse, several spin echoes and/or gradient echoes can be produced within a sequence, and can be coded for the image representation in the manner described above. One would speak here of multi-echo sequences (M-echo sequence). Depending upon whether one produces all required N echoes by means of one excitation and a single sequence, or whether the N echoes are collected in several successive sequences each with its own excitation pulse sequence, one speaks of a single-shot sequence or of multi-shot sequence methods.
In many applications of the MR imaging one is seeking to carry out the echo production and echo detection as rapidly as possible. In the last two decades, a large number of rapid imaging techniques have been proposed which are described extensively in the literature. Some of the methods described there have achieved wide use. From the methods conventional at the present time, the single-shot sequence variation of the so-called “echo planar imaging” (EPI) is the most rapid; here the whole total image information is obtained in a single sequence in the form of gradient echoes after a single excitation pulse by an ultra-fast sequence of read gradient reversals within 25 to 250 ms, so that image artefacts caused by movement are almost completely excluded. However, this method has the disadvantage of poor spatial resolution, since the number of the echoes measurable af
Haase Axel
Hahn Dietbert
Hillenbrand Claudia
Jakob Peter M.
Dennison, Scheiner & Schultz
Patidar Jay
Shrivastav Borij B.
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