Ardenkjær-Larsen, Jan Henrik

Gutte, Henrik

Hansen, Adam E.

Hansen, Rie Beck

Kjær, Andreas

Kristensen, Annemarie T.

Larsen, Majbrit M E

The aim of the present work is to address the challenge of optimal The aim of the present work is to address the challenge of optimal flflip angle calibration of ip angle calibration of C surface coils in C surface coils in hyperpolarization studies. To this end, we characterize the spatial pro hyperpolarization studies. To this end, we characterize the spatial profifile of the le of the flflip angle and demonstrate that it ip angle and demonstrate that it allows for a simple calibration improving the signal-to-noise ratio for hyperpolarized allows for a simple calibration improving the signal-to-noise ratio for hyperpolarized C magnetic resonance C magnetic resonancespectroscopic imaging

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Downloaded from orbit.dtu.dk on: Dec 18, 2017Characterization and flip angle calibration of 13C surface coils for hyperpolarizationstudiesHansen, Rie Beck; Gutte, Henrik; Larsen, Majbrit M E; Kristensen, Annemarie T.; Kjær, Andreas;Ardenkjær-Larsen, Jan Henrik; Hansen, Adam E.Publication date:2017Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Hansen, R. B., Gutte, H., Larsen, M. M. E., Kristensen, A. T., Kjær, A., Ardenkjær-Larsen, J. H., & Hansen, A. E.(2017). Characterization and flip angle calibration of 13C surface coils for hyperpolarization studies. Abstractfrom  ISMRM 25th Annual Meeting & Exhibition, Honolulu, United States.Characterization and ﬂip angle calibration of C surface coils forhyperpolarization studiesRie B Hansen , Henrik Gutte , Majbrit ME Larsen , Annemarie T Kristensen , Andreas Kjær , Jan H Ardenkjær-Larsen ,and Adam E HansenDepartment of Electrical Engineering, Technical University of Denmark, Kgs. Lyngby, Denmark, Department of Clinical Physiology, Nuclear Medicine& PET and Cluster for Molecular Imaging, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark, Department of Veterinary Clinical andAnimal Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Frederiksberg C, DenmarkSynopsisThe aim of the present work is  to address the chal lenge of optimal ﬂip angle cal ibration of  i      i     l l   i l  i  l  l i i   C surface coi ls  in  i l  ihyperpolarization studies.  To this end, we characterize the spatial  proﬁle of  the ﬂip angle and demonstrate that i tl i i  i .   i  ,   i   i l  l    i  l     ia l lows for a simple cal ibration improving the signal-to-noise rat io for hyperpolarized l l    i l  l i i  i i   i l i  i   l i  C magnetic resonance i  spectroscopic imaging.i  i i .IntroductionFor hyperpolarized C magnetic resonance spectroscopic imaging ( C-MRSI), ﬂip angle (FA) calibration is challenged. Therefore, a C enrichedphantom placed outside the imaged object is commonly used as reference. This does, however, give potential errors in case of RF transmit ﬁeld(B ) inhomogeneities directly aﬀecting the FA. The common use of single-channel surface coils with strong RF transmit proﬁles for C-MRSIenhances the importance of addressing FA calibration.The aim is to address optimal FA calibration of C surface coils. We characterize the spatial proﬁle of the FA and demonstrate that it allows fora simple calibration improving the SNR.MethodsTheory:: The B  proﬁle in the axial-axis of a surface loop coil is given by the Biot-Savart law. The coil transmitter voltage  required for constantFA as function of distance is:where  is an acquisition- and coil-dependent constant,  is the axial-axis distance, and  is the radius of the coil loop.From Eq. (1) the following expression can be stated:Using Eq. (2), with a 90° FA calibrated at a reference position through determination of , a calibration  can hereby be obtained atan arbitrary position  on the axial-axis.Phantom experiments: i : To verify Eq. (2) the FA proﬁle was characterized for three diﬀerent RF surface coils using the phantom setup inFigure 1.The coils were: 1) a dual-tuned, H/ C, transmit/receive 11 cm ﬂex coil, 2) a C, transmit/receive 12 cm loop coil, and 3) a C, transmit/receive20 cm loop coil (RAPID Biomedical). A 5.5 mL vial of 4.0 M C-urea was used for FA calibration. All scans were performed on the integrated 3TPET/MRI system (Siemens Biograph mMR).The vial was placed at a new position for each scan session, consisting of a localizer followed by FA calibration. For calibration a series of FIDmeasurements with increasing transmitter voltage (10 V steps) were acquired [TR/TE 10,000/0.35 ms, FA 90°, BW 3,000 Hz, transmitter voltage10 to 300 V (loop coils), transmitter voltage 10 to 150 V (ﬂex coil)]. Spectra were centered on C-urea. A sinusoidal ﬁt of the C-urea peakversus transmitter voltage was used to determine . Eq. (1) was ﬁtted to the calibration  versus position of the C-urea vial. Experimentswere repeated at diﬀerent days and with diﬀerent coil loads.In vivo experiment:I  i  i : The feasibility of the calibration method was demonstrated in a canine cancer patient (39 kg) with a biopsy-veriﬁedaxillar soft tissue sarcoma scanned as part of clinical staging work up. Patient handling and acquisition details are given in Gutte et al. 2015 .Hyperpolarized C-CSI was acquired with  V as measured using the calibration phantom positioned at the coil surface(equivalent to 90° at 2.3 cm). The CSI slice (13 mm slice thickness) was aligned parallel to the ﬂex coil at a depth of 4 cm. To compensate for theFA loss a second CSI was obtained with  V calculated by means of Eq. (2).Voxel-wise SNR was estimated as the total carbon signal (sum of absolute peak values for pyruvate, pyruvate-hydrate, lactate, alanine, andbicarbonate) divided by ﬁve times the spectral noise (standard deviation of the real spectrum in noise regions).Results131 2 3 3 2 121 23131313 13 13113131 VV(z) = c ⋅ (1)( + )z2 a2 3/2c z a(z) = ⋅ (2)V90 V90,REF ( )+z2 a2+z2REF a23/2V90,REF (z)V90z1 13 13 131313 13V90 V90 13113 = 87.4V90,REF= 136V90Figure 2 shows an example of FA calibration measurements and sinusoidal ﬁts at diﬀerent positions of the urea vial. The resulting calibrationvoltage  and ﬁts to Eq. (2) are shown in Figure 3. The ﬁtting coeﬃcients were: ﬂex coil,  V/cm  ( ); loop coil (12 cm),  V/cm  ( ); loop coil (20 cm),  V/cm  ( ). As can be appreciated in Figure 3 and by the -values,data obtained for diﬀerent days and loads follow the same curve.Figure 4 shows SNR for the two C-MRSI scans of the soft tissue sarcoma. The SNR improvement factor was 2.01. The noise level was constant.DiscussionThe study demonstrates feasibility of a FA calibration method for hyperpolarization studies using surface coils. Phantom experiments showedrobustness of the method. Feasibility of implementation is shown in the presented canine cancer patient. However, the SNR improvementestimated to a factor 2.01 is larger than the increase in FA voltage (a factor 1.56). Therefore, the signal changes between scans cannot solely beattributed to change in FA. Tissue perfusion changes during anesthesia could also play a role.ConclusionThe FA proﬁle of a surface loop coil can be utilized to improve SNR for hyperpolarized C-MRSI studies. Simple implementation and feasibilitywas demonstrated in a hyperpolarized C-MRSI canine cancer patient.AcknowledgementsNo acknowledgement found.References1. Gutte H, Hansen AE, Larsen MM, et al. Simultaneous Hyperpolarized 13C-Pyruvate MRI and 18F-FDG PET (HyperPET) in 10 Dogs with Cancer. JNucl Med. 2015;56(11):1786-1792.V90 c = 0.1847 3 = 0.9807r2c = 0.1697 3 = 0.9959r2 c = 0.1280 3 = 0.9681r2 r2131313FiguresFigure 1. (a) The phantom setup consisting of four Siemens H phantoms of 1.9 L each and the 5.5. mL C-urea vial viewed from above with theﬂex coil. The broken line indicates the cross-section sketched in (b). The sketch in (b) shows the vertical scale used for the measurements withthe construction in (c) in the center, which enables the change of position of the urea vial by means of moving the matchstick. The origin of thescale is located at the patient side of the coil surface.Figure 2. Flip angle calibration measurements for the 12 cm loop coil with the urea vial positioned at ﬁve diﬀerent distances on the axial-axis, ,from the coil loop center.1 13zFigure 3. Calibration voltage  as a function of axial distance from the coil surface. The solid lines correspond to the function ﬁts to Eq. (1).The loop center within each coil is located at -1.0 cm beneath the coil surface (see Figure 1). The reference position for the ﬂex coil is located at-4.3 cm, while the reference position for the loop coils is located at -4.2 cm. The diﬀerent symbols indicate diﬀerent acquisition dates for each ofthe coils. The blue downward triangles for the ﬂex coil indicate measurements using a smaller load.Figure 4. In vivo demonstration of the ﬂip angle calibration method; Total carbon CSI of canine tumor (a) without and (b) with a modiﬁedcalibration voltage. Mean SNR and standard deviations are given for a region-of-interest (ROI) within the tumor tissue. Mean SNR factorimprovement of 2.01.V90

Characterization and flip angle calibration of 13C surface coils for hyperpolarization studies

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Characterization and flip angle calibration of 13C surface coils for hyperpolarization studies

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