Magnetic resonance imaging method and apparatus

Abstract

Magnetic resonance imaging method and apparatus A method for generating magnetic resonance imaging (MRI) images of a subject, comprising: applying a magnetic field B0 to the subject; applying additional magnetic field gradients, including a plurality of first gradients and at least one second gradient, in addition to the magnetic field B0; measuring signal echoes generated by the subject in response to a plurality of the electromagnetic pulses and the first and second magnetic field gradients; acquiring image data at a first spatial resolution from the signal echoes generated by the subject in response to the electromagnetic pulses and the first and second magnetic field gradients; and combining the image data acquired from the signal echoes to generate at least one image of the subject at a second spatial resolution, the second spatial resolution being higher than the first spatial resolution. The first gradients are fully unwound between successive electromagnetic pulses, and at least one second gradient has a non-zero gradient time integral between successive electromagnetic pulses. The phase of at least one electromagnetic pulse in the sequence of electromagnetic pulses is different from another electromagnetic pulse in the sequence of electromagnetic pulses.

Description

[Technical field]

[0001] The present invention relates to magnetic resonance imaging (MRI), and in particular to methods and apparatus capable of providing high resolution images. [Background technology]

[0002] Magnetic resonance imaging (MRI) is also called nuclear magnetic resonance imaging. During an MRI scan, the subject being scanned is placed in a static magnetic field, which magnetizes nuclei in the subject with a net magnetization aligned parallel to the magnetic field. Often, it is the protons of the hydrogen atoms that are of interest, although nuclei of atoms other than hydrogen can be studied using MRI techniques. The subject is then exposed to an electromagnetic pulse, typically a radio frequency pulse, generated by a transmitter. The frequency and duration of the pulse are selected to perturb the net magnetization vector of the protons, in particular to tilt the net magnetization vector perpendicular to the static magnetic field. As the protons return to a lower energy state by relaxation, their precession creates a magnetic flux that can be detected by a receiver. From the magnetic flux, the pixels that make up the image of the scanned subject can be determined.

[0003] Typically, additional magnetic fields are applied to provide gradients in the overall magnetic field: these cause protons at different positions within the field to precess at different rates, so that specific volumes or slices of the subject can be selected to be imaged.

[0004] As such, MRI is a non-invasive imaging modality that does not rely on ionizing radiation and is often used in healthcare to produce images of tissues within the human body.

[0005] The signal produced by protons during an MRI scan is due to two relaxation processes. First, the component of the protons' net magnetization vector that is parallel to the static magnetic field applied to the subject increases. The rate at which this occurs is known as the constant T 1 Second, the component of the net magnetization vector of the protons perpendicular to the static magnetic field applied to the object is governed by a constant T2 The process is governed by T 1 and T 2 The value of depends on the properties of the material that makes up the object being scanned.

[0006] Often, radio frequency pulses are applied multiple times in a sequence. Thus, a time-resolved sequence of radio frequency pulses and magnetic field gradients is called a pulse sequence. 2 For sufficiently steep pulse sequences applied with pulse intervals significantly smaller than , the protons of interest do not have a sufficient period of time to relax to their thermal equilibrium state between pulses. Instead, the protons reach a dynamic equilibrium known as steady state. One such pulse sequence is refocused steady-state free precession (SSFP), which is a common method for producing images with high signal-to-noise ratios (SNR) in short acquisition times.

[0007] In an MRI scan, it is desirable to provide a high resolution image of a subject. However, many methods for obtaining higher resolution images require strong radio frequency pulses and strong magnetic field gradients, which can increase hardware performance requirements, reduce SNR efficiency, and lead to increased heating in the scanned subject. Summary of the Invention

[0008] To this end, a presently preferred embodiment of the present invention provides a method for producing a Magnetic Resonance Imaging, MRI, image of a subject, the method comprising exposing said subject to a magnetic field B 0 applying a series of electromagnetic pulses to the subject; and applying a magnetic field B 0applying a magnetic field gradient in addition to the first gradient, the magnetic field gradient including a plurality of first gradients and at least one second gradient; measuring signal echoes generated by the subject in response to a plurality of electromagnetic pulses and the first and second magnetic field gradients; acquiring image data at a first spatial resolution from the signal echoes generated by the subject in response to the electromagnetic pulses and the first and second magnetic field gradients; and combining image data acquired from the signal echoes to generate at least one image of the subject at a second spatial resolution, the second spatial resolution being higher than the first spatial resolution. The first gradient is fully unwound in the interval between successive electromagnetic pulses, while at least one second gradient has a non-zero gradient time integral in the interval between successive electromagnetic pulses. The phase of at least one electromagnetic pulse in the sequence of electromagnetic pulses is different from another electromagnetic pulse in the sequence of electromagnetic pulses.

[0009] In this way, the method provides an image of the object in which only specific locations in the object contribute to the signal echo. The first gradient allows the signal to be localized to a specific location in the object by a combination of at least one of slice selection, phase encoding, and frequency encoding. The second gradient provides a periodic signal modulation that depends on the off-resonance signal profile of protons in the object. The periodic spatial variation of magnetization is generated at steady state and can contain information about spatial frequencies above the Nyquist frequency. Thus, a series of images with nominal spatial resolution can be acquired, each with a different periodic variation of magnetization. These can be combined to generate an image with a spatial resolution higher than the nominal spatial resolution.

[0010] A further embodiment of the invention comprises a control device, the control device applying a magnetic field B 0 applying a series of electromagnetic pulses to the subject; and applying a magnetic field B 0and applying further magnetic field gradients in addition to the first and second magnetic field gradients, the magnetic field gradients comprising a plurality of first gradients and at least one second gradient, and measuring signal echoes generated by the subject in response to a plurality of electromagnetic pulses and the first and second magnetic field gradients. The controller is further configured to acquire image data at a first spatial resolution from signal echoes generated by the subject in response to the electromagnetic pulses and the first and second magnetic field gradients, and combine image data acquired from the signal echoes to generate at least one image of the subject at a second spatial resolution, the second spatial resolution being higher than the first spatial resolution. The first gradient is fully unwound in the interval between successive electromagnetic pulses, and at least one second gradient has a non-zero gradient time integral between successive electromagnetic pulses. A phase of at least one electromagnetic pulse in the sequence of electromagnetic pulses is different from another electromagnetic pulse in the sequence of electromagnetic pulses.

[0011] When a linear magnetic field gradient is applied along a given axis, the linear variation of the precession frequency results in a linear phase dispersion among the protons along that axis. Since the first gradients have zero integral between successive electromagnetic pulses, they do not produce any accumulation of phase dispersion in a static object. The second gradients have non-zero integrals and therefore produce cumulative phase dispersion throughout the application of successive electromagnetic pulses. The time integral of the second gradient between successive electromagnetic pulses is proportional to the gradient of this linear phase dispersion and determines the periodicity Δx of the modulation pattern in space at steady state. This is related to the gradient time integral as follows: G(t)dt = 2π / γΔx, where G is the gradient strength and γ is the gyromagnetic ratio. The smallest period that can be resolved is when Δx is equal to the nominal voxel spacing along that axis, so that the pattern is repeated once in each voxel.

[0012] MRI machines typically use a magnetic field B 0and a magnetic array suitable for generating a magnetic field gradient. MRI devices typically include a transmitter for transmitting electromagnetic pulses. MRI devices typically include a receiver for measuring signal echoes generated by the subject in response to the electromagnetic pulses.

[0013] The images may be merged voxel-by-voxel in the image domain, the image data may be synthesized in the frequency domain, and the data may be combined using artificial intelligence.

[0014] The first gradients can be fully unwound such that their gradient time integrals are zero between successive electromagnetic pulses.

[0015] The electromagnetic pulse may be a radio frequency pulse. The sequence of electromagnetic pulses and magnetic field gradients may be a steady state free precession (SSFP) pulse sequence.

[0016] This method may be repeated to obtain spatially or temporally separated images, in other words, images of other portions of the object may be acquired by adjusting the first gradient, second gradient, or electromagnetic pulse phase to provide an image of a desired location within the object, and imaging of the same portion of the object may be repeated to provide multiple images of the same portion separated by time.

[0017] The sequence of electromagnetic pulses can include at least a first set of electromagnetic pulses, where a first pulse of the first set has a first phase φ1 and each subsequent pulse in the first set has a phase that is incremented by a first interval, which may be fixed such that the phases of subsequent electromagnetic pulses in the set increase linearly.

[0018] By linearly incrementing the phase of the pulse between successive excitations, a translation of the signal modulation by a fraction of its periodicity is induced in the resulting images. This process can be repeated to generate images with different fractional translations of the signal modulation, generated by different fixed phase increments. These images can then be interleaved voxel-by-voxel before a one-dimensional deconvolution can be performed using the estimated off-resonance profile.

[0019] The sequence of electromagnetic pulses can include a second set of electromagnetic pulses that can occur before and after the first set of electromagnetic pulses, with a first pulse of the second set having a second phase φ2 and each subsequent pulse of the second set having a phase that is incremented by a second interval separate from the first interval. The sequence of electromagnetic pulses can include a third set of electromagnetic pulses. The sequence of electromagnetic pulses can include yet another set of electromagnetic pulses. The sequence of electromagnetic pulses can include as many settings of electromagnetic pulses as desired.

[0020] The phase of the first may be the same as the phase of the second, such that φ1 = φ2. The images derived from two settings of the electromagnetic pulse may be combined to produce a single image. Three or more images may be combined to create a single image.

[0021] Where the sequence of electromagnetic pulses includes at least a first set of electromagnetic pulses, where a first pulse of the first set has a first phase φ1, each subsequent pulse in the first set can have a phase that is incremented by a quadratically increasing interval.

[0022] Secondarily increasing the phase of successive electromagnetic pulses can produce a quasi-steady state in which the magnetization profile is slightly shifted between successive electromagnetic pulses. This can be continued to produce at least two images having a first spatial resolution. These images can be reordered and combined into a single image having a spatial resolution greater than the first spatial resolution.

[0023] Image data from two or more signal echoes may be measured in the intervals between successive pulses in a sequence of electromagnetic pulses. These data are 1 , T 2 or fitted to a signal model to estimate quantitative properties of the object, such as nuclear diffusion properties. Data from several echoes in an interval can be Fourier transformed to give an estimate of the off-resonance profile.

[0024] The steady or quasi-steady state is interrupted by at least one other magnetic field gradient or electromagnetic pulse, such as a reversal pulse, so that the signals are weighted by their quantitative properties. Upon return to the steady or quasi-steady state condition, a signal model can be fitted to estimate the quantitative properties of the subject.

[0025] The first gradient may comprise at least one of a phase encoding gradient, a slice selection gradient, and a frequency encoding gradient disposed along a first axis, and a gradient time integral of the second gradient may be non-zero along the first axis. The second gradient may have any shape that produces a non-zero time integral along any axis between successive electromagnetic pulses.

[0026] The first gradient can represent a non-Cartesian acquisition. The non-Cartesian acquisition may not have a fixed axis for at least one of frequency encoding, phase encoding, or slice selection. The first gradient or the second gradient can generate a non-linear magnetic field gradient.

[0027] The merged image may be deconvolved by the spatial profile. Image deconvolution may be performed in the frequency domain. The deconvolution may be performed by subtracting the magnetic field B generated by the electromagnetic wave. 1 This B may be informed by an estimate of the heterogeneity of 1 The estimate may be derived from the information itself. The merge of the image data is 0This B may be informed by an estimate of the heterogeneity of 0 The estimate may be derived from the data itself. The merging of image data may be informed by an estimate of the motion of the subject during acquisition. This motion estimate may be derived from the data itself.

[0028] Advantages of these embodiments are set forth below, and details and characteristics of each of these embodiments are defined in the accompanying dependent claims and at various points in the following detailed description. [Brief description of the drawings]

[0029] Various aspects of the present disclosure and configurations embodying those disclosures are described below by way of example with reference to the accompanying drawings. [Figure 1] FIG. 1 shows an example of a magnetic resonance imaging apparatus according to the present invention. [Diagram 2] FIG. 2 shows the magnetic field gradients applied by an MRI machine. [Diagram 3] FIG. 3 shows the magnetic field gradients applied by an MRI machine. [Figure 4] FIG. 4 shows the off-resonance frequency profile for a group of protons in an MRI machine. [Diagram 5] FIG. 5 shows the fractional shift in the off-resonance frequency profile when applying electromagnetic pulse settings with different linear phase increments. [Figure 6A] Figure 6A is an example image from an MRI device, Figures 6A-D show a simulated method, and Figures 7A-B are examples using real data. [Figure 6B] Figure 6B is an example image from an MRI device. Figures 6A-D show the method in simulation, while Figures 7A-B are examples using real data. [Figure 6C] Figure 6C is an example image from an MRI device. Figures 6A-D show the simulated method, while Figures 7A-B are examples using real data. [Figure 6D] Figure 6D is an example image from an MRI device. Figures 6A-D show the method in simulation, while Figures 7A-B are examples using real data. [Figure 7A] Figure 7A is an example image from an MRI device. Figures 6A-D show the simulated method, while Figures 7A-B are examples using real data. [Figure 7B] Figure 7B is an example image from an MRI device. Figures 6A-D show the simulated method, while Figures 7A-B are examples using real data. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] 1 is a block diagram of a magnetic resonance imaging (MRI) apparatus 100 according to the present invention. The MRI apparatus 100 comprises a chamber 110 that holds a subject to be scanned and is surrounded by a magnetic array 120 arranged to induce a magnetic field within the chamber 110. The functioning of the magnetic array 120 is controlled by a controller 130. The controller 130 also controls a transmitter 140 that is configured to broadcast radio frequency pulses within the chamber 110. A receiver 150 is configured to detect electromagnetic signals from the subject within the chamber 110 and provide these signals to the controller 130.

[0031] 2 is a diagram showing the operation of the MRI apparatus 100 when used for image acquisition using an enhanced SSFP (Steady State Free Precession) pulse sequence. FIG. 2 shows the pulses and magnetic fields applied during one repetition time TR.

[0032] In a refocused SSFP sequence, radio frequency pulses 210, 220 are repeatedly applied along with a magnetic field gradient that is completely unwound during the repetition time TR, which means that the time integral of the magnetic field gradient applied to the subject during the interval between two radio frequency pulses 210, 220 is zero.

[0033] At the beginning of a repetition time TR, a first radio frequency pulse 210 is applied to the object. The first radio frequency pulse 210 has an intensity α and a phase φ. The first radio frequency pulse 210 causes excitation of nuclei in the scanned object, rotating the net magnetization vectors of the nuclei in the object in the transverse plane by a flip angle α. The nuclei may be those of hydrogen atoms. The magnetization is then rotated by the main magnetic field B at a rate proportional to the local off-resonance frequency. 0 The magnetization of the object precesses around the time constant T 1 and T 2 This can be alleviated by:

[0034] FIG. 2 shows the magnetic field gradients applied during repetition times as part of slice selection (SS), phase encoding (PE), and frequency encoding (FE). The magnetic field gradients applied for slice selection 231, 232, 233, 234 generate corresponding gradients at the precession frequency of the object, so that a given slice of the object can be selected for scanning. In particular, the magnetic field gradients applied for slice selection 231, 232, 233, 234 can be varied in strength to select a desired slice width. Slice selection thus allows localization along a first axis within the object.

[0035] The magnetic field gradients applied for phase encoding (PE) 241, 242 cause dephasing in the precession of the nuclei. The magnetic field gradients applied for phase encoding 241, 242 can also be varied in strength to localize along a second axis in the subject perpendicular to the first axis. In some embodiments, phase encoding is used on two axes, with the second phase encoding PE2 complementing or replacing slice selection.

[0036] The magnetic field gradients applied to the frequency encoding (FE) 251, 252, 253 induce a frequency dispersion in the precession of the nuclei along a third axis perpendicular to the first and second axes within the subject, in order to localize along this third axis.

[0037] Thus, when signals are detected by receiver 150, signals relating to areas within the slice can be identified by their unique combination of frequency and phase encoding and used to generate pixels for use in the resulting image of the slice, typically by application of a Fourier transform.

[0038] The signal is detected during each TR centered around the echo time (TE), which is the time between the application of the radio frequency pulse and the center of the signal induced at the receiver 150. TE is typically equal to half the TR.

[0039] FIG. 3 illustrates the operation of the MRI apparatus 100 when used for image acquisition according to the present invention. In this operation, data acquisition similar to that used for refocused SSFP acquisition is used. In particular, multiple radio frequency pulses 310, 320 are applied to the object being scanned while it is in the chamber 110. At each TR, magnetic field gradients 331, 332, 333, 334, 341, 342, 351, 352, 353 are applied to the object to provide slice selection, phase encoding, and frequency encoding. Additional magnetic field gradients are applied with non-zero time integrals in specific directions. In the example shown in FIG. 3, an additional magnetic field gradient 343 is applied in the same direction as the phase encoding. An additional magnetic field gradient 343 is applied at each TR.

[0040] For a given material in the object to be scanned, the steady-state magnetization depends strongly on the off-resonance frequency Δf, which is the deviation from the frequency produced by an ideal uniform magnetic field. Figure 4 shows the dependence of the signal magnitude on the resonance frequency. In the limit of low flip angles such as α=1°, the magnitude of the off-resonance profile approximates a decaying comb function with a period of 1 / TR. Following a radio frequency pulse with a phase of φ, the next radio frequency pulse has a phase of φ+Δφ. By linearly incrementing the phase of successive radio frequency excitation pulses, the off-resonance profile is offset along the frequency dimension by Δφ / (2πTR) in the final image.

[0041] The off-resonance profile and the additional magnetic field gradient 343 result in a periodic modulation of the spatial magnetization along the direction of the additional magnetic field gradient, with the location of the peak in the periodic modulation depending on the value of the phase increment of the radio frequency pulse sequence, such that as the phase of each radio frequency pulse in the set is incremented by Δφ, the location of the peak in the periodic modulation is shifted slightly relative to the subject.

[0042] Figure 5 shows the shift of the peak in the periodic modulation as the linear phase increment is changed between sets of radio frequency pulses from Δφ=0 to Δφ=2π. Thus, the images derived from each set correspond to different fractional shifts of this periodic modulation. Images from each set can then be merged to generate a super-resolution image.

[0043] Figures 6A, 6B, 6C, and 6D show an exemplary application of the approach shown in Figure 3 when the MRI device 100 is used to create a cross-sectional scan of the brain. One set of image data is shown in Figure 6A, and the underlying magnetization profile in Figure 6A is shown in Figure 6B. When the phase increment between successive high-frequency excitations is changed, the modulation pattern shifts in the resulting image, as shown in Figure 6C, and therefore contains different information about high spatial frequencies. Seventeen sets of image data, including the image shown in Figure 6A, were simulated with different phase increments and merged to generate the super-resolution image shown in Figure 6D.

[0044] Figure 7A shows a single 2D SSFP acquisition with a nominal spatial resolution of 0.3 × 1.8 mm, using a flip angle α = 0.4° such that the periodicity of the modulation pattern is equal to 1.8 mm in the up and down direction, and including additional magnetic field gradients as shown in Figure 3. By voxel-by-voxel interleaving six images in which the spatial modulation pattern is shifted in six equidistant steps, a 0.3 × 0.3 mm image is generated in which super-resolution is achieved in one dimension, as shown in Figure 7B.

[0045] Because most of the signal is generated within the narrow peak of the off-resonance profile, as seen for example in Figure 6B, the use of low flip angles allows for simple deconvolution operations when merging images, which minimizes signal removal from magnetization elsewhere. As a result, the voxel interleaving and deconvolution operations work effectively using only the magnitude of the composite image. However, this approach can be used with higher flip angles as well if combined with a deconvolution operation that accounts for the complex integral of the broader off-resonance profile seen in Figure 2. This can further increase the SNR efficiency of the proposed super-resolution approach. However, the use of low radio frequency pulse flip angles may help minimize tissue heating.

[0046] Although the additional magnetic field gradient is positive in the phase encoding direction in Figure 3, it can have any shape and be equally applied in any direction as long as it has the required non-zero time integral. For example, an additional magnetic field gradient may be applied in the slice selection direction or the second phase encoding direction, and an additional magnetic field gradient may be applied in the frequency encoding direction.

Claims

1. 1. A method for generating a magnetic resonance imaging (MRI) image of a subject, comprising: A magnetic field B ０ applying applying a series of radio frequency electromagnetic pulses to the subject; The magnetic field B ０ applying further magnetic field gradients in addition to the first gradient, the magnetic field gradients including a plurality of first gradients and at least one second gradient, the first gradients enabling localization of signals to specific locations within the subject by a combination of at least one of slice selection, phase encoding, and frequency encoding; measuring signal echoes produced by the subject in response to the plurality of electromagnetic pulses and the first and second magnetic field gradients; obtaining image data from the signal echoes at specific locations within the object, the image data corresponding to a series of images at a first spatial resolution with periodic spatial variations in magnetization; combining the image data to generate at least one image of the object at a second spatial resolution, the second spatial resolution being greater than the first spatial resolution; the first gradients are fully unwound such that their gradient time integrals are zero between successive electromagnetic pulses, while the at least one second gradient has a non-zero gradient time integral between successive electromagnetic pulses to provide a periodic signal modulation that is dependent on an off-resonance signal profile of protons in the subject; a phase of at least one electromagnetic pulse in the series of electromagnetic pulses differs from another electromagnetic pulse in the series of electromagnetic pulses; the series of electromagnetic pulses and the plurality of first magnetic field gradients are a steady-state free precession pulse sequence; The periodic spatial variation of magnetization is generated in the steady state and contains information about spatial frequencies above the Nyquist frequency. A method for producing a magnetic resonance image of a subject.

2. A method for generating a magnetic resonance image of a subject as described in claim 1, wherein the series of electromagnetic pulses includes at least a first set of electromagnetic pulses, a first pulse of the first set having a first phase φ1, and each subsequent pulse of the first set having a phase incremented by a first interval.

3. A method for generating a magnetic resonance image of a subject as described in claim 2, wherein the series of electromagnetic pulses includes a second set of electromagnetic pulses, a first pulse of the second set having a second phase φ2, and each subsequent pulse of the second set having a phase incremented by a second interval separate from the first interval.

4. A method for generating a magnetic resonance image of a subject as described in claim 1, wherein the series of electromagnetic pulses includes at least a first set of electromagnetic pulses, a first pulse of the first set having a first phase φ1, and each subsequent pulse of the first set having a phase that is incremented at quadratically increasing intervals.

5. 5. A method for producing a magnetic resonance image of an object as claimed in any preceding claim, wherein the image data from a plurality of signal echoes is measured in intervals between successive pulses in the series of electromagnetic pulses.

6. 1. A magnetic resonance imaging (MRI) device comprising a controller, the controller controlling the MRI device to: Magnetic field B on the target ０ is applied, subjecting the subject to a series of radio frequency electromagnetic pulses; The magnetic field B ０ applying further magnetic field gradients in addition to the first gradient, the magnetic field gradients including a plurality of first gradients and at least one second gradient, the first gradients enabling localization of signals to specific locations within the subject by a combination of at least one of slice selection, phase encoding, and frequency encoding; configured to measure signal echoes generated by the subject in response to the plurality of electromagnetic pulses and the first and second magnetic field gradients; The controller further comprises: obtaining image data for specific locations within the object from the signal echoes, the image data corresponding to a series of images at a first spatial resolution with periodic spatial variations in magnetization; combining the image data to generate at least one image of the object at a second spatial resolution, the second spatial resolution being greater than the first spatial resolution; the first gradients are fully unwound such that their gradient time integrals are zero between successive electromagnetic pulses, while the at least one second gradient has a non-zero gradient time integral between successive electromagnetic pulses to provide a periodic signal modulation that is dependent on an off-resonance signal profile of protons in the subject; a phase of at least one electromagnetic pulse in the series of electromagnetic pulses differs from another electromagnetic pulse in the series of electromagnetic pulses; the series of electromagnetic pulses and the plurality of first magnetic field gradients are a steady-state free precession pulse sequence; The periodic spatial variation of magnetization is generated in the steady state and contains information about spatial frequencies above the Nyquist frequency.

7. The MRI apparatus of claim 6, wherein the series of electromagnetic pulses includes at least a first set of electromagnetic pulses, a first pulse of the first set having a first phase φ1, and each subsequent pulse of the first set having a phase incremented by a first interval.

8. The MRI apparatus of claim 7, wherein the series of electromagnetic pulses includes a second set of electromagnetic pulses, a first pulse of the second set having a second phase φ2, and each subsequent pulse of the second set having a phase incremented by a second interval different from the first interval.

9. The MRI apparatus of claim 6, wherein the series of electromagnetic pulses includes at least a first set of electromagnetic pulses, a first pulse of the first set having a first phase φ1, and each subsequent pulse of the first set having a phase that is incremented at quadratically increasing intervals.

10. 10. An MRI apparatus according to any one of claims 6 to 9, wherein the image data from a plurality of signal echoes is measured in intervals between successive pulses in the series of electromagnetic pulses.

Images