The present invention relates to a method of mapping a radiofrequency (RF) magnetic field (B1+) transmitted to a magnetic resonance imaging (MRI) specimen.
MRI has traditionally been used in clinical applications to acquire images of living tissue which distinguish between pathological tissue and normal tissue. MRI is also used in non-clinical applications to detect geological structures, for example in the oil industry.
The most well established MRI techniques are qualitative T1 (longitudinal relaxation time) and T2 (transverse relaxation time) weighted imaging. However, there are many circumstances where it is desirable to use quantitative imaging, that is to determine actual T1 and/or T2 values. Such quantitative imaging is generally hypothesized to provide improved sensitivity to tissue biochemical changes associated with disease pathogenesis.
Various methods exist to measure T1 and T2 values, but such conventional mapping methods suffer from lengthy scan times and poor spatial resolution and so have limited usefulness, for example in a clinical role. There is therefore a need for faster T1 and T2 mapping techniques.
Rapid T1 and T2 mapping is also desirable in non-clinical MRI applications, for example in situations such as underground drilling where it is necessary to situate imaging equipment on mobile structures and acquire images with minimum disturbance to movement of these structures.
Recently, a number of rapid methods have been proposed, which have acquisition times similar to routine clinical scans. Such methods for rapid voxel-wise T1 determination use steady-state imaging methods in which the magnetization is driven into dynamic equilibrium through application of low flip angle (angle of excitation: α), that is generally less than 30 degrees, radio-frequency (RF) pulses separated by short delays times (pulse sequence repetition time (TR) typically between 2 and 10 ms). These methods make it possible to quickly acquire high resolution T1 images. Depending on the specific steady-state sequence employed, the magnetization may be sampled either once equilibrium has been established, or during the transient phase preceding equilibrium, with the transverse magnetization either spoiled prior to each RF pulse with gradient or RF spoiling (or a combination of the two), or fully refocused.
Although these methods permit rapid T1 measurement, the accuracy of the derived T1 estimates depends strongly on correct knowledge of the transmitted flip angle. However, in many circumstances, the spatial homogeneity of the transmitted B1+ RF field cannot be ensured, resulting in the transmitted flip angle varying greatly from the prescribed value throughout the image. This is the case at high field strengths, such as at 3 Tesla (T) where the RF wavelength becomes similar in scale to the imaged object (for example a human head) and the dielectric properties of tissue cause RF shielding. RF inhomogeneity is also encountered (at any field strength) when non-symmetric surface transmit/receive RF coils are employed, such as for extremity (for example knee) imaging. High field scanners, as well as the use of surface coils, are becoming increasingly common in the clinical setting as they provide improved signal-to-noise ratio, allowing for high spatial-resolution imaging. However, even at moderate field strengths, such as 1.5 T, RF inhomogeneity can be problematic in large field-of-view imaging (such as abdominal imaging). In addition to these effects, imperfectly designed RF pulses result in non-uniform flip angle profiles across the two-dimensional (2D) slice or 3D slab, independent of field strength or RF coil. Finally, at all field strengths, a clinical MRI scanner performs an internal calibration at the beginning of every imaging examination, in part to determine the RF power required to transmit a certain flip angle. However, as this calibration is non-specific (i.e. averaged over the whole object) the result represents a global average. Consequently, the RF power requirements may be under- or over estimated in different regions of the object.
While a variety of methods have been proposed to account for, and correct, variations in the transmitted B1+ field, these require lengthy scan times, suffer large-scale geometric distortions, or require high power RF pulses, so are of limited use. Such methods include theoretical modeling of the transmitted field using finite element simulations of the coil and tissue compartments, the use of adiabatic or composite RF pulses which provide more uniform B1+ profiles and mapping the B1+ field from acquired image data.
For example, direct mapping of the transmitted field is appealing as it may be readily incorporated into an imaging experiment (in the form of a set of calibration scans run at the beginning of the session) and does not require a priori knowledge of the tissue and coil geometries or dielectric properties. Direct mapping methods generally involve acquisition of fully-relaxed (TR>>T1) spin-echo (SE) or gradient-echo (GE) images at two or three flip angles (generally either α and 2α, or α, 2α and 3α). From these data, B1+ can be determined via trigonometric relationships of the signal intensity values. However, such methods are slow due to the need to allow the spin system to fully recover between successive RF pulses, which reduces the practicality of B1+ mapping in large volume, three dimensional (3D) applications.
Although the use of echo-planar imaging (EPI) readout trains can alleviate these time concerns, SE-EPI and GE-EPI suffer susceptibility-induced geometric distortions and signal drop-outs, and are sensitive to main field (Bo) inhomogeneities, both of which require additional correction. Further, while these techniques permit compensation for B1+ errors related to dielectric effects, slice and slab profile effects are specific to the RF pulse shape which may vary between the multi-slice 2D SE B1+ correction sequence and the 3D spoiled gradient sequence used for T1 mapping.
An example of a T1 mapping method which suffers from the problems discussed above is Driven Equilibrium Single Pulse Observation of T1 (DESPOT1). (DESPOT1 can also be called variable nutation spoiled gradient recalled echo (SPGR) or the method of variable flip angles). The DESPOT1 method represents one of the most efficient (in terms of signal-to-noise per unit scan time) means of quantifying T1, but because of the problem of sensitivity to incorrect knowledge of the transmitted flip angle, the method has primarily been limited to lower field strengths, generally 1.5 T and below, where patient-specific B1+ variations due to tissue dielectric effects is small. While DESPOT1 has been successfully applied at higher fields, such as at 9.4 T, the fields of view utilized in these applications have been small enough to justify the assumption of a spatially uniform B; field. High field (3 T and above) large-volume (i.e. whole-brain) T1 mapping with DESPOT1, however, have remained a challenge.
In the DESPOT1 T1 mapping method, T1 is derived from a series of spoiled gradient recalled echo (SPGR) images (data sets) acquired over a range of flip angles (a) with constant repetition time (TR). By re-writing the general SPGR signal equation in the linear form Y=mX+b,
T1 and ρ may be readily determined from the slope and intercept of the SSPGR/sin α vs. SSPGR/tan α curve as,
T
1
=−TR/log(m) [2]
and
ρ=b/(1−m). [3]
In the above expressions, E1=exp(−TR/T1), ρ is proportional to the equilibrium longitudinal magnetization (and includes factors such as electronic amplifier gains and receive coil sensitivity effects), and αT is the transmitted flip angle defined by the applied B1+ field.
As T1 is derived directly from the slope of the SSPGR/sin α vs. SSPGR/tan α line, accurate knowledge of the transmitted flip angles is crucial for correct T1 determination. While it is conventionally assumed that the transmitted flip angle is equal to the prescribed value (αT=αP) and is spatially homogeneous throughout the image volume, as discussed above these assumptions are true only in a limited range of applications, such as at lower field strengths or with small fields of view. In fact, the transmitted flip angle is usually related to the prescribed value as αT=καP, where κ denotes the spatially varying B1+ field. Within the context of quantitative imaging, and T1 mapping via the conventional inversion recovery (IR) approach specifically, an approach often used to account for B1+ deviations is to include the flip angle as an additional parameter in the fitting routine. For example, by calculating the three-parameter fit of
S
IR(TI,TR)=ρ[1−βexp(−TI/T1(−TR/T1)], [4]
to multiple inversion time (TI), IR data for ρ, T1 and β, spatial variations in B1+ field are accounted for by the inversion efficiency term, β. Unfortunately, this approach may not always be used directly, for example in the case of DESPOT1, as is demonstrated in
In
A means of mapping the B1+ field is needed which addresses the problems with conventional approaches.
Embodiments of the invention relate to methods of mapping a radio frequency magnetic field transmitted to a magnetic resonance imaging specimen.
In one embodiment, a method comprises the steps of: applying a first radio frequency pulse having a first excitation angle to the specimen and at a first time period after applying the first pulse applying one or more second radio frequency pulses each having a second excitation angle to the specimen, with a second time period between second pulses, to obtain a first data set defining a first sample of an image space; applying one or more third radio frequency pulses each having a third excitation angle to the specimen, with a third time period between third pulses, to obtain a second data set defining a second sample of the image space; applying one or more fourth radio frequency pulses each having a fourth excitation angle to the specimen, with a fourth time period between fourth pulses, to obtain a third data set defining a third sample of the image space; wherein the fourth excitation angle is different to the third excitation angle and/or the fourth time period is different to the third time period; calculating a magnetic field map data from at the three data sets; and outputting the magnetic field map data.
An example of the present invention will now be described with reference to the accompanying drawings, in which:
a is a graph showing that for the conventional DESPOT1 method, for any assumed value of κ (spatial variance of B1+ field) a seemingly linear SSPGR/sin κα vs. SSPGR/tan κα curve can be generated;
b is a graph showing that when SSPGR vs. αT curves are calculated using T1 and ρ values derived from
a is a Pulse Timing Diagram for an example IR-SPGR sequence to acquire a data set for a plane in k-space, half a plane at a time;
b depicts a Pulse Timing Diagram for an example SPGR sequence;
a depicts residuals between predicted and measured IR-SPGR signal intensities as a function of κ;
b is an expanded view of the 0.5≦κ≦1.5 region of
a depicts tri-planar views of a uniform sphere phantom T1 maps without B1+ field correction;
b depicts tri-planar views of a uniform sphere phantom T1 maps with B1+ field correction;
c is a graph of the coronal profiles through the B1+ corrected and uncorrected maps;
d is a graph of the axial profiles through the B1+ corrected and uncorrected maps;
An example will be described in relation to the DESPOT1 T1 mapping approach discussed above.
This example comprises acquiring an additional inversion-prepared spoiled gradient echo (IR-SPGR) image alongside the conventional dual-angle DESPOT1 data. Therefore at least three data sets are acquired: a minimum of one IR-SPGR data set and DESPOT1 data which is two SPGR data sets. From this combined data, κ (the factor accounting for the B1+ field inhomogeneity) is found which means that both B1+ and T1 may be readily determined with high accuracy.
As shown in
b depicts an SPGR sequence which may be used to obtain the second and third data sets.
To eliminate T2 effects, the transverse magnetization is spoiled prior to each RF pulse. As the RF pulse train perturbs the recovery of the longitudinal magnetization, the measured IR-SPGR signal intensity is a complex function of T1, proton density, flip angle and RF pulse number. However, if low angle pulses (generally less than 15 degrees) are used such that their disturbing effect may be assumed to be negligible, the measured IR-SPGR signal can be approximated by the IR signal equation modulated by the sine of the low angle pulse,
S
IR-SPGR=π[1−INVexp(−TI/T1)+exp(−Tr/T1)] sin κα [5]
where INV=1−cos κπ, and Tr is the time between inversion pulses.
As mentioned above, the DESPOT1 T1 mapping method comprises acquiring at least two SPGR data sets, with sets of third and fourth pulses, over a range of flip angles (α) with constant repetition time (TR). By re-writing the general SPGR signal equation in the linear form Y=mX+b,
T1 and ρ may be readily determined from the slope and intercept of the SSPGR/sin α vs. SSPGR/tan α curve as,
T
1
=−TR/log(m) [2]
and
ρ=b/(1−m). [3]
From the combined multi-angle DESPOT1 and IR-SPGR data, a unique solution for κ, T1 and ρ can be found through the process of minimizing the residuals between the predicted and measured IR-SPGR and SPGR signal intensities. To simplify the fitting routine, it is possible to make use of the fact that for any value of κ, T1 and ρ can be determined from the multi-angle DESPOT1 data. The problem, therefore, can essentially be viewed as a single parameter fit for K with residuals calculated only with respect to the IR-SPGR data.
Determination of κ in this manner is demonstrated in
In addition to the global maxima centered at κ=1.00 shown in
In the method according to this example, which may be known as DESPOT1-HIFI, or, DESPOT1 with High-speed Incorporation of RF Field Inhomogeneities, the choice of inversion time may provide optimal T1 estimate accuracy and precision over a range of κ. Assuming nominal values of 1200 ms for T1 and ρ=1 (representing an average T1 of white and grey matter at 3 T, T1 accuracy and precision have been evaluated from combined theoretical DESPOT1-HIFI data comprised of two SPGR images with different flip angles and either one or two IR-SPGR data-sets with differing inversion times. The IR-SPGR data were generated over the TI range from 10 ms to 500 ms, while κ was varied from 0.3 to 1. Additional sequence-specific parameters were: IR-SPGR: αT=κ10° and Tr=192 ms+TI, SPGR: TR=5 ms and αT=κ3° and κ9°.
The results of this show that, to minimize the scan time for a single inversion time, the optimum inversion time is 250 ms. For dual inversion times, the T1 accuracy is maximised for all κ for the TI region between 250 ms and 350 ms. As it is generally desirable to maximize the signal difference between the two IP-SPGR measures, the optimum dual inversion times are 250 ms and 350 ms.
DESPOT1-HIFI data have been acquired for uniform sphere phantoms using the following IR-SPGR and SPGR parameters: IR-SPGR: TE/TR=1 ms/3.1 ms, TI=250 ms, Tr=448 ms, αP=10°, BW=±41.67 kHz, SPGR: TE/TR=1.4 ms/5.1 ms, αP=3° and 9°, BW=±27.7 kHz. FOV and matrix size of the DESPOT1-HIFI data were 25 cm×25 cm×18 cm and 256×256×180, respectively. To minimize the acquisition time, the IR-SPGR data were acquired with half the spatial resolution (in all 3 directions) of the SPGR data and zero-padded to the full resolution prior to Fourier reconstruction. Voxel-wise T1 values were estimated using the DESPOT1-HIFI approach, as well as with the conventional, non-B1+ corrected DESPOT1 method. From the sphere DESPOT1 and DESPOT1-HIFI T1 maps, profiles along all three orthogonal directions were calculated and compared. To evaluate the accuracy of the DESPOT1-HIFI T1 estimates, mean values where determined from regions of interest placed within each tube and compared with the reference FSE-IR values.
Reference T1 values were determined from data acquired using a single-slice, 2D inversion-prepared fast spin-echo (FSE-IR) sequence with the following parameters: 25 cm×25 cm×5 mm field of view (FOV), 128×128×1 matrix, echo time/repetition time (TE/TR)=9 ms/6000 ms, TI=(50, 150, 200, 400, 800, 1600, 3200) ms, bandwidth (BW)=±15.65 kHz and echo train length=2.
a shows T1 maps calculated from the uniform sphere phantom using the DESPOT1 method without B1+ correction and
To assess the in vivo performance of the method, sagittally-oriented whole-brain DESPOT1-HIFI data have been acquired of two healthy volunteers (ages: 24 and 26) with the following parameters: FOV=25 cm×19 cm×18 cm, matrix=256×192×180, IR-SPGR: TE/TR=1 ms/2.8 ms, TI=250 ms, Tr=430 ms. αP=10°, BW=±41.67 kHz, SPGR: TE/TR=1.3 ms/4.8 ms, αP=3° and 9°, BW=±31.3 kHz. Total imaging time for each volunteer was approx. 6.5 minutes, with the IR-SPGR collection requiring just over 1 minute. The IR-SPGR data were acquired with half the spatial resolution of the SPGR data and zero-padded prior to Fourier reconstruction. Reference T1 values for each volunteer were also determined from axially-oriented FSE-IR data acquired during the same scan session. Voxel-wise T1 values were calculated from the DESPOT1-HIFI and FSE-IR data and comparison were made between mean values calculated for frontal white matter, caudate nucleus, putamen, and globus pallidus.
In vivo volunteer results are shown in
This example provides a quick and unencumbered method to account for B1+ field variations in DESPOT1 involving the acquisition of one or more IR-SPGR data-sets in addition to the conventional dual-angle DESPOT1 data. Near perfect correction for flip angle variations is enabled while requiring minimal additional scan time (in the examples shown, less than 1 minute) and without adversely affecting the precision of the T1 estimates. Both the calculated B1+ field map and the corrected T1 map are obtained in a clinically feasible time of less than 10 minutes. More specifically it has been demonstrated that for DESPOT1-HIFI, whole-brain, high spatial resolution (1 mm3 isotropic voxels) combined B1+ and T1 maps are possible with a combined acquisition time of less than 10 minutes. Compared with reference FSE-IR measurements, mean error in the derived DESPOT1-HIFI T1 estimates is less than 7% with high reproducibility.
The B1+ field map obtained can be used to help correct signal inhomogeneities in subsequently acquired data. An example of this is when DESPOT1 is used in combination with DESPOT2 (Driven Equilibrium Single Pulse Observation of T2) for combined T1 and T2 mapping. In DESPOT2, T2 is determined from a series of fully-balanced steady-state free precession images acquired with constant TR and incremented flip angle. As with DESPOT1, accurate T2 determination with DESPOT2 relies on correct knowledge of αT. In this instance, the B1+ field map calculated with DESPOT1-HIFI may be directly used to determine the transmitted DESPOT2 flip angles.
The example method may be used solely to obtain the B1+ field map without using the T1 data also obtained in the process. If this is the case the resolution need not be as high as when the T1 data is also required. In both cases, the resolution required depends on the intrinsic B1 field variation. While the example method above calculates B1+ field map data by minimizing the residuals between predicted and measured IR-SPGR and SPGR signal intensities, alternative calculation methods may be used such as calculating the B1+ field map data from the at least three data sets acquired by performing a multi-parameter fit for all values for all of the data. The output B1+ field map data may be used to dynamically generate further RF pulses to minimise variation in B1+ field.
The example method may usually be performed with the underlying assumption that the spatial variations in the inversion pulse of IR-SPGR sequence are proportional to the variations in the lower angle pulses. In the example discussed above, similarly designed SLR RF pulses were employed for the inversion and low angle pulses, such that χ=κ, but the present invention is not limited to this case.
In cases where an adiabatic or composite inversion pulse is used, the assumption is not true and the deviations in the flip angle magnitudes become independent, i.e.,
S
IR-SPGR=ρ[1−(1−cos χπ)exp(−TI/T1)+exp(−Tr/T1)] sin κα [6]
where χ denotes the spatial variation in the inversion pulse, and χ≠κ. Under these conditions, it may be necessary to determine κ and χ independently. This process may be simplified in the case of a well-designed adiabatic pulse in which χ may be assumed to be approximately 1.00.
While the example described above uses a 180 degree inversion pulse and SPGR signals, the invention is not restricted to these examples. A first RF pulse with a flip angle of 90 degrees or above may be used, including an angle greater than 360 degrees. Although the optimum flip angle for the second RF pulses which are part of the IR-SPGR signal is less than 30 degrees, angles, for example, less than 100 degrees may be used. For the third and fourth RF pulses which are part of the DESPOT1 SPGR signals, flip angles of any angle may be used.
The example method can be used with any T1 weighted imaging protocol and does not have to comprise DESPOT1. The at least three data sets do not have to be acquired by IR-SPGR and two SPGR but may be acquired by other techniques known to the skilled person. Other techniques include Progressive Saturation, Look-Locker, accelerated Look-Locker, TOMROP, FLASH, inversion-prepared FLASH, snapshot FLASH (FLASH can also be called spoiled FLASH), inversion-prepared fully-balanced steady-state free precession (SSFP or TrueFISP or FISP or PSIF or FIESTA or FFSE), inversion recovery (inversion recovery echo planar imaging), saturation recovery (saturation recovery echo planar imaging). Such techniques have many different names and the present invention is not limited to any particular subset of these. The present invention is not limited to clinical techniques and can also be used with, for example, geophysical techniques.
It is not essential that the transverse magnetisation is spoiled and if the transverse magnetization is spoiled this does not have to be with a gradient magnetic field. Alternatively the transverse magnetisation may be spoiled by varying the phase of the subsequent RF pulse applied. In the above example, each data set is acquired with a different flip angle, but alternatively, the flip angle may remain constant and instead the repetition time may be varied. In the above example data sets are acquired directly defining samples in k-space, that is, directly giving the Fourier transform of the image, but any appropriate image space may be used. The samples in the image space may be defined by directly acquiring image data in a point by point fashion. Any method of filling the image space may be used, such as Cartesian filling for example by acquiring alternating lines in a linear fashion, or spiral filling starting from the center and spiraling outwards. Lines, planes or volumes in k-space may be acquired.
One example of data set acquisition which differs from the DESPOT1 example is acquisition using one second pulse following a first inversion pulse, in the form of, for example, an echo-planar readout, to acquire the whole of a k-space place plane at once. This is in contrast to the multi-shot approach described above. Second and third data sets may also each be acquired using one pulse, such as in the form of an echo-planar or spiral readout approaches as known by the skilled person. An echo-planar approach means that any flip angle may be used.
Although only three data sets are necessary, further data sets may be acquired. For the example of IR-SPGR+2 SPGR, further IR-SPGR data sets may be acquired with at least one of the following altered: flip angle for the first preparatory pulse, the time delay following the first pulse before the train of second pulses is applied, the time between the second pulses (repetition time) and the flip angle of the second pulses. Similarly, the number of second and third SPGR data sets acquired may be increased from two, varying at least one of the pulse repetition time and the flip angle.
While this example accounts for B1+ field effects, variations in the B1 receive field (BD can also cause signal intensity modulations throughout the image. Unlike B1+ effects, however, variations in B1− can be incorporated into the ρ term and therefore do not result in deviations of the derived T1 estimates. For applications where accurate proton density estimates are desired, these effects will require an addition correction, usually accomplished by the acquisition of two low spatial resolution images using a large homogeneous body coil and, in neuroimaging applications, a head coil.
The present invention enables a rapid approach for B1+ field mapping, which may be incorporated into a rapid approach for combined B1+ field and T1 mapping. This allows the highly efficient T1 mapping methods to be performed at high field strengths, such as 3 T and above, or with small non-symmetric surface RF coils.
Number | Date | Country | Kind |
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0619269.4 | Sep 2006 | GB | national |
The present application is a National Phase entry of PCT Application No. PCT/GB2007/003665, filed Sep. 26, 2007, which claims priority from Great Britain Application Number 0619269.4, filed Sep. 29, 2006, the disclosures of which are hereby incorporated by reference herein in their entirety.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB07/03665 | 9/26/2007 | WO | 00 | 10/20/2010 |