System and method for correcting effects of temperature on magnetic resonance

Abstract

A system and method is disclosed that relates to magnetic resonance (MR) systems in the field of medical imaging, and particularly the evaluation of MR parameters in phantoms in such systems. The system and method are suitable for measuring the temperature in phantoms with MR, calibrating MR parameters as a function of temperature, and correcting the MR parameters for temperature variations. The system and method correct the effect of temperature on one or more MR parameters and render a process of scanning the phantom more commercially deployable.

Description

[0001] System and Method for Correcting Effects of Temperature on Magnetic Resonance Parameters

[0002] Field of the Invention

[0003] This invention relates to magnetic resonance (MR) systems in the field of medical imaging, and particularly the evaluation of MR parameters in phantoms in such systems.

[0004] Phantoms are objects with known composition that are scanned to evaluate the performance of MR scanners and pulse sequences. With MR pulse sequences, various MR parameters can be measured, including T1 and T2 relaxation times. If the MR parameter measured in a phantom with a given pulse sequence is reproducible, then one can conclude that the MR scanner and pulse sequence are performing well. Hence, one can be confident that the MR scanner and pulse sequence will perform well when used for scanning human subjects. This is particularly helpful for medical applications, where phantoms are routinely scanned for evaluating the performance of MR pulse sequences developed commercially.

[0005] Unlike human subjects, who maintain a consistent body temperature via homeostasis, the temperature of phantoms is dependent on the external environment and is more challenging to regulate. Temperature fluctuations in phantoms increase the variability of measurements, including T1 and T2, decreasing the reproducibility. This can cause a pulse sequence to be deemed to be not performing well on a particular MR scanner, when the variation is caused by the phantom itself.

[0006] It would be useful to determine an accurate estimate of the relaxation parameters of a phantom at a reference temperature (i.e. 21.1 °C) even if the phantom data are acquired at a different temperature. This accurate estimate would be used in testing and quality assurance of MR scanners to ensure their measurement performance meets our specifications. For some measurement tasks, tight specifications are necessary to deploy our quantitative methods consistently, and consequently to use the same human reference ranges across multiple scanners such that those scanners perform equivalently. In practice, this would reduce unnecessary time and cost associated with repeating quality assurance scans due to the phantom not being at the reference temperature (i.e. 21.1 °C).

[0007] Ethylene glycol (ethane- 1,2-diol (CEEOH^) is a chemical that can be used in phantoms. Previous studies have shown that the temperature of ethylene glycol can be measured in an MR system via measurement of the chemical shift between the CEE and OH groups (Van Geet et al. 1968, Ammann et al. 1982). Notably, the temperature of ethylene glycol can be calculated from the resonance frequency and chemical shift. In particular, the frequency offset A between the OH and CH2 peaks of ethylene glycol is given by Eq. 1 :

[0008] / o(466.5 - T) 102.0 x 106where f0is the resonance frequency (Hz) and T is the temperature (kelvin) (Ammann et al. 1982).

[0009] It has also been shown that multi-echo gradient echo sequences can be used for measuring the temperature of ethylene glycol (Sprinkhuizen et al. 2010, McDaniel et al. 2017, Odeen et al. 2022).

[0010] Calibrations of MR parameters, including T1 and T2, as a function of temperature have been performed in MR scanners. One work involves the temperature measurement of ethylene glycol with a diffusion-weighted sequence (Spees et al. 2012). In other prior art, temperature measurements were acquired with an external device instead of MR (Hardy et al. 2020).

[0011] Following temperature calibration of MR parameters, temperature correction has been performed in phantoms for T1 (Vassiliou et al. 2016, Captur et al. 2020), and both T1 and T2 (Zhang et al. 2021, Patent W02020234570A1, Statton et al. 2021). Additionally, temperature calibration of both T1 and T2 (Captur et al. 2016, Captur et al. 2020, Topriceanu et al. 2023) has been performed without temperature correction.

[0012] The prior art does not combine the features of measuring the temperature of a temperature reference vial with MR, calibrating MR parameters as a function of the temperature of the temperature reference material , and correcting for the MR parameters based on the temperature. of the Invention

[0013] In an embodiment of the system and method there is provided a method for correcting the effect of temperature on one or more MR parameters comprising the steps of: providing an MR phantom with a plurality of vials, comprising at least one temperature reference vial, and at least one MR parameter vial, where the temperature reference vial and the MR parameter vial are thermally coupled; stabilising the temperature of the MR phantom and measuring the stabilised temperature of the temperature reference vial using an MR pulse sequence at multiple echo times; determining one or more MR parameters of the MR parameter vial at the measured temperature using an MR pulse sequence; adjusting the temperature of the MR phantom and repeating the stabilising and determining steps one or more times to obtain the measured temperature reference vial temperature and the determined MR parameters for each adjusted temperature; determining one or more temperature coefficients of the at least one MR parameter as a function of the measured temperature of the temperature reference vial, for each adjusted temperature; calibrating the one or more temperature coefficients of the at least one MR parameter as a function of the at least one MR parameter at the measured temperature to generate one or more calibration coefficients for the MR parameter; adjusting the at least one MR parameter to a reference temperature using the one or more calibration coefficients and the measured temperature.

[0014] Preferably, the temperature reference vial is a vial of Ethylene Glycol.

[0015] In a preferred embodiment of the system and method, the MR pulse sequence for determining the one or more MR parameters is performed at multiple echo times.

[0016] Further preferably, the MR pulse sequence for determining the one or more MR parameters is performed at multiple inversion times.

[0017] In a preferred embodiment of the system and method the calibration of the at least one MR parameter as a function of the verified temperature of the temperature reference vial is a linear calibration.

[0018] Preferably, the reference temperature is between 15-25°C. Further preferably, the reference temperature is between 20-22°C.

[0019] In a preferred embodiment of the system and method, the temperature of the MR phantom is adjusted between 0-40°C.

[0020] Preferably, the MR phantom further comprises a flood solution to thermally couple the temperature reference vial and the MR parameter vial. Further preferably, the flood solution is a water based flood solution. In a further preferred embodiment of the system and method, the flood solution is a solution of at least one of Nickel Chloride and Sodium Chloride.

[0021] Preferably, the at least one MR parameter vial comprises agar gel and Nickel Chloride. Further preferably, the at least one MR parameter vial comprises a plurality of vials with different concentrations of agar gel and Nickel Chloride.

[0022] In an alternative embodiment of the system and method the at least one MR parameter vial comprises biological material. Preferably, the temperature reference vial and the MR parameter vial are formed of the same material. Preferably, the vials are formed of polypropylene.

[0023] In a preferred embodiment of the system and method, the one or more MR parameters comprise at least one of: Tl, T2. Further preferably, the one or more MR parameters further comprise at least one of: PDFF, T2*.

[0024] Preferably, the MR pulse sequence is at least one of: 2D multi-echo spoiled FLASH, shMOLLI, T2 prepared TurboFLASH, Multi-echo Fast-spin-echo.

[0025] In a preferred embodiment of the system and method the MR pulse sequence is a gradient echo pulse sequence or a spoiled gradient echo pulse sequence.

[0026] Further preferably, the measured temperature of the Ethylene Glycol vial is calculated based on the resonant frequency and a frequency offset between the OH and CH2 peaks of the Ethylene Glycol.

[0027] In a further embodiment of the system and method there is also provided a Computer program capable of execution by a computer apparatus and configured to: measure the temperature of a temperature reference vial in a MR phantom with a plurality of vials, comprising at least one temperature reference vial and at least one MR parameter vial, where the temperature reference vial and the MR parameter vial are thermally coupled, where the temperature is determined using an MR pulse sequence at multiple echo times; determine one or more MR parameters of the MR parameter vial at the measured temperature using an MR pulse sequence; repeat the measuring and determining steps one or more times to obtain the measured temperature reference vial temperature and the determined MR parameters , after the temperature of the MR phantom has been adjusted, for each adjusted temperature; determine one or more temperature coefficients of the at least one MR parameter as a function of the measured temperature of the temperature reference vial, for each adjusted temperature; calibrate the one or more temperature coefficients of the at least one MR parameter as a function of the at least one MR parameter at the measured temperature to generate one or more calibration coefficients for the MR parameter; adjust the at least one MR parameter to a reference temperature using the one or more calibration coefficients and the measured temperature.

[0028] In a yet further embodiment of the system and method there is also provided a system for performing one or more MR scans, comprising an MR scanner, and a processor configured to control the MR scanner and execute the following steps: measure the temperature of a temperature reference vial in a MR phantom with a plurality of vials, comprising at least one temperature reference vial and at least one MR parameter vial, where the temperature reference vial and the MR parameter vial are thermally coupled, where the temperature is determined using an MR pulse sequence at multiple echo times; determine one or more MR parameters of the MR parameter vial at the measured temperature using an MR pulse sequence; repeat the measuring and determining steps one or more times to obtain the measured temperature reference vial temperature and the determined MR parameters , after the temperature of the MR phantom has been adjusted, for each adjusted temperature; determine one or more temperature coefficients of the at least one MR parameter as a function of the measured temperature of the temperature reference vial, for each adjusted temperature; calibrate the one or more temperature coefficients of the at least one MR parameter as a function of the at least one MR parameter at the measured temperature to generate one or more calibration coefficients for the MR parameter; adjust the at least one MR parameter to a reference temperature using the one or more calibration coefficients and the measured temperature.

[0029] Preferably, the MR scanner operates at 3T.

[0030] In a further embodiment of the system and method there is provided a phantom for use with an MR scanner comprising a vial of temperature reference material and at least one MR parameter vial of material for measuring an MR parameter. Preferably, the temperature reference material is Ethylene Glycol. Further preferably, wherein the at least one MR parameter vial comprises agar gel and Nickel Chloride. In a preferred embodiment of the system and method the at least one MR parameter vial comprises a plurality of vials with different concentrations of agar gel and Nickel Chloride. Preferably, the at least one MR parameter vial is thermally coupled to the at least one temperature reference vial, via a water based flood solution.

[0031] By placing a temperature reference vial inside a phantom and measuring the temperature of the material within the temperature reference vial with MR, no external device such as a temperature probe is needed to measure temperature. This is advantageous to minimise the amount of equipment needed and render the process of scanning the phantom more commercially deployable.

[0032] The calibration of MR parameters as a function of temperature allows for the characterisation of the temperature effects in a controlled experiment, and hence the development of a generalised temperature correction. By applying a retrospective temperature correction on the MR parameters, one can scan the phantom without the need to control for the temperature, which simplifies the process of scanning the phantom and avoids the complications associated with controlling the scanning environment.

[0033] Thus, the following problem(s) has (have) been resolved, by the present system and method.

[0034] The current system and method presents a system and method for measuring the temperature in phantoms with MR, calibrating MR parameters as a function of temperature, and correcting the MR parameters for temperature variations. The temperature of a temperature reference vial within the phantom is measured with an MR system, and hence no external device such as a temperature probe is necessary. Moreover, the temperature correction relies on a calibration that is generalisable for phantoms with varying concentrations of material resulting in varying MR parameters. The invention will now be described, by way of example only, with reference to the accompanying figures in which:

[0035] Figure 1 is shows a schematic of an MR system used in an embodiment of the method of this invention;

[0036] Figure 2(a) is a side view of an MR phantom as used in an embodiment of this invention;

[0037] Figure 2(b) is a top view of an MR phantom as used in an embodiment of this invention;

[0038] Figure 3 shows a flow chart of some of the steps of the method of this invention;

[0039] Figure 4 shows a further flow chart of the steps for temperature measurement in an embodiment of the method of this invention;

[0040] Figure 5 shows a further flow chart of the steps for temperature calibration in an embodiment of the method of this invention;

[0041] Figure 6 shows a further flow chart of the steps for temperature coefficient calibration and temperature correction in an embodiment of the method of this invention;

[0042] Figure 7 shows examples of MR pulse sequence parameters as used in different embodiments of the invention;

[0043] Figure 8 shows example MR images showing the temperature reference vial in the MR phantom, in an embodiment of the invention ;

[0044] Figure 9 shows an example of the simulation of the ethylene glycol signal ; Figure 10 shows examples of simulated ethylene glycol signal parameters as used in the example of figure 9 ;

[0045] Figure 11 shows an example of correlation between simulated and acquired signals for the ethylene glycol temperature measurement;

[0046] Figure 12 shows an example graph used for temperature calibration for the T1 MR parameter in an embodiment of the invention;

[0047] Figure 13(a) shows an example graph used for temperature coefficient calibration for T1 MR parameter;

[0048] Figure 13(b) shows example parameters determined from the graph of figure 13(a);

[0049] Figure 14(a) and 14(b) shows an example of the temperature correction for T1 MR parameter;

[0050] Figure 15 shows an example of the temperature calibration for T2 MR parameter;

[0051] Figure 16(a) shows an example graph for temperature coefficient calibration for T2 MR parameter;

[0052] Figure 16(b) shows example parameters determined from the graph of figure 16(a);

[0053] Figures 17(a) and (b) shows an example of the temperature correction for T2 MR parameter; Figure 18 shows an example pulse sequence diagram for a 2D multi-echo spoiled gradient echo pulse sequence;

[0054] Figure 19 shows an example pulse sequence for a T2 prepared TurboFLASH pulse sequence;

[0055] Figure 20 shows an example TurboFLASH readout in a T2 prepared TurboFLASH pulse sequence;

[0056] Figure 21 shows example RF pulses for a shMOLLI pulse sequence;

[0057] Figure 22 is an example of a magnified view of TrueFISP readout for a shMOLLI pulse sequence.

[0058] In the figures, like reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.

[0059] Detailed

[0060] The figures illustrate an example of a system, method and computer program for controlling a system for correcting the effect of temperature on one or more magnetic resonance parameters, and an MR phantom for use with such a system and method.

[0061] We have developed a system and method for correcting variations in one or more MR parameters due to temperature in MR phantoms. In the present embodiment, T1 and T2 are the MR parameters that are corrected, though the system and method can be extended to other MR parameters, such as PDFF or T2*. As shown in figure 1, the system includes an MR scanner 102 and a phantom 104. In an embodiment of the system and method, the scanner is a 3T MR scanner, but scanners with other field strengths may alternatively be used. A preferred embodiment of the system and method also includes a phantom for use with an MR scanner comprising a vial of temperature reference material and at least one MR parameter vial of material for measuring an MR parameter .The phantom is defined as an ensemble containing at least one temperature reference vial for measuring the temperature with MR, and at least one vial for characterising MR parameters; all the separate vials in the phantom are thermally coupled to one another, via a flood solution. Preferably, the temperature reference material is Ethylene Glycol. Further preferably, the at least one MR parameter vial comprises agar gel and Nickel Chloride. In a further preferred embodiment of the system and method, the at least one MR parameter vial comprises agar gel and Nickel Chloride. Further preferably, the at least one MR parameter vial comprises a plurality of vials with different concentrations of agar gel and Nickel Chloride.

[0062] Figures 2(a) and (b) show different views of the phantom 104. Preferably, the phantom is a substantially spherical shell 134, but other shapes may also be provided. Figure 2(a) shows a side view of phantom 104 and figure 2(b) shows a top view of phantom 104. As shown, the phantom comprises at least vial 110 for MR parameter measurement, and vial 120, containing a reference material for measurement of the temperature of the phantom. Preferably, the temperature of the MR phantom is adjusted between 0-40°C.The vials are held in position by a vial holder 130 at the base of the phantom, and a vial holder 132 that secures the tops of the vials. Preferably, the reference material for temperature measurement is ethylene glycol, for measurement of the temperature within the phantom. In a preferred embodiment of the system and method, the vial 110 for MR parameter measurement is filled with agar gel and Nickel Chloride. In a further preferred embodiment of the system and method, a plurality of vials 110 are filled with different concentrations of agar gel and Nickel Chloride. Alternatively, the MR parameter vial may be biologically derived material, derived from living tissue for example. In the present embodiment of the system and method, the phantom 104 is filled with a large volume of water-based flood solution 150 which is thermally stable and ensures an even temperature distribution to the vials, so that the vials are all thermally coupled. A temperature reference vial 120 preferably filled with ethylene glycol is used for measuring the temperature of the phantom with MR. In an embodiment of the system and method, the flood solution 150 is preferably doped with at least one of Nickel Chloride and Sodium Chloride. Preferably, all the vial containers 110, 120 are made of the same material to ensure equivalent heat transfer. Preferably the vials are all made of polypropylene. The time constants for heat exchange between the flood solution 150 and the vials 110, 120 has been determined and is very short compared to the time constant of the entire phantom 104.

[0063] If the phantom 104 is used for the temperature calibration, a heating source 106 (e.g. a microwave oven, warm water bath etc.) or a cooling source (e.g. a refrigerator, ice bath etc.) may be used for varying the temperature of the phantom 104.

[0064] In an embodiment of the system and method, the method utilises the following elements:

[0065] (1) Measuring temperature in an ethylene glycol reference vial 120 with a gradient-echo MR pulse sequence at multiple echoes 400;

[0066] (2) Calibrating one or more MR parameters (e.g. T1 and T2) 500 as a function of the temperature measured from ethylene glycol;

[0067] (3) Calibrating the temperature coefficient (slope) as a function of MR parameters, to obtain a calibration that is generalisable for phantoms 104 with the same materials at different concentrations;

[0068] (4) Correcting for the effect of temperature on MR parameters based on the temperature coefficient calibration from (3) Figure 3 shows a flow diagram of the steps in an example embodiment of the method 300 of this system and method. In a preferred embodiment of the system and method, the method allows for correcting the effect of temperature on one or more MR parameters. Step 302 requires the measurement of temperature in vial 120 of temperature reference material, preferably ethylene glycol, with a gradient-echo MR pulse sequence. Preferably, the method includes providing an MR phantom with a plurality of vials, comprising at least one temperature reference vial, and at least one MR parameter vial, where the temperature reference vial and the MR parameter vial are thermally coupled; stabilising the temperature of the MR phantom and measuring the stabilised temperature of the temperature reference vial using an MR pulse sequence at multiple echo times; This is followed by step 304 the calibration of an MR parameter as a function of the temperature measured from the ethylene glycol in step 302. At step 306 the temperature coefficient is calibrated as a function of the MR parameter from step 304. Finally, in step 308 a correction for the effect of temperature on the MR parameter is performed. Preferably, the method includes determining one or more MR parameters of the MR parameter vial at the measured temperature using an MR pulse sequence; adjusting the temperature of the MR phantom and repeating the stabilising and determining steps one or more times to obtain the measured temperature reference vial temperature and the determined MR parameters for each adjusted temperature; determining one or more temperature coefficients of the at least one MR parameter as a function of the measured temperature of the temperature reference vial, for each adjusted temperature; calibrating the one or more temperature coefficients of the at least one MR parameter as a function of the at least one MR parameter at the measured temperature to generate one or more calibration coefficients for the MR parameter; and adjusting the at least one MR parameter to a reference temperature using the one or more calibration coefficients and the measured temperature.

[0069] Each of the steps in the method 300 is determined in more detail below: (1) Figure 4 shows the steps required for measuring temperature in a temperature reference vial with a gradient-echo MR pulse sequence with multiple echoes 400.

[0070] The overall method 400 start at step 402 with the vial 120 of temperature reference material, preferably, ethylene glycol placed in the phantom 104. The phantom contains one or more other vials 110 that contain material for which the MR parameters are to be characterised. As mentioned above, in a preferred embodiment of the system and method, the vial 110 for MR parameter measurement is filled with agar gel and Nickel Chloride. Preferably, the at least one MR parameter vial comprises agar gel and Nickel Chloride. Further preferably, at least one MR parameter vial comprises a plurality of vials with different concentrations of agar gel and Nickel Chloride. In a further preferred embodiment of the system and method, a plurality of vials 110 are filled with different concentrations of agar gel and Nickel Chloride. In a preferred embodiment of the system and method the concentration of Nickel Chloride varies between OmM to 2mM and the concentration of agar gel is constant at 3% g / g Agar, to obtain a plurality of vials 110 that vary in Tl. In a further preferred embodiment, the concentration of the agar gel varies between 0.7%-5% g / g Agar and the concentration of Nickel Chloride is constant at 0.013% g / g, to obtain a plurality of vials 110 that vary in T2. The phantom 104 is allowed to come to thermal equilibrium so the temperature is stabilised over the entire phantom. Typically, the phantom will rest for approximately 10-30 mins, to allow the phantom to reach thermal equilibrium.

[0071] At step 404 MR images of the phantom 104 are acquired with a gradient-echo pulse sequence at a plurality of echo times. Example MR images acquired at this stage are shown in figures 8(a) and (b). In a preferred embodiment of the system and method, one image of the phantom is acquired at each echo time with a 2D multi-echo spoiled gradient-echo sequence, for a total of twelve images. An example of this sequence is the 2D multi-echo spoiled Fast Low Angle Shot (2D multi-echo spoiled FLASH) sequence implemented on a Siemens 3T Prisma MR scanner, shown in figure 18, with parameters as detailed in figure 7. A radiofrequency (RF) pulse is deployed at a flip angle of 4°. At the same time, the trapezoidal gradients in the z-direction (Gz) are deployed for slice selection, for a total of 3 slices with a slice thickness of 8 mm. In the y-direction, the Gygradients change in amplitude for each phase encoding line, for a total of 116 phase encoding lines. The first echo with an echo time (TE) of 1.1 ms is formed when the area under the positive Gxgradient lobe is equal to the area under the negative Gxgradient lobe. Subsequent echoes are formed similarly, for a total of 12 echoes, separated by an echo spacing of 1.1 ms. The 12 echoes are followed by spoiling in the RF and gradient axes, then the cycle is resumed with the next RF pulse. The time between two RF pulses is the repetition time (TR) of 15 ms. Of the 3 slices acquired, only the second slice is analysed. There are 12 images of the second slice, with one image at each TE. In each image, a region of interest (ROI) is drawn over the area representing the temperature reference vial 120. The ROI is formed of pixels with signal intensities.

[0072] In a preferred embodiment of the system and method, the MR pulse sequence is at least one of: 2D multi-echo spoiled FLASH, shMOLLI, T2 prepared TurboFLASH, Multi-echo Fast-spin-echo. Further preferably, the MR pulse sequence is a gradient echo pulse sequence or a spoiled gradient echo pulse sequence. In the present embodiment, a 2D multi-echo spoiled gradient-echo (2D multi-echo spoiled Fast Low Angle Shot (FLASH)) pulse sequence on a Siemens 3T scanner is used, acquiring 12 echoes. Figure 8(a) shows the MR image 802 acquired at the first echo time of 1.1 ms. Figure 8(b) shows the MR image 804 acquired at the 12th echo time of 13.2 ms.

[0073] At step 406 a region of interest (ROI) 810, 812 representing the temperature reference vial 120 in the phantom images is drawn. For each echo time, a region of interest is drawn on the MR image over the region representing the temperature reference vial 120, and the mean signal within the region of interest is determined for each echo time. Figure 8(c) shows the region of interest for the ethylene glycol vial 120 from the MR acquisition of figure 8(a). Figure 8(d) shows the region of interest for the ethylene glycol vial 120 from the MR acquisition of figure 8(b). The regions of interest in figures 8(c) and 8(d) are processed to determine the mean signal in the temperature reference vial 120 for all 12 echo times. The mean signal intensity is taken over all pixels in the ROI, and this is repeated for each of the twelve ROIs. This amounts to a vector of twelve mean signal intensities for the temperature reference vial 120. The vector of mean signal intensities at all echo times is normalized to obtain the normalized acquired signal in ethylene glycol. The normalized acquired signal A is normalised as a unit vector, as follows: where there are 12 components for the 12 echoes.

[0074] At step 408 the normalised mean signal at all echo times is compared against a dictionary of simulated ethylene glycol signals at varying temperatures. Preferably, the measured temperature of the Ethylene Glycol vial is calculated based on the resonant frequency and a frequency offset between the OH and CH2 peaks of the Ethylene Glycol. Notably, the resonant frequency shift A / between the OH and CH2 peaks of ethylene glycol is given by Eq. 1 :

[0075] / o(466.5 - T) 102.0 x 106where fQis the resonance frequency (Hz) and T is the temperature (kelvin) (Ammann et al. 1982). The simulated signals S are constructed, via Eq. 2:

[0076] TE TE

[0077] 5 = A0HeT2OH + C ■ ACH2el2n& TEeT2CH2 where A0Hand ACH2are the amplitudes of the OH and CH2 peaks, TE is the echo time, and C is the CH2 amplitude due to differences in T2 or T1 weighting of the two peaks.

[0078] Figure 9 shows the simulated ethylene glycol signal, S, is generated at a range of input temperatures T. The temperature affects the resonant frequency shift A / between the OH and CH2 peaks of ethylene glycol. The resonant frequency shift is one of the variables in the simulated signal equation, along with the T2’s of OH and CH2 (T20H, T2CH2the signal amplitudes of OH and CH2 G40H, -dCH2), and an additional signal amplitude factor for CH2 (C).

[0079] From Eq. 1 and Eq. 2, T, C, T20H, T2CH2are simulated over a range of values, generating a dictionary of simulated signals for a range of temperatures. Figure 10 shows the parameters that are used for the simulated ethylene glycol signals. The simulated signals S are normalised as a unit vector, as follows: where there are 12 components for the 12 echoes.

[0080] The normalised dot product (NDP) is found between the normalised simulated signals and the normalised acquired signal, as a measure of correlation:

[0081] NDP = ||S|| ■ ||71||

[0082] The NDP is calculated for the dictionary of ||S|| generated from varying input parameters specified in figure 10. Subsequently, the maximum NDP over the dictionary of US’ || is found, and the associated || || and input temperature are noted.

[0083] Finally, at step 410 the temperature of the normalised simulated signal for ethylene glycol with the highest correlation (maximum NDP) to the normalised acquired signal is outputted. Figure 11 shows the NDP between the normalised simulated and acquired signals for the ethylene glycol temperature measurement. The graph shows the echo time in ms, against the normalised signal. The normalised acquired ethylene glycol signal (figure 8) is compared against the normalised simulated ethylene glycol signals (determined as shown in figure 9). Out of the generated dictionary of normalised simulated signals, the normalised simulated ethylene glycol signal displayed in figure 11 correlates the most highly with the normalised acquired ethylene glycol signal, with an NDP of 0.9998. The temperature used as an input in the normalised simulated ethylene glycol signal is noted as the temperature of ethylene glycol in vial 120. In the example shown in figure 11, the temperature is 20.4°C.

[0084] In this method, and the steps as outlined above, the temperature of temperature reference vial 120, preferably ethylene glycol is measured as a proxy to the temperature of the entire phantom 104, including the one or more MR parameter vials 110, assuming that the temperature reference vial 120 is at the same temperature as the rest of the phantom 104.

[0085] Figure 7 shows the parameters of the different MR pulse sequences that may be used in an embodiment of the system and method, for imaging the temperature reference vial 120 , as well as for determining the MR parameters of the one or more MR parameter vials 110. For measuring the MR parameter Tl, a T1 mapping sequence called Shortened Modified Look-Locker Inversion Recovery (shMOLLI) on a Siemens 3T MR scanner is used, as shown in Figures 21 and 22. Other manufacturers and field strengths may also be used in alternative embodiments of the system and method. Figure 21 demonstrates that the pulse sequence is cardiac triggered from a simulated electrocardiogram (ECG) signal with an RR interval of 1000 ms. The acquisition scheme follows a 5(3)3 pattern, meaning that the acquisition occurs for 5 heartbeats, there is a rest for 3 heartbeats, and the acquisition occurs for another 3 heartbeats. The acquisition involves a 180° inversion pulse, followed by a train of 35° excitation pulses; in the 5 heartbeat section, there are five 35° excitation pulses and in the 3 heartbeat section, there are three 35° excitation pulses. The inversion time (TI) is the time between the inversion pulse and the excitation pulse; the first inversion time for the 5 heartbeat section is 100 ms, and the first inversion time for the 3 heartbeat section is 180 ms. Further inversion times are incremented by 1000 ms, which is the RR interval. Within each 35° excitation pulse is a collection of 72 alternating excitation pulses, where 72 is the number of segments. These excitation pulses are alternating in that a 35° excitation pulse is followed by a -35° excitation pulse. The time between two of these alternating pulses is the small TR. The big TR of 280.56 ms is the time for the collection of alternating excitation pulses. Figure 22 is a magnified view of the collection of alternating excitation pulses, with a TrueFISP readout. The areas under the gradients are balanced in terms of positive and negative polarities. The echo time (TE) is the time from the excitation pulse to the echo. After each excitation pulse, the area under the Gygradients is changed to sample a different segment of k-space, for a total of 72 segments.

[0086] For measuring the MR parameter T2, a T2-prepared ultrafast spoiled gradient-echo sequence on a Siemens 3T MR scanner is used, namely called a T2-prepared Turbo Fast Low Angle Shot (TurboFLASH), shown in figure 19. This pulse sequence is characterised by a T2 preparation section with a collection of Malcolm Levitt pulses. The pulses include a 90° pulse, at least one 180° pulse depending on the T2 preparation duration, and a -90° pulse. After spoiling, the TurboFLASH readout begins, which is a train of 58 single-echo FLASH readouts, for 58 segments. The TurboFLASH readout is shown in more detail in figure 20 . The small TR is measured between two successive RF pulses in the train; the big TR is measured between the first and last excitation pulse for the entire TurboFLASH readout. The T2 preparation duration is changed to 0, 30 and 55 ms to obtain different T2 weightings.

[0087] (2) Calibrating at least one MR parameter (e.g. T1 and T2) as a function of the temperature measured from the temperature reference vial.

[0088] Figure 5 shows the steps of method 500, required for the temperature calibration. Using the ethylene glycol temperature measurement as determined using the method of figure 4, the MR parameter for the MR parameter vial 110 in the phantom 104 is calibrated as a function of the temperature.

[0089] The method 500 comprises the following steps At step 502 the phantom 104 is prepared with at least one temperature reference vial 120, and at least one MR

Claims

Claims:

1. A method for correcting the effect of temperature on one or more MR parameters comprising the steps of: providing an MR phantom with a plurality of vials, comprising at least one temperature reference vial, and at least one MR parameter vial, where the temperature reference vial and the MR parameter vial are thermally coupled; stabilising the temperature of the MR phantom and measuring the stabilised temperature of the temperature reference vial using an MR pulse sequence at multiple echo times; determining one or more MR parameters of the MR parameter vial at the measured temperature using an MR pulse sequence; adjusting the temperature of the MR phantom and repeating the stabilising and determining steps one or more times to obtain the measured temperature reference vial temperature and the determined MR parameters for each adjusted temperature; determining one or more temperature coefficients of the at least one MR parameter as a function of the measured temperature of the temperature reference vial, for each adjusted temperature; calibrating the one or more temperature coefficients of the at least one MR parameter as a function of the at least one MR parameter at the measured temperature to generate one or more calibration coefficients for the MR parameter; and adjusting the at least one MR parameter to a reference temperature using the one or more calibration coefficients and the measured temperature.

2. A method as claimed in claim 1 wherein the temperature reference vial is a vial of Ethylene Glycol.

3. A method as claimed in claim 1 or claim 2 wherein the MR pulse sequence for determining the one or more MR parameters is performed at multiple echo times.

4. A method as claimed in claim 1 or claim 2 wherein the MR pulse sequence for determining the one or more MR parameters is performed at multiple inversion times.

5. A method as claimed in any preceding claim wherein the calibration of the at least one MR parameter as a function of the verified temperature of the temperature reference vial is a linear calibration.

6. A method as claimed in any preceding claim wherein the reference temperature is between 15-25°C.

7. A method as claimed in claim 6 wherein the reference temperature is between 20-22°C.

8. A method as claimed in any preceding claim wherein the temperature of the MR phantom is adjusted between 0-40°C.

9. A method as claimed in any preceding claim wherein the MR phantom further comprises a flood solution to thermally couple the temperature reference vial and the MR parameter vial.

10. A method as claimed in claim 9 wherein the flood solution is a water based flood solution11. A method as claimed in claim 9 or claim 10, wherein the flood solution is a solution of at least one of Nickel Chloride and Sodium Chloride.

12. A method as claimed in any preceding claim wherein the at least one MR parameter vial comprises agar gel and Nickel Chloride.

13. A method as claimed in claim 12 wherein the at least one MR parameter vial comprises a plurality of vials with different concentrations of agar gel and Nickel Chloride.

14. A method as claimed in any of claims 1 to 11 wherein the at least one MR parameter vial comprises biological material.

15. A method as claimed in any preceding claim wherein the temperature reference vial and the MR parameter vial are formed of the same material.

16. A method as claimed in claim 15 wherein the vials are formed of polypropylene.

17. A method as claimed in any preceding claim wherein the one or more MR parameters comprise at least one of: Tl, T2.

18. A method as claimed in claim 17 wherein the one or more MR parameters further comprise at least one of: PDFF, T2*.

19. A method as claimed in any preceding claim wherein the MR pulse sequence is at least one of: 2D multi-echo spoiled FLASH, shMOLLI, T2 prepared Turbo FLASH, Multi-echo Fast-spin-echo.

20. A method as claimed in any preceding claim wherein the MR pulse sequence is a gradient echo pulse sequence or a spoiled gradient echo pulse sequence.

21. A method as claimed in claim 2, or any claim dependent on claim 2, wherein the measured temperature of the Ethylene Glycol vial is calculated based on the resonant frequency and a frequency offset between the OH and CH2 peaks of the Ethylene Glycol.

22. A Computer program capable of execution by a computer apparatus and configured to: measure a temperature of a temperature reference vial in a MR phantom with a plurality of vials, comprising at least one temperature reference vial and at least one MR parameter vial, where the temperature reference vial and the MR parameter vial are thermally coupled, where the temperature is determined using an MR pulse sequence at multiple echo times; determine one or more MR parameters of the MR parameter vial at the measured temperature using an MR pulse sequence; repeat the measuring and determining steps one or more times to obtain the measured temperature reference vial temperature and the determined MR parameters , after the temperature of the MR phantom has been adjusted, for each adjusted temperature; determine one or more temperature coefficients of the at least one MR parameter as a function of the measured temperature of the temperature reference vial, for each adjusted temperature; calibrate the one or more temperature coefficients of the at least one MR parameter as a function of the at least one MR parameter at the measured temperature to generate one or more calibration coefficients for the MR parameter; adjust the at least one MR parameter to a reference temperature using the one or more calibration coefficients and the measured temperature.

23. A system for performing one or more MR scans, comprising an MR scanner, and a processor configured to control the MR scanner and execute the following steps:measure a temperature of a temperature reference vial in a MR phantom with a plurality of vials, comprising at least one temperature reference vial and at least one MR parameter vial, where the temperature reference vial and the MR parameter vial are thermally coupled, where the temperature is determined using an MR pulse sequence at multiple echo times; determine one or more MR parameters of the MR parameter vial at the measured temperature using an MR pulse sequence; repeat the measuring and determining steps one or more times to obtain the measured temperature reference vial temperature and the determined MR parameters , after the temperature of the MR phantom has been adjusted, for each adjusted temperature; determine one or more temperature coefficients of the at least one MR parameter as a function of the measured temperature of the temperature reference vial, for each adjusted temperature; calibrate the one or more temperature coefficients of the at least one MR parameter as a function of the at least one MR parameter at the measured temperature to generate one or more calibration coefficients for the MR parameter; adjust the at least one MR parameter to a reference temperature using the one or more calibration coefficients and the measured temperature.

24. A system as claimed in claim 23 wherein the MR scanner operates at 3T.

25. A phantom for use with an MR scanner comprising a vial of temperature reference material and at least one MR parameter vial of material for measuring an MR parameter.

26. A phantom as claimed in claim 25 wherein the temperature reference material is Ethylene Glycol.

27. A phantom as claimed in claim 25 or 26 wherein the at least one MR parameter vial comprises agar gel and Nickel Chloride.

28. A phantom as claimed in any of claims 25 to 27 wherein the at least one MR parameter vial comprises a plurality of vials with different concentrations of agar gel and Nickel Chloride.

29. A phantom as claimed in any claims 27 to 28 wherein the at least one MR parameter vial is thermally coupled to the at least one temperature reference vial, via a water based flood solution.

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