System for non-contrast enhanced MR anatomical imaging

A system for Non-Contrast Agent enhanced MR imaging, includes an MR image acquisition device that acquires first and second datasets representing first and second image slabs individually comprising multiple image slices acquired at fast and slow blood flow portions of a heart cycle and oriented substantially perpendicular in at least one axis to direction of vasculature blood flow, in response to a heart cycle synchronization signal. An image data processor processes imaging datasets representing the first and second image slabs to provide first and second volume datasets representing a 3D volume imaged at the fast blood flow portion and the slow blood flow portion respectively and for providing a difference dataset representing an image difference between the first and second volume datasets and enhancing arterial blood flow. A display processor provides data representing an image showing the enhanced arterial blood flow.

FIELD OF THE INVENTION

This invention concerns a system for Non-Contrast Agent enhanced MR (magnetic resonance) imaging, by acquiring image slices oriented substantially perpendicular to direction of vasculature blood flow, at a relatively fast blood flow portion of a heart cycle and at a relatively slow blood flow portion of the heart cycle and by determining a difference between the slices.

BACKGROUND OF THE INVENTION

Known systems employ ECG-gated (Turbo) spin echo (TSE) imaging for MR Angiography using an intrinsic contrast mechanism by exploiting differences in the velocity of arterial blood during a cardiac cycle. In this type of imaging, an MR signal from stationary tissue and non-pulsatile venous blood is canceled by image subtraction, while an MR signal from arterial blood is preserved. This is because, faster flowing blood has a low MR signal on TSE images due to de-phasing effects, while slow flowing blood has a higher signal. Known systems acquire two datasets during diastolic and systolic cardiac phase as illustrated inFIG. 1corresponding to fast and slow flow periods of the cardiac cycle using ECG triggering. The datasets are subtracted and post processed using maximum intensity projection (MIP). The known systems employ readout methods including SPACE based readout (using a variable flip angle method) and Turbo Spin Echo readout including HASTE (Half Fourier Acquisition STE) as identified inFIG. 2.

The readout direction of known systems for limb imaging, for example, is along the direction of blood flow i.e. longitudinally along a limb as illustrated inFIG. 3because this is the longest dimension of the anatomy and ensures efficient data acquisition. In order to obtain adequate recovery of magnetization between successive RF (radio frequency) excitation pulses, data is typically collected in one or more cardiac cycles. Many k-space lines (typically k-space lines of an entire slice (partition)) need to be collected per echo train to image a 3D volume within a reasonable scan time as illustrated inFIG. 4. Specifically,FIG. 4illustrates acquisition of a slice every two heart cycles. Further, in order to achieve a larger field of view (FOV) coverage, known MR systems typically use a coronal orientation together with a localization scan to find vessels prior to an MRA scan which is time-consuming and operator dependent.

In known MR imaging systems, an entire 3D volume is excited with RF excitation fields over multiple cardiac cycles and typically two 3D datasets are acquired with one being acquired during a fast blood flow (e.g. systolic) cardiac period and another being acquired during a slow blood flow (diastolic) cardiac period. For example, known system perform imaging using a single shot SPACE or TSE sequence, with 80 partitions (slices) for 3D volume imaging, triggered by an RR waveform so that 160 RRs are needed, which is time consuming and therefore sensitive to respiratory motion disturbance. A system according to invention principles addresses the deficiencies of known MR imaging systems and related problems.

SUMMARY OF THE INVENTION

A system provides user friendly MR device operation with ECG triggering and Spin Echo sequence based Non Contrast Enhanced MR Angiography using transversal orientation acquisition and continuous patient table or electronic FOV movement to accelerate 3D image volume acquisition. A system for Non-Contrast Agent enhanced MR imaging, includes an MR image acquisition device. The MR image acquisition device acquires over multiple heart cycles, first and second datasets representing first and second image slabs individually comprising multiple image slices oriented substantially perpendicular in at least one axis to direction of vasculature blood flow, in response to a heart cycle synchronization signal. Within an individual heart cycle, a slice of a first image slab and a slice of a second image slab are acquired with one slice of one slab being acquired at a relatively fast blood flow portion of the heart cycle and the other slice of the other slab being acquired at a relatively slow blood flow portion of the heart cycle. An image data processor processes imaging datasets representing the first and second image slabs to provide first and second volume datasets representing a 3D volume imaged at the fast blood flow portion and the slow blood flow portion respectively and for providing a difference dataset representing an image difference between the first and second volume datasets and enhancing arterial blood flow. A display processor provides data representing an image showing the enhanced arterial blood flow.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

An inversion recovery (IR) pulse inverts longitudinal magnetization from the positive z-axis by 180 degrees to the negative z-axis. IR pulses are used as preparation pulses prior to a main imaging pulse sequence to achieve different kinds of MR contrast (such as T1 weighted, T2 weighted). Adiabatic IR pulses are used to give more uniform contrast throughout an imaging volume than non-adiabatic RF pulses.

iPAT (integrated Parallel Acquisition Techniques) comprises “parallel imaging”. It enables faster scanning through reduced phase encoding and addition of RF coil information. An iPAT factor of 2 enables scanning about twice as fast, iPAT factor of 3 enables scanning about three times as fast and so on.

TI=inversion time, the time between an inversion recovery pulse and the next RF excitation pulse. TI determines the image contrast.

T2=the transverse (or spin-spin) relaxation time T2is the decay constant for a proton spin component.

TR=repetition time, the time between successive RF excitation pulses.

A system according to invention principles advantageously acquires a 3D imaging volume over multiple cardiac cycles by acquiring first and second image slices within individual heart cycles of the multiple heart cycles in response to a heart cycle synchronization signal.FIG. 6shows acquisition sequences603and605employed by the system for acquisition of first and second slab image data in fast flow and slow flow periods. A 3D volume (80 slices) is divided into first and second slabs acquired by sequences603and605, each comprising 40 slices. In sequence603a first slab comprises 20 slices, for example, including first, second . . . twentieth. slices (including slices607,609) acquired at a relatively fast blood flow (e.g. systolic) portion of a cycle and in sequence605the first slab comprises 20 slices, for example, including first, second . . . twentieth. slices (including slices621,623) acquired at a relatively slow blood flow (e.g. diastolic) portion of a cycle. Similarly, in sequence603a second slab comprises 20 slices, for example, including first, second . . . twentieth. slices (including slices611,613) acquired at a relatively slow blood flow (e.g. diastolic) portion of a cycle and in sequence605the second slab comprises 20 slices, for example, including first, second . . . twentieth. slices (including slices617,619) acquired at a relatively fast blood flow (e.g. systolic) portion of a cycle. The system acquires a slice for slab1and a slice for slab2in a single cardiac cycle. The system acquires two 3D slabs in 40 heart cycles (RR (R wave to R wave) intervals). The system employs a total number of 80 heart cycles using two slab RF excitation and the system advantageously halves the time required for acquisition and reduces sensitivity to respiratory motion. The fast and slow datasets are subtracted and post-processed using maximum intensity projection (MIP) to provide images with enhanced vasculature visualization. The system provides user friendly MR device operation with ECG triggered and Spin Echo sequence based Non Contrast Enhanced MR Angiography using transversal orientation acquisition and continuous patient table or electronic FOV movement to accelerate 3D image volume acquisition.

FIG. 7illustrates MR image data acquisition at three stations (station1, station2and station3) using transversal orientation acquisition. The system performs multi-station MR Angiography acquisition of a whole peripheral vessel tree by advantageously automatically performing continuous patient table movement.FIG. 10illustrates positioning a patient table for advancing through an MR imaging device. Alternatively, instead of (or as well as) moving a patient table, the system electronically shifts a field of view (FOV) in another embodiment. The system employs coronal, sagittal or transversal orientated 3D imaging. The multi-station interleaved two-slab acquisition further speeds up image data acquisition time. InFIG. 7, for individual stations1,2and3, a first slab (slab1) is acquired during a fast blood flow period, and a second slab (slab2) is acquired during a slow blood flow period. In response to completion of image data acquisition for a whole 3D volume comprising stations1,2and3, the system swaps the acquisition order and acquires slab1during slow blood flow period and acquires slab2during a fast blood flow period. The system advantageously reduces the total scan time by a factor2. The system also advantageously uses transversal orientation 3D acquisition (substantially perpendicular to the blood flow direction) where the phase encoding direction is A-P (Anterior-Posterior), the 3D encoding direction is F-H (Foot-Head), frequency encoding direction is R-L (Right-Left).

FIG. 5shows system10for Non-Contrast Agent enhanced MR imaging. In system10a basic field magnet1generates a strong magnetic field, which is constant in time, for the polarization or alignment of the nuclear spins in the examination region of an object, such as, for example, a part of a human body to be examined. The high homogeneity of the basic magnetic field required for the magnetic resonance measurement is provided in a spherical measurement volume M, for example, into which the parts of the human body to be examined are brought. In order to satisfy the homogeneity requirements and especially for the elimination of time-invariant influences, shim-plates made of ferromagnetic material are mounted at suitable positions. Time-variable influences are eliminated by shim coils2, which are controlled by a shim-current supply15.

In the basic magnetic field1, a cylinder-shaped gradient coil system3is used, which consists of three windings, for example. Each winding is supplied with current by an amplifier14in order to generate a linear gradient field in the respective directions of the Cartesian coordinate system. The first winding of the gradient field system3generates a gradient Gxin the x-direction, the second winding generates a gradient Gyin the y-direction, and the third winding generates a gradient Gzin the z-direction. Each amplifier14contains a digital-analog converter, which is controlled by a sequence controller18for the generation of gradient pulses at proper times.

Within the gradient field system3, radio-frequency (RF) coils4are located which convert the radio-frequency pulses emitted by a radio-frequency power amplifier16via multiplexer6into a magnetic alternating field in order to excite the nuclei and align the nuclear spins of the object to be examined or the region of the object to be examined. In one embodiment, RF coils4comprise a subset or substantially all of, multiple RF coils arranged in sections along the length of volume M corresponding to the length of a patient. Further, an individual section RF coil of coils4comprises multiple RF coils providing RF image data that is used in parallel to generate a single MR image. RF pulse signals are applied to RF coils4, which in response produces magnetic field pulses which rotate the spins of the protons in the imaged body by ninety degrees or by one hundred and eighty degrees for so-called “spin echo” imaging, or by angles less than or equal to 90 degrees for so-called “gradient echo” imaging. In response to the applied RF pulse signals, RF coils4receive MR signals, i.e., signals from the excited protons within the body as they return to an equilibrium position established by the static and gradient magnetic fields. The MR signals comprising nuclear spin echo signals received by RF coils4as an alternating field resulting from the processing nuclear spins, are converted into a voltage that is supplied via an amplifier7and multiplexer6to a radio-frequency receiver processing unit8of a radio-frequency system22.

The radio-frequency system22operates in an RF signal transmission mode to excite protons and in a receiving mode to process resulting RF echo signals. In transmission mode, system22transmits RF pulses via transmission channel9to initiate nuclear magnetic resonance in volume M. Specifically, system22processes respective RF echo pulses associated with a pulse sequence used by system computer20in conjunction with sequence controller18to provide a digitally represented numerical sequence of complex numbers. This numerical sequence is supplied as real and imaginary parts via digital-analog converter12in the high-frequency system22and from there to a transmission channel9. In the transmission channel9, the pulse sequences are modulated with a radio-frequency carrier signal, having a base frequency corresponding to the resonance frequency of the nuclear spins in the measurement volume M.

The conversion from transmitting to receiving operation is done via a multiplexer6. RF coils4emit RF pulses to excite nuclear proton spins in measurement volume M and acquire resultant RF echo signals. The correspondingly obtained magnetic resonance signals are demodulated in receiver processing unit8of RF system22in a phase-sensitive manner, and are converted via respective analog-digital converters11into a real part and an imaginary part of the measurement signal and processed by imaging computer17. Imaging computer17reconstructs an image from the processed acquired RF echo pulse data. The processing of RF data, the image data and the control programs is performed under control of system computer20. In response to predetermined pulse sequence control programs, sequence controller18controls generation of desired pulse sequences and corresponding scanning of k-space. In particular, sequence controller18controls the switching of the magnetic gradients at appropriate times, transmission of RF pulses with a determined phase and amplitude and reception of magnetic resonance signals in the form of RF echo data. Synthesizer19determines timing of operations of RF system22and sequence controller18. The selection of appropriate control programs for generating an MR image and the display of the generated nuclear spin image is performed by a user via terminal (console)21, which contains a keyboard and one or more screens. System10uses magnetic field gradients and radio frequency excitation to create an image. System computer20translates acquired k-space data onto a Cartesian grid and a Three-Dimensional Fourier Transform (3DFT) method is used to process the data to form a final image. K-space is the temporary image space in which data from digitized MR signals is stored during data acquisition and comprises raw data in a spatial frequency domain before reconstruction. When k-space is full (at the end of an MR scan), the data is mathematically processed to produce a final image.

System computer20automatically (or in response to user command entered via terminal21) employs and directs the MR imaging device of system10in Non-Contrast Agent enhanced MR imaging. The MR image acquisition device of system10acquires over multiple heart cycles, first and second datasets representing first and second image slabs individually comprising multiple image slices oriented substantially perpendicular in at least one axis, to direction of vasculature blood flow, in response to a heart cycle synchronization signal. Within an individual heart cycle a slice of a first image slab and a slice of a second image slab are acquired with one slice of one slab being acquired at a relatively fast blood flow portion of the heart cycle and the other slice of the other slab being acquired at a relatively slow blood flow portion of the heart cycle. Image data processor95in computer20processes imaging datasets representing the first and second image slabs to provide first and second volume datasets representing a 3D volume imaged at the fast blood flow portion and the slow blood flow portion respectively and for providing a difference dataset representing an image difference between the first and second volume datasets and enhancing arterial blood flow. Display processor93in computer20provides data representing an image showing the enhanced arterial blood flow. Patient support table controller97automatically advances a patient support table to change an imaged field of view in response to a pulse sequence used for image acquisition.

FIG. 8illustrates MR transversal orientation image data acquisition of a cross-section805of a patient limb803. Successive cross-section images are acquired down the limb in accordance with the image acquisition timing sequences described herein. A rectangular FOV up to 35%-50% of the volume may be used and the Phase encoding (PE) direction is Anterior-Posterior of a limb in cross-section. A rectangular FOV is used to reduce the number of phase encoding lines acquired and reduce acquisition time, and is also advantageously used in particular cases to reduce T2 decay time, such as for a single shot EPI (Echo Planar Imaging) pulse sequence or a single shot TSE sequence, for example. System10uses a rectangular FOV (comprising the scanned region) in frequency and phase encoding directions and reduces the number of measurement lines acquired. A rectangular image is obtained because there are fewer rows than columns. System10reduces the FOV in the phase encoding direction (foldover direction) to reduce scan time by decreasing spatial resolution by undersampling.

FIG. 9illustrates MR transversal orientation data acquisition identifying a FOV903. System10in one embodiment uses parallel imaging (SENSE or GRAPPA) in PE and 3D direction (along the limb) to accelerate imaging. In another embodiment system10employs a slice turbo factor and multiple slice acquisitions are performed for an individual RF pulse excitation. A rectangular FOV or integrated parallel imaging method (iPAT) is used in the PE direction to reduce the total image data acquisition time. Further, a scout sequence for imaging of peripheral limb arteries is not necessary since transversal orientation is used.

FIG. 11shows a pulse sequence employed by the system for acquisition of first, second, third and fourth slab image data in fast blood flow and slow blood flow periods to reduce cross-talk. Cross talk artifacts resulting from use of just two slabs per 3D imaging volume acquisition are reduced by using 4 slabs per 3D imaging volume, which also reduces image data acquisition time.FIG. 11shows acquisition sequences923and925employed by the system for acquisition of first, second, third and fourth slab image data in fast flow and slow flow periods. A 3D volume910(80 slices) is divided into first, second third and fourth slabs acquired by sequences923and925, each comprising 20 slices. In sequence923, first and second slabs individually comprising 10 slices, including slices (partitions)927,931are acquired at a relatively fast blood flow (e.g. systolic) portion of a cycle and third and fourth slabs individually comprising 10 slices, including slices (partitions)929,933are acquired at a relatively slow blood flow (e.g. diastolic) portion of a heart cycle. Similarly, in sequence925, first and second slabs individually comprising 10 slices, including slices (partitions)949,953are acquired at a relatively slow blood flow (e.g. diastolic) portion of a cycle and third and fourth slabs individually comprising 10 slices, including slices (partitions)947,951are acquired at a relatively fast blood flow (e.g. systolic) portion of a heart cycle.

System10(FIG. 5) acquires two slices for corresponding different slabs in a single cardiac cycle. The system acquires four 3D slabs in 40 heart cycles (RR (R wave to R wave) intervals). The system employs a total number of 80 heart cycles using four slab RF excitation and the system advantageously halves the time required for acquisition and reduces sensitivity to respiratory motion. The fast and slow datasets are subtracted and post processed using maximum intensity projection (MIP) to provide images with enhanced vasculature visualization. The system in this embodiment uses a repetition time (TR) comprising the time between successive RF excitation pulses of two heart cycles advantageously reducing cross talk relative to a two slab embodiment. The system provides user friendly MR device operation with ECG triggered and Spin Echo sequence based Non Contrast Enhanced MR Angiography using transversal orientation acquisition and continuous patient table or electronic FOV movement to accelerate 3D image volume acquisition. The system is also applicable for 2D imaging.

FIG. 12shows a flowchart of a process performed by system10(FIG. 5) for Non-Contrast Agent enhanced MR imaging. In step812following the start at step811, the MR image acquisition device of system10acquires over multiple heart cycles, first and second datasets representing first and second image slabs individually comprising multiple image slices oriented substantially perpendicular in at least one axis to direction of vasculature blood flow, in response to a heart cycle synchronization signal and automatic advancement of a patient table substantially along patient vasculature substantially synchronously with image slab acquisition completion. Within an individual heart cycle a slice of a first image slab and a slice of a second image slab are acquired with one slice of one slab being acquired at a relatively fast blood flow portion of the heart cycle and the other slice of the other slab being acquired at a relatively slow blood flow portion of the heart cycle. The relatively fast flow portion of the heart cycle comprises a systolic portion and the relatively slow flow portion of the heart cycle comprises a diastolic portion. The first and second image slabs comprise adjacent respective first and second volumes encompassing a vessel structure of a patient and comprise a rectangular area.

In one embodiment, the MR image acquisition device alternates order of acquisition of the slice of the first image slab and the slice of the second image slab. The MR image acquisition device acquires first and second datasets representing first and second image slabs respectively using at least one of, a parallel imaging pulse sequence and a spin echo based pulse sequence. In one embodiment the spin echo based pulse sequence is a Turbo Spin Echo pulse sequence acquiring multiple slices per RF excitation. Further, in one embodiment, the MR image acquisition device alternates order of acquisition by acquiring slices of the first slab at the relatively fast blood flow portion of the heart cycle for a first portion of slices of the first slab and acquiring slices of the first slab at the relatively slow blood flow portion of the heart cycle for a second portion of slices of the first slab. Also, the MR image acquisition device alternates order of acquisition by acquiring slices of the second slab at the relatively fast blood flow portion of the heart cycle for a first portion of slices of the second slab and acquiring slices of the second slab at the relatively slow blood flow portion of the heart cycle for a second portion of slices of the second slab.

Patient support table controller97automatically advances the patient table substantially along patient vasculature to acquire image slabs substantially covering limb vasculature of a patient region of interest and synchronously with image acquisition. The acquired image slabs individually comprise multiple image slices oriented substantially perpendicular in at least one axis to direction of vasculature blood flow. In one embodiment, controller97automatically moves the patient table to acquire third and fourth image slabs substantially adjacent to the region comprising the first and second image slabs of a patient region of interest. The third and fourth image slabs individually comprise multiple image slices oriented substantially perpendicular in at least one axis to direction of vasculature blood flow. A region comprising the third and fourth image slabs overlaps a region comprising the first and second image slabs. In one embodiment, the MR image acquisition device automatically electronically advances a field of view substantially along patient vasculature to acquire image slabs substantially covering limb vasculature of a patient region of interest and synchronously with image acquisition. The acquired image slabs individually comprise multiple image slices oriented substantially perpendicular in at least one axis to direction of vasculature blood flow.

In step817image data processor95processes (e.g., by combining and subtracting) imaging datasets representing the first and second image slabs to provide first and second volume datasets representing a 3D volume imaged at the fast blood flow portion and the slow blood flow portion respectively. In step820, processor95provides a difference dataset representing an image difference between the first and second volume datasets and enhancing arterial blood flow. Image data processor95provides the difference dataset using maximum intensity projection (MIP) processing and applies the maximum intensity projection (MIP) processing in saggital or coronal orientation to acquired datasets. Image data processor95combines image datasets representing the first and second image slabs to provide first and second volume datasets representing the 3D volume imaged at the fast blood flow portion and the slow blood flow portion respectively. In step823display processor93provides data representing an image showing the enhanced arterial blood flow. The process ofFIG. 12terminates at step831.

A processor as used herein is a device for executing machine-readable instructions stored on a computer readable medium, for performing tasks and may comprise any one or combination of, hardware and firmware. A processor may also comprise memory storing machine-readable instructions executable for performing tasks. A processor acts upon information by manipulating, analyzing, modifying, converting or transmitting information for use by an executable procedure or an information device, and/or by routing the information to an output device. A processor may use or comprise the capabilities of a computer, controller or microprocessor, for example, and is conditioned using executable instructions to perform special purpose functions not performed by a general purpose computer. A processor may be coupled (electrically and/or as comprising executable components) with any other processor enabling interaction and/or communication there-between. A user interface processor or generator is a known element comprising electronic circuitry or software or a combination of both for generating display images or portions thereof. A user interface comprises one or more display images enabling user

A user interface (UI), as used herein, comprises one or more display images, generated by a user interface processor and enabling user interaction with a processor or other device and associated data acquisition and processing functions. The UI also includes an executable procedure or executable application. The executable procedure or executable application conditions the user interface processor to generate signals representing the UI display images. These signals are supplied to a display device which displays the image for viewing by the user. The executable procedure or executable application further receives signals from user input devices, such as a keyboard, mouse, light pen, touch screen or any other means allowing a user to provide data to a processor. The processor, under control of an executable procedure or executable application, manipulates the UI display images in response to signals received from the input devices. In this way, the user interacts with the display image using the input devices, enabling user interaction with the processor or other device. The functions and process steps herein may be performed automatically or wholly or partially in response to user command. An activity (including a step) performed automatically is performed in response to executable instruction or device operation without user direct initiation of the activity.

The system and processes ofFIGS. 5-12are not exclusive. Other systems, processes and menus may be derived in accordance with the principles of the invention to accomplish the same objectives. Although this invention has been described with reference to particular embodiments, it is to be understood that the embodiments and variations shown and described herein are for illustration purposes only. Modifications to the current design may be implemented by those skilled in the art, without departing from the scope of the invention. The system advantageously performs multi-stage transversal orientation MR Angiography image acquisition of a whole peripheral (e.g., limb) vessel tree by automatically advancing a patient table or by electronically shifting a field of view (FOV). Further, the processes and applications may, in alternative embodiments, be located on one or more (e.g., distributed) processing devices on a network linking the units ofFIG. 5. Any of the functions and steps provided inFIGS. 5-12may be implemented in hardware, software or a combination of both.