Proceedings of the meeting - Department of Physics - University of ...

Low field, low cost multi-nuclear imaging research systemsEugeny Krjukov, James Wild, Martyn PaleyAcademic Radiology, University of Sheffield, Royal Hallamshire Hospital, Sheffield, UKP13IntroductionMagnetic resonance imaging (MRI) is a widely used and powerfulmethod for in-vivo diagnosis of disease. Unfortunately physicalrestrictions limit its application. At room temperature nuclearpolarisation is low and as a consequence MR signal amplitudes at lowfield strength are very weak. In order to increase image contrast tonoise ratio, high magnetic fields are usually applied. For instance 3Twhole body magnets are routinely used in medical practice andmagnets up to and beyond 7T are used in research. However, they areexpensive to manufacture and require cryostats and liquid helium as acoolant which increases cost dramatically. Another potential problemis nonuniform magnetic susceptibility of the object being imaged. Thisis especially significant for high magnetic fields and can reduce imagecontrast and produce distortion. Hyperpolarized contrast agents suchas 3-He or 129-Xe can be an effective solution for these problems forcertain applications where gases can be used.EquipmentMagnets: Use of parallel techniques with hyperpolarised gases shouldbe very useful as the number of RF pulses can be minimised ropreserve polarisation. Two low field resistive magnets were designedand built in order to apply the MAMBA and SPIRIT parallel imagingmethods respectively (1, 2). A corrected solenoidal magnet (S) withdiameter of 20cm and length of 100cm shown in Fig. 1a was designedfor industrial hyperpolarised gas flow chamber’ applications. A sixcoil open magnet (H) with a main coil diameter of 60 cm and anintercoil distance of 15cm, shown in Fig. 1b and 1c was designed forneonatal lung hyperpolarised 3-He imaging. Homogeneous magneticfields of 0.008T (S) and 0.015T (H) were obtained at the centre ofboth magnets respectively with stable temperatures below 60 o C (1) asreported previously (3, 4).Spectrometer: A four channel direct detection MR spectrometer wasbuilt from standard Mini-Circuit components linked to a NationalInstruments data acquisition and control system programmed inLabView software. Solenoidal RF coils and passive TR switches wereused for initial testing of the system.ResultsFigure 2a shows FIDs of 3-He and 1-H at 450 kHz in the H magnet,central and top curves respectively using the same RF coil andadjusting for gyromagnetic ratio by ramping the field. Flip angle wasclose to 90 o for 1-H and about 2 o for 3-He. 3-He nuclei were 30%optically prepolarized. The bottom curve shows the 1-H FID at 350kHz in the S magnet. Receiver amplification and spectrometer noiselevels were the same for all curves. In all cases an 18 mm diameter, 70mm long water phantom doped with CuSO4 was used with T1 ~100ms. Decay curves show good homogeneity over the phantomvolume. A strong FID could be acquired over a 300mm length of the Smagnet, as predicted, which should be sufficient for gas flow imagingapplications.Figure 2b-d show sagittal and coronal 1-H proton images and a 3-Hesagittal image. For the 1-H images, a letter F profile object withsection thickness 2mm was put inside the phantom in order todemonstrate image resolution. The 3-He image used a larger FOV asweaker gradient fields were required in order to prevent signal lossdue to diffusion. However, the spatial resolution was still similar tothat used for in vivo lung imaging on whole body systems.ConclusionWe have demonstrated operation of two simple MR systems with lowweight, low cost as well as low energy consumption which could beuseful in mobile MRI. They could be effectively used in industrialapplications (S) and in medical practice (H) where cost and/or safetyare the dominant factors. Imaging of neonatal lungs at the cotsidewhere low acoustic noise, low RF absorption and low static fieldprovide a major advantage when linked to high SNR hyperpolarised 3-He imaging should form an important future application.AcknowledgementWe acknowledge support from the Royal Society Paul InstrumentFund and DH-NEAT for this workFig. 1 a – Solenoidal magnet; 1b and 1c – Helmholtz magnet.FID (V)1050-5-101aHe 3 450kHzH 1 450kHzH 1 350kHz0 5 10 15 20 25time (ms)2cFig. 2 a – Fid’s from 3-He and 1-H at 450 kHz in the H magnet and 1-H at350 kHz in the S magnet;2b and 2c – sagittal and coronal 1-H phantomimages; 2d – sagittal 3-He image, all acquired in the H magnet.References2a1b1c(1) Paley M et al. Mag. Res. Med. 48, 1043-1050 (2002)(2) Paley M et al. Mag. Res. Imag. 24, 557-562 (2006)(3) Fichele S et al. Proc. BC ISMRM., 2005.(4) Paley M et al., Proc. BC ISMRM, 2005.2b2d