TECHNICAL FIELD

The present invention relates generally to snapshot Magnetic Resonance Imaging (MRI) based Proton Resonance Frequency (PRF) thermometry, and, more particularly, to image-based retrospective gating of snapshot images to mitigate the impact of between-frame motion during snapshot image acquisition for PRF thermometry.

BACKGROUND

Proton Resonance Frequency (PRF) thermometry is a widely used MRI-based technique to monitor changes in tissue temperature in response to thermal therapy. The PRF thermometry is indispensable for thermal therapies (e.g., High Intensity Focused Ultrasound (HIFU) and Laser based heating) to ensure delivery of desired thermal dose to the target tissue, and to minimize unintended damage to the normal tissue.

During the thermal treatment, a patient is placed in an MRI scanner, which is used to locate a targeted tissue to be ablated, monitor a temperature change using MM thermometry data, and produce detailed images of the targeted tissue, so that the physician can observe the treatment situation in real time.

The temperature rise causes a change in the MR phase images. The proton resonance frequency shift (PRFS) based Magnetic Resonance (MR) thermometry method captures this temperature change by subtracting a phase image acquired prior to the thermal therapy (also called a “baseline”) from a phase image with temperature rise during thermal therapy, and the temperature difference (ΔT) is calculated by Equation 1 below:

Δ⁢T=∅therm-∅baseα⁢γ⁢B0⁢T⁢E=Δ⁢∅α⁢γ⁢B0⁢T⁢E(1)

In Equation 1, γ is the gyromagnetic ratio, TE is echo time, Øthermand Øbaseare a phase during a thermal therapy, and a phase at a baseline (i.e., prior to thermal therapy), respectively.

However, PRF thermometry of moving organs, such as the liver and the heart, is especially challenging due to motion within and between the acquisition of images (i.e., frames) used for thermometry. Between-frame motion causes misalignment between images used for phase subtraction, and therefore degrades the accuracy of temperature difference estimates (because temperature difference ΔT is proportionally related to phase difference ΔΦ between images).

The existing approaches to minimize misalignment between snapshot MR phase images include navigator-gated acquisition, physiological signal-gated acquisition, and intensity-based image registration.

In the navigator-gated acquisition, the motion-state of the organ of interest during its periodic motion can be determined by means of a navigator. Image acquisition is triggered when the organ is in the desired state of motion as estimated using the navigator. This approach lacks versatility, because pulse sequence and the reconstruction program need to be adapted to enable navigator-gated acquisition. Additionally, navigator signals only allow a crude estimation of the state of motion, because the navigator signals capture only a small sub-portion of k-space (typically around the k-space center).

In the physiological signal-gated acquisition, the motion-state of the organ of interest during its periodic motion can be determined using one or more physiological signals, such as an electrocardiogram (ECG) recording and/or respiratory bellows signals. Image acquisition is triggered when the organ is in the desired state of motion as estimated using physiological signals. This approach is negatively impacted by irregular patterns/artifacts during physiological signal detection and unreliability of certain physiological signals, e.g., respiratory bellows signal. Additionally, physiological signal variation may not have 1-1 correspondence to organ displacement.

In the intensity-based image registration, the acquired images are geometrically transformed to match a template image. This is a versatile approach since it is independent of the imaging sequence. However, it requires close initial alignment between the images to be registered. The through-plane motion may not be corrected for 2D acquisitions in case of poor initial alignment.

The present disclosure is directed to overcoming these and other problems of the prior art.

SUMMARY

Embodiments of the present invention address and overcome one or more of the above shortcomings and drawbacks, by providing methods, systems, and articles of manufacture for selecting thermal images for generating a temperature difference map through PRF thermometry.

Embodiments provide a computer-implemented method for selecting thermal images for generating a temperature difference map through proton resonance frequency (PRF) thermometry, comprising: acquiring a set of baseline images prior to a thermal treatment of an organ of interest; identifying a subset of baseline images in a most stable motion state from the set of baseline images; averaging the subset of baseline images to generate a template image; determining an acceptance threshold based on an image similarity measure (ISM) between each of the set of baseline images and the template image; acquiring a set of thermal images during the thermal treatment; and selecting a subset of thermal images from the set of thermal images, wherein each of the subset of thermal images has the image similarity measure above the acceptance threshold.

Embodiments further provide a computer-implemented method, if the organ of interest is a liver, the most stable motion state is exhalation; if the organ of interest is a heart, the most stable motion state is in diastole.

Embodiments further provide a computer-implemented method, wherein the subset of baseline images include the most mutually similar baseline images.

Embodiments further provide a computer-implemented method, wherein the most mutually similar baseline images are identified based on correlation or negated mean-squared difference between the set of baseline images.

Embodiments further provide a computer-implemented method, wherein the set of baseline images are temporally sorted to reflect mutual similarity between the set of baseline images in a correlation matrix.

Embodiments further provide a computer-implemented method, wherein the subset of baseline images account for a ratio R of the set of baseline images, the acceptance threshold is (1−R)*100th percentile ISM value, and the subset of thermal images are expected to account for the ratio R of the set of thermal images.

Embodiments further provide a computer-implemented method, wherein the ISM is a correlation between each of the set of baseline images and the template image, and a correlation threshold is determined based on the ratio R.

Embodiments further provide a computer-implemented method, wherein the correlation threshold is dynamically adapted to ensure that the ratio R is a fixed ratio over time.

Embodiments further provide a computer-implemented method, further comprising smoothing each of the subset of baseline images.

Embodiments further provide a computer-implemented method, further comprising registering each of the subset of baseline images.

Embodiments further provide a computer-implemented method, further comprising registering the selected subset of thermal images to the template image.

Embodiments provide a snapshot magnetic resonance imaging system configured to select thermal images for generating a temperature difference map through proton resonance frequency (PRF) thermometry, comprising: a magnetic resonance imaging (MRI) device for monitoring a thermal treatment; and a computer system configured to: acquire a set of baseline images prior to the thermal treatment of an organ of interest; generate a pairwise similarity matrix between the set of baseline images; identify a subset of baseline images in a most stable motion state from the set of baseline images based on the pairwise similarity matrix; average the subset of baseline images to generate a template image; calculate an image similarity measure value between each of the set of baseline images and the template image; determine an acceptance threshold to ensure that a ratio R of the set of baseline images have an image similarity measure value above the acceptance threshold, wherein the subset of baseline images account for the ratio R of the set of baseline images; acquire a set of thermal images during the thermal treatment; and select a subset of thermal images from the set of thermal images, wherein each of the subset of thermal images has the image similarity measure value above the acceptance threshold.

Embodiments further provide a snapshot magnetic resonance imaging system, wherein the set of baseline images are temporally sorted to reflect mutual similarity between the set of baseline images in the correlation matrix.

Embodiments further provide a snapshot magnetic resonance imaging system, wherein the subset of thermal images are expected to account for the ratio R of the set of thermal images.

Embodiments further provide a snapshot magnetic resonance imaging system, wherein the acceptance threshold is dynamically adapted to ensure that the ratio R is a fixed ratio over time.

Embodiments further provide a snapshot magnetic resonance imaging system, wherein the computer system is further configured to: register each of the subset of baseline images; and register the selected subset of thermal images to the template image.

Embodiments provide an article of manufacture for selecting thermal images for generating a temperature difference map through proton resonance frequency (PRF) thermometry, the article of manufacture comprising a non-transitory, tangible computer-readable medium holding computer-executable instructions for performing a method comprising: acquiring a set of magnetic resonance (MR) images of an organ of interest, wherein the set of MR images includes a plurality of baseline images acquired prior to a thermal treatment and a plurality of baseline images acquired during the thermal treatment; generating a correlation matrix between the set of MR images; identifying a subset of MR images in a most stable motion state from the set of MR images based on the correlation matrix; averaging the subset of MR to generate a template image; calculating a correlation between each of the set of MR images and the template image; determining a correlation threshold to ensure that a ratio R of the set of MR images are above the correlation threshold, wherein the subset of MR images account for the ratio R of the set of MR images; acquiring a set of thermal images during the thermal treatment; and selecting a subset of thermal images from the set of thermal images, wherein each of the subset of thermal images has the correlation above the correlation threshold, wherein the subset of thermal images account are expected to account for the ratio R of the set of thermal images.

Embodiments further provide an article of manufacture, wherein the correlation threshold is dynamically adapted to ensure that the ratio R is a fixed ratio over time.

Embodiments further provide an article of manufacture, the method further comprising smoothing each of the subset of baseline images.

Embodiments further provide an article of manufacture, the method further comprising: registering each of the subset of baseline images; and registering the selected subset of thermal images to the template image.

Additional features and advantages of the invention will be made apparent from the following detailed description of illustrative embodiments that proceeds with reference to the accompanying drawings.

