The invention furthermore relates to a magnetic resonance imaging apparatus comprising a magnet for generating a location-dependent magnetic field on an object, and a transmitter for transmitting a Larmor frequency signal to said object; and to signals for using the imaging technique in the imaging apparatus.

By now such a magnetic resonance imaging technique, briefly called MRI technique, and a corresponding imaging apparatus are generally known. Introductory information on the subject can be found in, for example, the book entitled :"Magnetic Resonance Imaging", by D. D. Stark and W. G. Bradley Jr., ISBN 0-8016-4930-7. According to said technique, an object, usually a patient's tissue to be examined, is placed in a magnetic field, which field has a gradient, as a result of which the generated magnetic field gets a location-dependent field strength. This results in a location-dependent precession or Larmor frequency. When subsequently a Radio Frequency (RF) signal having a frequency around the Larmor frequency of an object part in question is transmitted to the patient, the precession is influenced, and a relaxation signal is formed after termination of the RF signal, which is emitted into the excited tissue in particular by the hydrogen atoms. Said relaxation signal is picked by receiving coils and subsequently processed into an overall image of the tissue by signal processing equipment.

A drawback of the known method is the fact that it takes quite some time in practice before images of the various slices or parts of a patient's body have been obtained at successive locations. This makes the known method costly and, in addition, leads to long waiting times.

The object of the present invention is to provide an improved method by which the time during which the MRI apparatus is used can be reduced whilst retaining the desired quality of the image being obtained.

In order to accomplish that object, the MRI technique according to the invention is characterized in that said signal is a signal having several Larmor frequencies, which excites more than one object part simultaneously.

When several-viz. two or more-Larmor frequencies are supplied to the object in one and the same RF signal, as many slices or object parts can be excited simultaneously with one and the same RF signal. In that case the resulting relaxation signal will contain the combined information of each of the-two or more-slices without more transmitting or receiving coils than those that are already present as standard being required.

Briefly put, in principle the hardware, such as the magnet and the coils, can remain largely unchanged, which is of major practical advantage, since it is readily possible to continue to use existing MRI equipment. It is furthermore of practical importance that the processing speed of the imaging technique is increased and that the MRI time per patient is reduced, so that more patients can be treated with existing MRI equipment. The existing MRI equipment only needs to be adapted as far as the software-implemented processing of the combined signal is concerned in order to ensure that the images of the individual slices can be derived from the relaxation signal that has been picked up.

An important advantage of the imaging technique according to the invention is furthermore the fact that it can be combined with all the MRI techniques that are currently known, such as image forming and image processing MRI techniques. Examples of this are frequency and phase coding techniques for generating 3D images, selection of the width of object parts, existing pulse sequences (such as T1 and T2 MRI techniques), proton-hydrogen techniques and imaging techniques based on other core elements, etc.

One embodiment of the imaging technique according to the invention is characterized in that the multiple Larmor frequency signal is built up of more than one spectrum, which spectrums are distributed round the Larmor frequencies of various object parts.

The location-dependent magnetic field as applied provides slices having different Larmor frequencies, which can all be excited simultaneously by one and the same signal having said RF frequencies.

Another embodiment of the imaging technique is according to the invention characterized in that said multiple Larmor frequency signal is built up of several substantially block-shaped frequency spectrums.

The block waves can be arranged beside each other in the frequency domain, as it were, whilst such a signal nevertheless forms one signal, seen in the time domain.

Yet another embodiment of the imaging technique according to the invention is characterized in that the overall frequency spectrum of the Larmor frequency signal is determined by the minimum and maximum values of the location-dependent magnetic field.

In this way it is even possible to group two, three, four... Larmor frequency block waves beside each other in the frequency domain, depending on the distance between the

minimum and maximum values of the magnetic field in the frequency domain, as a result of which as many object parts or slices can be excited simultaneously. As a result, the production capacity of the imaging apparatuses in question increases accordingly, in terms of patients to be examined, which is of singular practical and social importance, whilst the cost of the required hardware remains substantially the same.

Another embodiment of the imaging technique according to the invention is characterized in that separate signals representative of the individual parts of the object are derived from the signal that is received back from the excited object.

Said deriving takes place in excitation signal processing and image receiving MRI equipment that is known per se.

Characteristic of the imaging technique according to the invention is the fact that said separate signals are derived by filtering, in particular by analog and/or digital filtering. In particular said digital filtering usually does not require any analog components, which are usually fairly voluminous, whilst the excitation signals from the various object parts or slices can be separated from each other in a simple manner, for the purpose of deriving separate images therefrom, by means of known digital filter design techniques.

It is of practical advantage that the imaging technique according to the invention can be used with one or more, if desired, radio frequency transmitting and/or receiving coils.

In practice a number of known coils will be used for that purpose, such as gradient coils, surface coils,"shim" coils, volume coils etc.

The magnetic resonance imaging technique according to the present invention will now be explained in more detail with reference to the figures below, in which like parts are indicated by the same numerals. In the drawing: Fig. 1 shows an assembly of sub-images l (a), l (b), l (c) and l (d), with reference to which the prior art magnetic resonance imaging technique will be explained ; Fig. 2 shows an assembly of graphs 2 (a) and 2 (b), with reference to which the magnetic resonance imaging technique according to the invention will be explained ; and Fig. 3 is a schematic representation of a magnetic resonance imaging apparatus according to the invention.

Fig. 1 comprises sub-images l (a)... l (d). An object O is placed in a magnetic field whose field strength is location-dependent. This is achieved by means of a- usually strong-magnet (not shown), which produces a magnetic field, whereon a superposition is provided for example in the form of a gradient field, as a result of which a connection, for example a linear connection, is created between position-in the z-direction in this case - and the field strength. The protons of, for example, the hydrogen present in the tissue of the object exhibit a Larmor precession that depends on the local field strength, in accordance with the formula: OL = y B wherein OL is the Larmor or precession frequency, y is the so-called gyromagnetic ratio and B is the local field strength. When a high frequency or radio frequency (RF) signal in the form of a pulse around the Larmor frequency having a frequency shift as illustrated in Fig. l (c) is fed to a an object part S1 (not shown to scale) in a linear magnetic field, the cores in question in the object part Si are excited and the precession is disturbed. The RF

amplitude time image of such a block-shape in the frequency domain, which is obtained after Fourier Transformation (FT), is shown in Fig. l (d). Upon termination of the RF signal, the precession resumes its initial position in a relaxation pattern that is typical of the tissue of the object part. A relaxation signal is delivered, which is picked up by a receiving coil 1, which signal is subjected to image transformation and processing techniques that are known per se so as to obtain an MRI image. In order to create a distinction between locally differing precessing cores in the x-and y-directions as well, frequency encoding and/or phase encoding may be used, for example, in order to eventually generate a 3D MRI image exhibiting sufficient intensity differences, so that medical analyses can be based thereon.

Making one recording for each slice S1 in this manner, or several consecutive recordings, if necessary, for each slice S1 in order to obtain a satisfactory overall MRI image, takes relatively much valuable time, which limits the number of patients that can make use of the MRI equipment.

Said number can be increased by transmitting an RF signal, possibly with the same transmitting coil (not shown), to the object in the magnetic field, which contains. several different Larmor frequencies. As a result, several slices or object parts S1 can be excited simultaneously with each recording, after all, as a result of the location-dependent magnetic field each slice has its own specific Larmor frequency, to which it is only sensitive for that part of the RF signal.

Fig. 2 (a) shows the frequency spectrum of the signal having three Larmor frequency bands, which signal is suitable for exciting three neighbouring object parts Si, S2 and S3 at the same time. The time image of such a signal is shown in Fig. 2 (b). The individual Larmor blocks (three in this case) can be arranged close together in the

frequency domain. A block having a frequency around a particular Larmor frequency of a slice will excite only that block which-in accordance with the above formula- is sensitive to that frequency. In other words, the excitations that result from the various frequency blocks do not disturb each other. The eventual excitation signal that is transmitted after termination of the RF signal and picked up by the receiving coil 1 contains all the components of the separate excitations.

It stands to reason that it is possible to use a multiple Larmor frequency signal comprising two, three, four, five, generally i different frequency blocks in order to be able to excite in principle the same number of object parts Si simultaneously and, if possible, with only one multiple signal. The advantages of the imaging technique increase proportionately. Technically, the total width of the frequency spectrum of the Larmor frequency signal and thus the maximum of i is determined by the minimum and maximum values of the location-dependent magnetic field, and in particular by the difference between said values divided by the width or partial bandwidth of a frequency block. In practice a desired partial bandwidth will be taken into account, of course, in view of the slice thickness that is related thereto. It is also possible, however, to modify the steepness of the gradient of the magnetic field in a known manner for the purpose of adjusting the slice thickness.

In the above, frequency blocks have been described which are schematically shown in Fig. 2 (a). The frequency blocks may be practically approximated blocks, such as frequency sync blocks or, for example, Gaussian (bell- shaped) blocks, which do not disturb neighbouring blocks (too much) and which therefore comprise spectral components having a relatively low level, which are present in addition to the spectral components having a high level.

Generally, the bandwidth of the frequency blocks in the

frequency domain is usually kept within predefined bounds, so that the eventual separation of the signals from the various slices can be effected in a simple manner without very costly investments being required.

Fig. 3 schematically shows the receiving coil 1 that picks up the emitted excitation signal. After multiple excitation by the multiple Larmor frequency signal, the signals representative of the individual object parts or slices of the object can be derived by simple filtering in filters fl, f2, f3.... After all, each slice delivers its own characteristic signal. Although the use of analog filters is in principle possible, frequency-separation of the individual signals of each of the object parts by means of digital filtering will be preferred in practice. Each individual signal is subsequently processed in a manner that is known per se so as to create the desired MRI image.

Said processing can take place simultaneously for each individual object part signal, or be carried out in succession, by the same processing and imaging unit. The MRI magnet and the MRI apparatus itself are not needed for such electronic processing, however, so that a much greater number of patients can benefit from the use of said apparatus.

If desired, the imaging technique may be used with one and the same radio-frequency transmitting and/or receiving coil 1. It is also possible to use more than one coil, possibly of different types, of course.