Method for relaxation-compensated fast multi-slice chemical exchange saturation transfer MRI
A volumetric APT imaging sequence is provided that acquires multi-slice images immediately after a single long continuous wave (CW) RF irradiation, wherein the relaxation-induced loss of CEST contrast is compensated for during post-processing. Thus, a fast volumetric pH-weighted APT imaging technique is provided.
1. A method for producing an image of a subject with a magnetic resonance imaging (MRI) system the steps comprising:
a) acquiring, with the MRI system reference image data using a first pulse sequence that saturates a selected labile spin species by applying a radiofrequency (RF) irradiation at a reference frequency,
b) acquiring, with the MRI system, labeled image data using a second pulse sequence that saturates a selected labile spin species by applying an RF irradiation at a labeling frequency;
c) reconstructing a reference image from the acquired reference image data and a labeled image from the acquired labeled image data; and
d) producing a corrected ratio image by subtracting the reconstructed labeled image form the reference image to correct for relaxation-induced losses by recovering saturation of the selected labile spin species.
2. The method of claim 1 wherein the applied RF irradiation is a continuous wave RF irradiation.
3. The method of claim 1 wherein the applied RF irradiation is a series of RF pulses.
4. The method of claim 1 further comprising:
e) acquiring, with the MRI system, signal data indicative of a relaxation parameter; and
wherein step d) includes producing, from the acquired signal data, a relaxation parameter image and producing a first ratio image from the reconstructed reference and labeled images.
5. The method of claim 4 wherein the corrected ratio image is produced by multiplying voxel values in the first ratio image by corresponding voxel values in the relaxation parameter image.
6. The method of claim 4 wherein the relaxation parameter is a longitudinal relaxation rate.
7. The method of claim 1 wherein the selected labile spin species includes amide protons.
8. The method of claim 1 wherein at least one of the first and second pulse sequence is configured to intrinsically compensate for relaxation-induced losses by recovering saturation of a labile spin species after each image slice is acquired.
9. The method of claim 1 wherein the first pulse sequence includes a continuous wave RF irradiation at the reference frequency, wherein the reference frequency is −Δf s , and a saturation pulse at the reference frequency, −Δf s .
10. The method of claim 9 wherein the second pulse sequence includes a continuous wave RF and a saturation pulse applied at the labeling frequency, wherein the labeling frequency is Δf s .
11. A method for producing an image of a subject with a magnetic resonance imaging (MRI) system, the method comprising the steps of:
a) acquiring, with the MRI system, T1 measurement data;
b) producing a T1 map from the acquired T1 measurement data;
c) acquiring, with the MRI system, reference image data and labeled image data from the subject;
d) reconstructing a reference image from the reference image data and a labeled image from the labeled image data;
e) producing a CEST Ratio (CESTR) map by subtracting the labeled image from reference image; and
f) producing a compensated CESTR (cCESTR) map using the CESTR map and T1 map by correcting for longitudinal magnetization relaxation effects on a voxel-by-voxel basis.
12. The method of claim 11 wherein step a) includes performing a Look-Locker EPI pulse sequence with the MRI system to acquire the T 1 measurement data as transverse magnetization recovers to equilibrium.
13. The method of claim 11 wherein step c) includes performing an imaging pulse sequence including:
a labeling section, wherein RF labeling is performed;
a preparation section, which includes an outer volume suppression module and fat suppression;
a data acquisition section; and
an idle period.
14. The method of claim 13 wherein the imaging pulse sequence is performed a first time to acquire the reference image data and the imaging pulse sequence is performed a second time to acquire labeled image data.
15. The method of claim 14 wherein the imaging pulse sequence includes apply a continuous wave RF at a labeling frequency, Δf s and a reference frequency, −Δf s .
16. The method of claim 15 wherein the labeling frequency, Δf s and reference frequency, −Δf s are selected to be symmetric about a bulk water resonance frequency.
17. The method of claim 15 wherein the labeling frequency is selected as a chemical shift difference between bulk water and the labile protons.
18. The method of claim 11 wherein step f) wherein the cCESTR map is calculated according to:
cCESTR
(
i
,
j
)
=
CESTR
(
i
,
j
)
·
ⅇ
t
T
1
w
(
i
,
j
)
,
where cCESTr(i,j) indicates a voxel value in the cCESTR map at voxel location (i,j), CESTr(i,j) indicates a voxel value in the CESTR map at a corresponding voxel location (i,j), T 1w (i,j) indicates a voxel value in the T 1 map at the corresponding voxel location (i,j), and t is the duration of time after RF irradiation at which a signal corresponding to a given image was acquired.
19. A method for producing an image of a subject with a magnetic resonance imaging (MRI) system, the method comprising the steps of:
a) acquiring, with the MRI system, T1 measurement data;
b) producing a T1 map from the acquired T1 measurement data;
c) acquiring, with the MRI system, reference image data and control image data from the subject;
d) reconstructing a reference image from the reference image data and a control image from the control image data;
e) producing a Chemical Exchange Saturation Transfer Ratio (CESTR) map using the reference image and the control image; and
f) producing a compensated CESTR (cCESTR) map using the CESTR map and T1 map by correcting for longitudinal magnetization relaxation effects on a voxel-by-voxel basis.
20. The method of claim 19 wherein step e) includes normalizing the CESTR map by dividing the CESTR map by the control image.