- Email: [email protected]
NMR imaging versus nuclear medicine
308
Magnetic
Resonance
Imaging
2-D multislice include most efficient with long-TR, short-TE sequences; high resolution performed with reasonable scan times; gaps between slices; and reduced motion artifacts. The advantages of 3-D methods include improved signal-to-noise (S/N) ratio proportional to the square root of the number of slices; thinner slices more easily achieved; longer TE’s and shorter TR’s more easily achieved; best efficiency with short-TR sequences; no gaps between slices; reduced paradoxical enhancement; and images are at the same phase of the cardiac cycle. The selective 3-D method limits the 3-D acquisition to a selected “slab” of tissue. This method is useful when a limited number of thin, conteguous slices are needed such as in surface-coil applications. Single spin-echo sequences are useful in T, weighted scans where the maximum number of slices per 2-D multislice acquisition can be obtained. For maximum efficiency, multiecho sequences in 2-D multislice are best used with long TR. The first echo in a long-TR scan will be primarily N (H) weighted with each additional echo being more T, weighted. Multiecho sequences in 3-D acquisitions with shorter TR’s can be obtained so that the first echo is primarily T, weighted and each additional echo is more T, weighted. Inversion spin echo is more efficient with 2-D multislice acquisitions because of the need for a longer TR. For demyelinating disease, dysmyelinating disease, and screening exams, our choice is a 2-D multislice acquisition and a multiecho pulse sequence (2-4 echoes). If a suspected infarct, hemorrhage or tumor is detected then either an inversion spin-echo sequence for T, weighting or a 3-D exam is performed. For patients with suspected tumor, infarct, or hemorrhage, we use a 3-D multiecho with 8 echoes of TE 30-240 and TR 800. Using this method, acute, subacute, and remote infarcts, acute and subacute hemorrhage, and low-grade and high-grade tumors may be distinguished. Additional specificity may be achieved with a variety of other lesions. Most acoustic neuromas are better identified on N (H) weighted images. Other acoustic neuromas are better seen on T, weighted images. Therefore, a multiecho multislice with both N (H) and T2 weighting is used. Careful planning of the MR examination will result in a more efficient, sensitive, and specific study. NMR Imaging Versus Nuclear Medicine Peter Pfannenstiel Departments for Nuclear Medicine and NMR, Deutsche Klinik f iir Diagnostik, Aukammallee 33, 6200 Weisbaden, FRG The latest development of diagnostic imaging modalities were influenced by the impact of alternative
0 Volume 3, Number
3, 1985
methods such as ultrasonography, digital subtraction angiography and, most recently, the technique of nuclear magnetic resonance (NMR). Even so it seems clear that the new NMR equipment will not be installed as rapidly as X-ray CT equipment, since the NMR method must compete with the widespread CT experience; however, there is no question, that NMR will in the near future impact the number of other examinations currently performed. As the technology of nuclear medicine becomes more complex with positron and single-photon tomography the future will be influenced by these techniques. Even though NMR is no competitor in the study of regional biochemistry when compared to nuclear medicine, NMR will have a negative impact on nuclear medicine by reducing the number of examinations currently performed. Clearly the image quality of NMR is superior to PET and SPECT scans. Today, in routine clinical work, nuclear medicine seems to be superior to NMR in studying organ function and in vivo metabolism, and it is also superior to organ selectivity. However, the future of NMR imaging in medicine seems bright. Absence of known biological hazards and its ability to measure multiple tissue parameters will make it the study of choice in many clinical situations. Its ability to create detailed tomographic images in any plane with both excellent anatomic delineation and tissue specificity is a revolution in diagnostic imaging. The additional information gained by providing in vivo data concerning human biochemistry and pathophysiology may change our entire understanding of health and disease. A combination of both NMR tomography and NMR spectroscopy is envisioned. NMR spectroscopy is a possible quantitative method of measuring biochemical processes in vivo, for instance “P in different phosphorous metabolites in patients with enzyme defects. In the future there will be a relationship between nuclear medicine and NMR techniques. The two techniques will act in the same direction by adding structural and functional data that have not been obtained before by any other technique. In this way nuclear medicine and NMR will work together rather than in opposition. The promise of NMR imaging is based upon its capability of combining three parameters: the image quality of CT scanning, the safety of ultrasound and the selectivity of nuclear medicine. Since NMR has a number of points in common with nuclear medicine and promises to be a powerful complementary modality, the participation of nuclear medicine physicians, physicists, chemists and technologists in NMR training and research efforts should be encouraged. Clinical efficiency studies will be defining the role and limitations of NMR techniques in comparison with various other imaging and diagnostic
Abstracts
modalities over the next 5-10 years. It seems to be important to work closely together during this evaluation period. Low-Field Magnetic Experiences
Resonance
Imaging-Clinical
R. E. Sepponen Helsinki University Central Hospital, NMR Laboratory and Instrumentarium Corp., Helsinki, Finland The effect of the field strength on the contrast of MR images is well recognized. The increase of the strength of the polarizing magnetic field increases the signalto-noise ratio and improves the resolution of the final image [I]. However, the effectivity of various relaxation mechanisms changes with the resonance frequency. The relaxation processes due to the macromolecular movements are effective at low resonance frequencies [ 21. The diagnostic effectivity of low-field imaging has been demonstrated [3,4, 51. A prototype unit (ACUTSCANB, manufacturer Instrumentarium Corp., Helsinki, Finland) operating at the field strength 0.02 T (corresponding proton resonance frequency 0.8 MHz) was used. Inversion recovery and spin-echo sequences were used in order to generate T, and T, weighted images. The imaging method was 2-DFT. The data acquisition was carried out as a spin echo. The selective excitation method was used to define the imaging plane. In order to reduce the imaging time the multislice technique was utilized. The benefits of the low-field operation may be summarized as follows: (1) Relaxation time T, of tissues at low field reflects macromolecular differences of tissues. The tissue contrast is demonstrated in the diagnosis of intracranial hematomas. At the operating field strength of the unit the relaxation time T, of freshly extravasated blood is shorter than the T, of the surrounding brain tissue. This facilitates the differential diagnosis of acute hemorrhage from other pathologies which in the most cases has long relaxation times T, and T, [5]. At 0.2 T and above the relaxation time of blood is equal or longer than that of the surrounding brain tissue [5, 61. (2) The fringe fields are small and the missile effects are virtually eliminated. The 0.5 mT field extends 1.2 m from the ends of the solenoid and is usually well confined in the installation room. Thus usually no special safety measures are needed against inadvertent magnetic field exposures (e.g. pacemaker patients). (3) The unit may be constructed based on inexpensive resistive magnet technology. The magnet is a solenoid with end-correction coils. The absolute inhomogeneities are small and thus long data collection
309
time may be used in order to maximize the signalto-noise ratio. (4) The unit may be installed without extensive site preparation. The weight of the magnet is 850 kg and the total system weight is 1550 kg. The total power consumption is 7 kW and typical water consumption for cooling of the system is 4-6 l/min. Due to the small fringe fields magnetic shielding is not needed. I. Hart, H.R., Bottomley, P.A., Adelstein, W.A., et al., AJR 141:1195-1201, 1983. 2. Borcard, B., Prog. Nucl. Med. 8~41-54, 1984. 3. Hutchison, J.M.S., Smith, F.W., Partain, CL.. James, A.E., Rollo, F.D., Price, R.R., eds. “Nuclear Magnetic Resonance (NMR) Imaging”, pp. 23 1-249. Philadelphia, PA: W.P. Saunders, 1983. 4. Sepponen, R.E., Sipponen, J.T., Sivula, A., J. Comput. Assist. Tomogr. 9~237-241, 1985. 5. Sipponen, J.T., Sepponen, R.E., Tanttu, J.I., Sivula, A., J. Comput. Assist. Tomogr. (In press) 1985. 6. DeLaPaz, R.L., New, P.F.J., Buonanno, F.S., et al., J. Comput. Assist. Tomogr. 8599-607, 1984.
Chemical Shift Imaging, Technical Requirements Derek Shaw I.G.E. Medical Systems, England
260, Bath Road, Slough,
The technical requirements for chemical shift imaging, which for the purposes of this talk will be defined as procedures which produce localised spectroscopic information (i.e. M, x, y, 6) can be summarised as follows: a high, homogeneous magnetic field, a method of localisation, and a method of display. The first of these requirements is self-evident; the field must be high enough to produce sufficient chemical shift dispersion (>l T) and uniform enough to permit the lines of interest to be resolved (to.5 ppm). The third requirement depends on whether the data is displayed as an image of a specific chemical shift, i.e. M, (x, y) or as a spectrum from a specific location, i.e. M,,(6). The next most challenging requirement is the method of localisation. The talk will concentrate on this aspect of the topic. The first major subdivision of localisation methods which can be made is between those methods which use linear field gradients (i.e. imaging techniques) to provide the required localisation and those which do not. Non-Linear Field Gradient Localisation Techniques Techniques in this group depend basically on the properties of surface coils [ 11. The size and shape of the coil provide the primary source of iocalisation, the signal arising from an approximately hemispherical region with the same radius as the coil (for a circular coil). The problem axis with the surface coils is the one perpendicular to the plane. Depth resolution with surface coils can be achieved by simple surface nulling
Magnetic
Resonance
Imaging
2-D multislice include most efficient with long-TR, short-TE sequences; high resolution performed with reasonable scan times; gaps between slices; and reduced motion artifacts. The advantages of 3-D methods include improved signal-to-noise (S/N) ratio proportional to the square root of the number of slices; thinner slices more easily achieved; longer TE’s and shorter TR’s more easily achieved; best efficiency with short-TR sequences; no gaps between slices; reduced paradoxical enhancement; and images are at the same phase of the cardiac cycle. The selective 3-D method limits the 3-D acquisition to a selected “slab” of tissue. This method is useful when a limited number of thin, conteguous slices are needed such as in surface-coil applications. Single spin-echo sequences are useful in T, weighted scans where the maximum number of slices per 2-D multislice acquisition can be obtained. For maximum efficiency, multiecho sequences in 2-D multislice are best used with long TR. The first echo in a long-TR scan will be primarily N (H) weighted with each additional echo being more T, weighted. Multiecho sequences in 3-D acquisitions with shorter TR’s can be obtained so that the first echo is primarily T, weighted and each additional echo is more T, weighted. Inversion spin echo is more efficient with 2-D multislice acquisitions because of the need for a longer TR. For demyelinating disease, dysmyelinating disease, and screening exams, our choice is a 2-D multislice acquisition and a multiecho pulse sequence (2-4 echoes). If a suspected infarct, hemorrhage or tumor is detected then either an inversion spin-echo sequence for T, weighting or a 3-D exam is performed. For patients with suspected tumor, infarct, or hemorrhage, we use a 3-D multiecho with 8 echoes of TE 30-240 and TR 800. Using this method, acute, subacute, and remote infarcts, acute and subacute hemorrhage, and low-grade and high-grade tumors may be distinguished. Additional specificity may be achieved with a variety of other lesions. Most acoustic neuromas are better identified on N (H) weighted images. Other acoustic neuromas are better seen on T, weighted images. Therefore, a multiecho multislice with both N (H) and T2 weighting is used. Careful planning of the MR examination will result in a more efficient, sensitive, and specific study. NMR Imaging Versus Nuclear Medicine Peter Pfannenstiel Departments for Nuclear Medicine and NMR, Deutsche Klinik f iir Diagnostik, Aukammallee 33, 6200 Weisbaden, FRG The latest development of diagnostic imaging modalities were influenced by the impact of alternative
0 Volume 3, Number
3, 1985
methods such as ultrasonography, digital subtraction angiography and, most recently, the technique of nuclear magnetic resonance (NMR). Even so it seems clear that the new NMR equipment will not be installed as rapidly as X-ray CT equipment, since the NMR method must compete with the widespread CT experience; however, there is no question, that NMR will in the near future impact the number of other examinations currently performed. As the technology of nuclear medicine becomes more complex with positron and single-photon tomography the future will be influenced by these techniques. Even though NMR is no competitor in the study of regional biochemistry when compared to nuclear medicine, NMR will have a negative impact on nuclear medicine by reducing the number of examinations currently performed. Clearly the image quality of NMR is superior to PET and SPECT scans. Today, in routine clinical work, nuclear medicine seems to be superior to NMR in studying organ function and in vivo metabolism, and it is also superior to organ selectivity. However, the future of NMR imaging in medicine seems bright. Absence of known biological hazards and its ability to measure multiple tissue parameters will make it the study of choice in many clinical situations. Its ability to create detailed tomographic images in any plane with both excellent anatomic delineation and tissue specificity is a revolution in diagnostic imaging. The additional information gained by providing in vivo data concerning human biochemistry and pathophysiology may change our entire understanding of health and disease. A combination of both NMR tomography and NMR spectroscopy is envisioned. NMR spectroscopy is a possible quantitative method of measuring biochemical processes in vivo, for instance “P in different phosphorous metabolites in patients with enzyme defects. In the future there will be a relationship between nuclear medicine and NMR techniques. The two techniques will act in the same direction by adding structural and functional data that have not been obtained before by any other technique. In this way nuclear medicine and NMR will work together rather than in opposition. The promise of NMR imaging is based upon its capability of combining three parameters: the image quality of CT scanning, the safety of ultrasound and the selectivity of nuclear medicine. Since NMR has a number of points in common with nuclear medicine and promises to be a powerful complementary modality, the participation of nuclear medicine physicians, physicists, chemists and technologists in NMR training and research efforts should be encouraged. Clinical efficiency studies will be defining the role and limitations of NMR techniques in comparison with various other imaging and diagnostic
Abstracts
modalities over the next 5-10 years. It seems to be important to work closely together during this evaluation period. Low-Field Magnetic Experiences
Resonance
Imaging-Clinical
R. E. Sepponen Helsinki University Central Hospital, NMR Laboratory and Instrumentarium Corp., Helsinki, Finland The effect of the field strength on the contrast of MR images is well recognized. The increase of the strength of the polarizing magnetic field increases the signalto-noise ratio and improves the resolution of the final image [I]. However, the effectivity of various relaxation mechanisms changes with the resonance frequency. The relaxation processes due to the macromolecular movements are effective at low resonance frequencies [ 21. The diagnostic effectivity of low-field imaging has been demonstrated [3,4, 51. A prototype unit (ACUTSCANB, manufacturer Instrumentarium Corp., Helsinki, Finland) operating at the field strength 0.02 T (corresponding proton resonance frequency 0.8 MHz) was used. Inversion recovery and spin-echo sequences were used in order to generate T, and T, weighted images. The imaging method was 2-DFT. The data acquisition was carried out as a spin echo. The selective excitation method was used to define the imaging plane. In order to reduce the imaging time the multislice technique was utilized. The benefits of the low-field operation may be summarized as follows: (1) Relaxation time T, of tissues at low field reflects macromolecular differences of tissues. The tissue contrast is demonstrated in the diagnosis of intracranial hematomas. At the operating field strength of the unit the relaxation time T, of freshly extravasated blood is shorter than the T, of the surrounding brain tissue. This facilitates the differential diagnosis of acute hemorrhage from other pathologies which in the most cases has long relaxation times T, and T, [5]. At 0.2 T and above the relaxation time of blood is equal or longer than that of the surrounding brain tissue [5, 61. (2) The fringe fields are small and the missile effects are virtually eliminated. The 0.5 mT field extends 1.2 m from the ends of the solenoid and is usually well confined in the installation room. Thus usually no special safety measures are needed against inadvertent magnetic field exposures (e.g. pacemaker patients). (3) The unit may be constructed based on inexpensive resistive magnet technology. The magnet is a solenoid with end-correction coils. The absolute inhomogeneities are small and thus long data collection
309
time may be used in order to maximize the signalto-noise ratio. (4) The unit may be installed without extensive site preparation. The weight of the magnet is 850 kg and the total system weight is 1550 kg. The total power consumption is 7 kW and typical water consumption for cooling of the system is 4-6 l/min. Due to the small fringe fields magnetic shielding is not needed. I. Hart, H.R., Bottomley, P.A., Adelstein, W.A., et al., AJR 141:1195-1201, 1983. 2. Borcard, B., Prog. Nucl. Med. 8~41-54, 1984. 3. Hutchison, J.M.S., Smith, F.W., Partain, CL.. James, A.E., Rollo, F.D., Price, R.R., eds. “Nuclear Magnetic Resonance (NMR) Imaging”, pp. 23 1-249. Philadelphia, PA: W.P. Saunders, 1983. 4. Sepponen, R.E., Sipponen, J.T., Sivula, A., J. Comput. Assist. Tomogr. 9~237-241, 1985. 5. Sipponen, J.T., Sepponen, R.E., Tanttu, J.I., Sivula, A., J. Comput. Assist. Tomogr. (In press) 1985. 6. DeLaPaz, R.L., New, P.F.J., Buonanno, F.S., et al., J. Comput. Assist. Tomogr. 8599-607, 1984.
Chemical Shift Imaging, Technical Requirements Derek Shaw I.G.E. Medical Systems, England
260, Bath Road, Slough,
The technical requirements for chemical shift imaging, which for the purposes of this talk will be defined as procedures which produce localised spectroscopic information (i.e. M, x, y, 6) can be summarised as follows: a high, homogeneous magnetic field, a method of localisation, and a method of display. The first of these requirements is self-evident; the field must be high enough to produce sufficient chemical shift dispersion (>l T) and uniform enough to permit the lines of interest to be resolved (to.5 ppm). The third requirement depends on whether the data is displayed as an image of a specific chemical shift, i.e. M, (x, y) or as a spectrum from a specific location, i.e. M,,(6). The next most challenging requirement is the method of localisation. The talk will concentrate on this aspect of the topic. The first major subdivision of localisation methods which can be made is between those methods which use linear field gradients (i.e. imaging techniques) to provide the required localisation and those which do not. Non-Linear Field Gradient Localisation Techniques Techniques in this group depend basically on the properties of surface coils [ 11. The size and shape of the coil provide the primary source of iocalisation, the signal arising from an approximately hemispherical region with the same radius as the coil (for a circular coil). The problem axis with the surface coils is the one perpendicular to the plane. Depth resolution with surface coils can be achieved by simple surface nulling