System and method for measuring pH using magnetic resonance
Systems and methods for indicating pH in a subject using a magnetic resonance imaging (MRI) system are provided. The method includes selecting a contrast agent, generating a chemical exchange saturation transfer (CEST) pulse sequence, performing the pulse sequence with the saturating pulse at a first power level, and performing the pulse sequence again with the saturating pulse at a power level different from the first power level. The method also includes generating values indicating pH of the subject, and generating a report indicating the pH using those values.
1. A method for assessing pH in a subject using a magnetic resonance imaging (MRI) system, comprising:
a) generating, using the MRI system, a chemical exchange saturation transfer (CEST) pulse sequence comprising a saturating magnetic field used to saturate protons of the subject by irradiating the subject with the field, wherein the field has a frequency offset from a Lamar frequency of water protons, and the frequency offset is related to a contrast agent associated with the subject;
b) performing the pulse sequence wherein the saturating magnetic field is at a first radio-frequency (RF) power level (ω 11 ) and a first set of MR signals of the subject are acquired;
c) performing the pulse sequence wherein the saturating magnetic field is at a second RF power level (ω 12 ) different from ω 11 and a second set of MR signals are acquired;
d) generating first inverse CEST ratio and a second inverse CEST ratio of the subject using the first and the second set of MR signals; and
e) generating a report indicating pH using the values based on a labile proton exchange rate, where the labile proton exchange rate is derived using RF-power based inverse CEST ratio (PRICESTR) between the first inverse CEST ratio and the second inverse CEST ratio.
2. The method as recited in claim 1 , wherein the sequence generated in a) comprises:
a first subpart including irradiating protons in the subject at a first frequency offset (Δω) equal to a chemical shift difference between a proton exchange site of the contrast agent and the Lamar frequency, and acquiring labeled MR signals (I label );
a second subpart including irradiating the protons at a second frequency offset (−Δω) that is opposite of the first frequency offset, and acquiring reference MR signals (I ref ); and
a third subpart including turning off saturating magnetic field, and acquiring control MR signals I 0 .
3. The method as recited in claim 2 , wherein step e) includes:
calculating an inverse CEST ratio (CESTR ind ) as CESTR ind =I 0 /I label −I 0 /I ref using each of the first and the second set of MR signals to generate the first inverse CEST ratio CESTR ind (ω 11 ) for the first set of MR signals and the second inverse CEST ratio CESTR ind (ω 12 ) for the second set of MR signals;
calculating the PRICESTR as a ratio of CESTR ind (ω 11 ) and CESTR ind (ω 12 );
deriving the labile proton exchange rate based on the PRICESTR using a preset equation; and
deriving the values indicating the pH based on the labile proton exchange rate.
4. The method as recited in claim 3 , wherein the preset equation is:
k
sw
=
R
2
s
2
+
4
ω
11
2
·
ω
12
2
·
1
-
PRICESTR
ω
12
2
·
PRICESTR
-
ω
11
2
-
R
2
s
2
,
where k sw is the labile proton exchange rate and R 2s is a transverse relaxation rate of a solute.
5. The method as recited in claim 4 , wherein a labile proton ratio is derived as:
f
r
=
R
1
w
·
(
ω
1
2
+
k
sw
(
k
sw
+
R
2
s
)
)
k
sw
·
ω
1
2
·
CESTR
ind
(
ω
1
)
,
where ω 1 denotes the first RF power level ω 11 or the second RF power level ω 12 , θ=atan(ω 1 /Δω), and R 1w is a longitudinal relaxation rate of water.
6. The method as recited in claim 5 , further comprising:
deriving a concentration based on the labile proton ratio.
7. The method as recited in claim 2 , wherein step e) includes:
calculating a CEST ratio (CESTR) as CESTER=(I ref −I label )/I 0 using each of the first and the second set of MR signals to generate a CESTR(ω 11 ) for the first set of MR signals and a CESTR(ω 12 ) for the second set of MR signals;
calculating an RF-power based CEST ratio (rCESTR) as a ratio of CESTR(ω 11 ) and CESTR(ω 12 );
deriving a labile proton exchange rate based on the rCESTR using a second preset equation; and
deriving the values indicating the pH based on the labile proton exchange rate.
8. The method as recited in claim 7 , wherein the second preset equation is:
k
sw
=
R
2
s
2
+
4
(
w
12
2
-
w
11
2
(
(
1
-
σ
)
w
12
(
1
-
σ
)
w
11
)
·
w
12
2
w
11
2
·
rCESTR
-
1
-
w
11
2
)
-
R
2
s
2
,
where k sw is the labile proton exchange rate and R 2s is a transverse relaxation rate of a solute, and (1−σ) indicates a spillover factor.
9. The method as recited in claim 7 , wherein the first and the second RF power levels ω 11 and ω 12 are optimized by maximizing a relative contrast-to-noise ratio (rCNR), where
rCNR
=
CNR
SNR
I
0
,
CNR is a contrast-to-noise ratio of the rCESTR, SNR I 0 is a signal-to-noise ratio of the control MR signals I 0 .
10. The method as recited in claim 7 , wherein
the contrast agent has two or more exchangeable sites;
a frequency offset of the pulse sequence applied in step b) correspond to a chemical shift of a first exchangeable site or a first chosen offset;
a frequency offset of the pulse sequence applied in step c) correspond to a chemical shift of a second exchangeable site or a second chosen offset.
11. A magnetic resonance imaging (MRI) system, comprising:
a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject arranged in the MRI system;
a magnetic gradient system including a plurality of magnetic gradient coils configured to apply at least one magnetic gradient field to the polarizing magnetic field;
a radio frequency (RF) system configured to apply an RF field to the subject and to receive magnetic resonance (MR) signals therefrom;
a computer system programmed to:
a) generate, using the MRI system, a chemical exchange saturation transfer (CEST) pulse sequence comprising a saturating magnetic field used to saturate protons of the subject by irradiating the subject with the field, wherein the field has a frequency offset from a Lamar frequency of water protons, and the frequency offset is related to a contrast agent associated with the subject;
b) perform the pulse sequence wherein the saturating magnetic field is at a first radio-frequency (RF) power level (ω 11 ) and a first set of MR signals of the subject are acquired;
c) perform the pulse sequence wherein the saturating magnetic field is at a second RF power level (ω 12 ) different from ω 11 and a second set of MR signals are acquired;
d) generate first inverse CEST ratio and a second inverse CEST ratio of the subject using the first and the second set of MR signals; and
e) generate a report indicating pH using the values based on a labile proton exchange rate, where the labile proton exchange rate is derived using RF-power based inverse CEST ratio (PRICESTR) between the first inverse CEST ratio and the second inverse CEST ratio.
12. The system as recited in claim 11 , wherein the sequence generated in a) comprises:
a first subpart including irradiating protons in the subject at a first frequency offset (Δω) equal to a chemical shift difference between a proton exchange site of the contrast agent and the Lamar frequency, and acquiring labeled MR signals (I label );
a second subpart including irradiating the protons at a second frequency offset (−Δω) that is opposite of the first frequency offset, and acquiring reference MR signals (I ref ); and
a third subpart including turning off the saturating magnetic field and acquiring control MR signals I 0 .
13. The system as recited in claim 12 , wherein d) includes
calculating an inverse CEST ratio CESTR ind , as CESTR ind =I 0 /I label −I 0 /I ref using each of the first and the second set of MR signals to generate the first inverse CEST ratio CESTR ind (ω 11 ) for the first set of MR signals and the second inverse CEST ratio CESTR ind (ω 12 ) for the second set of MR signals;
calculating the PRICESTR as a ratio of CESTR ind (ω 11 ) and CESTR ind (ω 12 );
deriving the labile proton exchange rate based on the PRICESTR using a preset equation; and
deriving the values indicating the pH based on the labile proton exchange rate.
14. The system as recited in claim 13 , wherein the labile proton exchange rate in relation with the PRICESTR is characterized by the preset equation:
k
sw
=
R
2
s
2
+
4
ω
1
a
2
·
ω
1
b
2
·
1
-
PRICESTR
ω
1
b
2
·
PRICESTR
-
ω
1
a
2
-
R
2
s
2
,
where k sw is the labile proton exchange rate and R 2s is a transverse relaxation rate of a solute.
15. The system as recited in claim 14 , wherein a labile proton ratio is derived as:
f
r
=
R
1
w
·
(
ω
1
2
+
k
sw
(
k
sw
+
R
2
s
)
)
k
sw
·
ω
1
2
·
CESTR
ind
(
ω
1
)
,
where ω 1 denotes the first RF power level ω 11 or the second power level ω 12 , θ=atan(ω 1 /Δω), and R 1w is a longitudinal relaxation rate of water.
16. The system as recited in claim 15 , wherein a concentration is derived based on the labile proton ratio.
17. The system as recited in claim 12 , wherein d) includes
calculating a CEST ratio (CESTR) as CESTR=(I ref −I label )/I 0 using each of the first and the second set of MR signals to generate a CESTR(ω 11 ) for the first set of MR signals and a CESTR(ω 12 ) for the second set of MR signals;
calculating an RF-power based CEST ratio (rCESTR) as a ratio of CESTR(ω 11 ) and CESTR(ω 12 )
deriving a labile proton exchange rate based on the rCESTR using a second preset equation; and
deriving the values indicating the pH based on the labile proton exchange rate.
18. The system as recited in claim 17 , wherein the labile proton exchange rate in relation with the rCESTR is represented by the second preset equation:
k
sw
=
R
2
s
2
+
4
(
w
12
2
-
w
11
2
(
(
1
-
σ
)
w
12
(
1
-
σ
)
w
11
)
·
w
12
2
w
11
2
·
rCESTR
-
1
-
w
11
2
)
-
R
2
s
2
,
where k sw is the labile proton exchange rate and R 2s is a transverse relaxation rate of a solute, and (1−σ) indicates a spillover factor.
19. The system as recited in claim 17 , wherein the first and the second RF power levels ω 11 and ω 12 are optimized by maximizing a relative contrast-noise ratio (rCNR), where
rCNR
=
CNR
SNR
I
0
,
CNR is a contrast-to-noise ratio of the rCESTR, SNR I 0 is a signal-to-noise ratio of the control MR signals I 0 .
20. The system as recited in claim 17 , wherein
the contrast agent has two or more exchangeable sites;
a frequency offset of the pulse sequence applied in b) correspond to a chemical shift of a first exchangeable site or a first chosen offset;
a frequency offset of the pulse sequence applied in c) correspond to a chemical shift of a second exchangeable site or a second chosen offset.