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A multisection, two-dimensional, Fourier transform, double-gradient-echo magnetic resonance imaging sequence with partial flip-angle excitation and section doubling by radiofrequency encoding approximately doubles the signal-to-noise ratio obtainable from a conventional spin-echo sequence at low field strength, while maintaining essentially equivalent contrast.

The open architecture of a permanent magnet MR system, a thin solenoidal coil and the addition of a simple positioner permit knee imaging under extreme degrees of flexion.

Three-dimensional Fourier transform (3-DFT) MR imaging Offers advantages in terms of SIN per unit of time for the case where a large number of slices is desired. This advan- sidered briefly in this paper. The first, contrast, is prob- ably the single most important one. Nevertheless, the at- tage is enhanced when T, is short. Because time limitations in tention Of the community is focused On the 3-DFT imaging force the use of short TR times, lesion contrast second One, and in MR in particular, the perception is often undesirable at mid- and high-field strength even when the SIN is good. At low fields, where T, values are short, high S/Nand contrast can both be achieved with 3-DFT MR images. The conceptual and analytical aspects of low-field 3-DFT MRI are presented and demonstrated at 640 G. Of its relation to field strength has important COIlSequenCeS in terms of cost.

Switched magnetic fields (in addition to the usual pulsed magnetic gradient fields) aiding and/or opposing the usual constant static magnetic field Bo are utilized so as to increase the signal-to-noise ratio for given available imaging sequence times and/or to provide special imaging effects.

A total of 58 multiple sclerosis lesions from 16 patients were used to characterize the performance of a low field system for the detection of edematous (e.g., water-elevating) brain lesions. Contrast, signal-to-noise, and their product (signal difference to noise) were measured for two-dimensional and three-dimensional Fourier transform techniques at low field strength (640 G) and compared to two-dimensional sequences at mid field strength (3,500 G). The results showed numerically that low-field strength magnetic resonance imaging can reliably detect multiple sclerosis lesions, and, by extension, other water-elevating lesions, although with lower confidence levels.

In magnetic resonance imaging (MRI), the signal intensity depends indirectly on particular physical and chemical characteristics of the tissues being imaged. These tissue properties influence the behavior of the nuclei undergoing resonance and their behavior is what directly affects the MRI signal. The parameters of interest that describe this behavior are the relaxation times Tl and T2, the spin density [for hydrogen, N(H)], and the microscopic (diffusion) and macroscopic (flow, motion) motional states of the nuclei. As has often been repeated, different imaging techniques result in different responses to these magnetic resonance (MR) parameters. Early speculation suggested that the MR parameters, in particular the relaxation times, would provide very specific diagnostic information on the state of tissue within the body. In fact the results in this area have been disappointing. While MRI has been extremely sensitive to disease within the body’s soft tissue, the specificity is much poorer. It is often difficult to make specific comments on the character of the lesion based on the MR data alone. Yet when properly used, the MR parameters can provide valuable information.

An electromagnet shim coil is utilized for temporarily altering the shape of a volume in which there is provided a substantially homogeneous NMR polarizing field. By temporarily energizing the electromagnet shim coil and thus altering the shape of the volume, magnetic resonance imaging can take place in other than a substantially spherical volume (e.g., in an elongated ellipsoidal-like volume extending axially along a patient so as to encompass a longer section of the spinal column). In the exemplary embodiment, the electromagnet shim coil takes the form of a pancake-like coil with windings positioned so as to create fourth power spherical harmonic in a transverse magnet-type of MRI system.

An array of plural magnetic resonance imaging (MRI) RF coils is provided having different and overlapping fields of view. Controllable switches are connected with each individual coil of the array and are capable of selectively conditioning any one of the coils for individual usage in an MRI procedure. Either mechanical or electrical (e.g., PIN diode) switching control may be utilized. Preferably, controllable electrical switches are located at points having approximately zero RF potential. Distributed capacitance is also preferably employed for reducing terminal inductance, preventing the establishment of spurious magnetic fields and facilitating the use of electrical switching diodes and/or varactor capacitance elements. Such distributed capacitances are also dimensioned so as to cause the terminal inductance of each coil to be within the tuning/matching range of a common tuning/matching RF circuit.

A bridge conductor (104) for the turns of an MRI RF coil may be connected serially within a connector joint area of an inductive coil (100) so as to selectively increase its physical size (e.g., so as to accommodate larger patient volumes to imaged therewithin). Serial capacitance may be included in at least one of the bridging conductors so as to substantially reduce the net inductive impedance of the added bridge conductors such that the standard coil RF tuning and impedance matching circuits may still operate within their normal predetermined adjustable ranges.

At least one extra NMR measurement cycle is performed without any imposed magnetic gradients during readout and recordation of the NMR RF response. Calibration data derived from this extra measurement cycle or cycles can be used for resetting the RF transmitter frequency and/or for phase shifting other conventionally acquired NMR RF response data to compensate for spurious changes in magnetic fields experienced during the NMR data measuring processes. Some such spurious fields may be due to drifting of the nominally static magnetic field. Another source of spurious fields are due to remnant eddy currents induced in surrounding conductive structures by magnetic gradient pulses employed prior to the occurrence of the NMR RF response signal. Special procedures can be employed to permit the compensation data itself to be substantially unaffected by relatively static inhomogeneities in the magnetic field and/or by differences in NMR spectra of fat and water types of nuclei in imaged volumes containing both.

N UCLEAR magnetic resonance (NMR), the exciting new imaging modality, promises to combine the advantages of CT and ultrasound by offering excellent resolution of its tomographic image, the use of nonionizing radiation, and the capability of imaging in any desired plane. Like nuclear medicine, it is expected to supply information about. metabolic processes and about the physiologic status of tissues and organs. It does not appear to have any biologic risks in the imaging energy range.’

Nuclear magnetic resonance (NMR) images of live rats with sterile and pyogenic abscesses, hematomas, and various implanted and spontaneous neoplasms demonstrated good contrast differentiation between pathologic and surrounding normal tissues. This differentiation was maximal when both the T1 and T2 tissue relaxation times were used as criteria. Neoplasms have a broad range of T1 and T2 values and may be confused with abscesses or hematomas. Tissue rate constants (1/T1 and 1/T2) are mainly dependent on total water content, the exception being fat, which has a 1/T2 value much shorter than that expected on the basis of water content alone.

Practical constraints make it difficult to build large-aperture echo-planar magnetic resonance (MR) imagers. The implementation of a pediatric imager and its performance are described. Spatial resolution and signal-to-noise levels comparable to those of 1982 state-of-the-art MR imagers have been achieved in imaging times of 0.05-0.15 seconds. T1 and T2 information are obtainable in the echo-planar mode. A major issue is that of chemical-shift displacements.

Thirty healthy subjects and 15 patients with a variety of musculoskeletal disorders were examined by conventional radiography, computed tomography (CT), and nuclear magnetic resonance (NMR). NMR proved capable of demonstrating important anatomic structures in the region of the lumbosacral spine. Lumbar disk protrusion was demonstrated in three patients with CT evidence of the disease. NMR appeared to differentiate annulus fibrosus from nucleus pulposus in intervertebral disk material. Avascular necrosis of the femoral head was demonstrated in two patients. The cruciate ligaments of the knee were well defined by NMR. Muscles, tendons and ligaments, and blood vessels could be reliably differentiated, and the excellent soft-tissue contrast of NMR proved useful in the evaluation of bony and soft-tissue tumors. NMR holds promise in the evaluation of musculoskeletal disorders.

Computational techniques allow the simulation of NMR images as well as the construction of tools for evaluating the impact of changing parameters. Use of these tools assumes the acquisition of a minimal set of patient data from which the tissue properties T1, T2 and hydrogen density H, can be taken. Changes in technique, imaging parameters such as TE, TR or TI, and field can be simulated and examined individually as well as concurrently for their effects on the contrast between a lesion and its surrounding tissue. This evaluation is done by plotting the absolute difference in signal intensities as a function of the imaging parameters of the technique. Retrospective calculations of intensity images at new values of the pulse timing parameters may show improved diagnostic utility.

Although cross-sectional magnetic resonance examination of the head and body is useful for screening large regions of tissue, subsectional regions of the head and body often need to be examined. Orthogonally directed, selectively irradiated planes with different flip angles produce a spatially limited signal region from which two- or three-dimensional volume images can be reconstructed. Images with limited fields-of-view can be acquired in reduced imaging time. We present a general description of this technique. These subsectional or "inner volume" images eliminate respiratory motion artifacts by excluding moving tissues from the imaged volume. A result of this technique is a high signal from rapid pulsatile blood flow, produced without cardiac gating the pulse sequence.

NMR imaging of rats with implanted hepatomas in the liver demonstrates that under the imaging conditions of this study tumors of over 8 mm2 in area can be detected with high accuracy. Compared to normal liver, approximately 70% of these tumors had a combination of relaxation times (T1-T2) that could be uniquely identified as tumors, while the other 30% demonstrated relaxation time combinations that overlapped those previously found for abscesses, brain, and hematomas.

Theoretical analysis predicts that performing magnetic resonance (MR) imaging with partial (less than 90 degrees) flip angles can reduce imaging times two- to fourfold when lesions with elevated T1 values are being examined. This time savings occurs because repetition time (TR) is reduced when imaging is performed with partial flips. Partial flip MR imaging can also improve signal-to-noise ratio (S/N) in fast body imaging. For this study, analytical tools were used to predict image contrast and S/N for short TR, partial flip sequences. Experimental implementation of the short TR, partial flip sequences that analytical work had predicted would be optimal supported the analytical predictions and demonstrated their validity. Partial flip MR imaging is applicable to reducing imaging time only when the ratio of signal differences to noise exceeds threshold values in conventional MR images. Partial flip sequences can be used to advantage in MR imaging of both the head and the body, and the observed effects are predictable through theoretical analysis.

A study of 24 rats implanted in the hind leg with mammary adenocarcinomas and five sham-implanted rats, followed from the second to the eight week postimplantation, showed nuclear magnetic resonance imaging capable of detecting all the tumors without yielding any false-positives in the control rats. The T1 relaxation time of tumors overlapped that of muscle, and the T2 times overlapped fat, but the combination was unique when comparing tumors to muscle and fat. Necrotic regions of the tumor and the bladder contents tended to have very long T1 and T2 relaxation times. The difference in relaxation time between tumors and muscle could be accounted for in terms of water content, which was approximately 8% higher for the tumors. The study corroborates data from previous studies indicating that NMR imaging is a highly sensitive modality, although T1 and T2 times are not exclusive indicators of malignancy.

Fibrosarcoma tumor cells were implanted in the hind legs of 25 rats and studied from the first to the sixth week postimplantation. Nuclear magnetic resonance imaging detected all tumors and did not yield any false-positives in five control rats. The T1 relaxation values of tumors overlapped those of muscle, and the T2 values overlapped those of fat, but the combination of the two values allowed discrimination of each of the three tissues with no overlap. The difference in relaxation time between tumor and muscle could be accounted for on the basis of water content, which was approximately 14% higher in the tumors. This study confirms data from previous studies suggesting that nuclear magnetic resonance imaging is a highly sensitive modality, but that T1 and T2 values are not specific for individual pathologic conditions.

Human images of excellent quality have been obtained with Nuclear Magnetic Resonance (NMR), using two different techniques: projection reconstruction (PR) and two-dimensional Fourier transformations (2DFT). It has been shown in CT that the reconstruction algorithms will multiply the noise in the projections and change the noise spectrum. A comparison of the noise of the two techniques is presented under the constraint of equal imaging conditions using both computer simulations and data obtained with the UCSF human imager. The computer simulations show that the two techniques produce approximately equal signal to noise values when S/N ~ 15. Below that value small differences are observed. For both simulations and actual data, the 2DFT shows a uniform spatial frequency response while the PR method shows a roll-off at low spatial frequencies. The 2DFT method is shown to be more resistant to artifact formation.

Ten patients with definite multiple sclerosis underwent hydrogen nuclear magnetic resonance imaging with a 3.5 kilogauss superconducting magnet, using the inversion recovery and spin-echo techniques of signal acquisition. Results were compared with high-resolution x-ray computed tomography. Spin-echo images demonstrated abnormal regions as areas of variably increased signal intensity. The contrast between abnormal and normal white matter improved as the intervals between sequential radiofrequency pulses and between pulse administration and signal sampling were increased. Inversion recovery images demonstrated abnormal areas as regions of decreased signal intensity but did not visualize lesions as well as spin-echo imaging. Spin-echo and inversion recovery imaging each demonstrated more extensive abnormalities than did computed tomography.

Eleven Sprague-Dawley rats with experimentally produced infarction of the lower extremities were imaged by nuclear magnetic resonance (NMR) immediately after infarction, and 6 and 24 hours later. Contrast between the infarcted and control muscles was noted immediately and continued to increase through 24 hours. These changes corresponded to a significant increase in the water content of the infarcted extremity at 24 hours. These results suggest promise for NMR imaging in the identification of myocardial infarction and should encourage further investigation in this area.

This article provides some examples of what the previous article outlined. The sections on Image Display and Myocardial Characterization contain numerous illustrations of gating, cross-sectional images, flow signals, and ischemic injury. The possibility of metabolic imaging with NMR and quantitating blood flow is also considered.

A technique is described for obtaining tomographic images of hydrogen distribution in animals using nuclear magnetic resonance (NMR). Resonant frequency is proportional to magnetic field strength, so that spatial resolution is achieved by frequency selection and magnetic field shaping. The results of scanning a phantom and two rats are presented.

To test if changing magnetic fields and radiofrequency fields used in nuclear magnetic resonance imagers could induce electrical currents capable of causing localized tissue heating in metal surgical clips and prostheses, steel surgical clips, copper wire clips, and hip prostheses were exposed to fields greater than those used in the nuclear magnetic resonance imager. Observations indicated that no significant heating should be expected from implanted surgical clips during exposure. The heating of larger metallic implants should be further investigated.

The sequence of radiofrequency and magnetic field gradients used to form an NMR image produces changes in the signals emitted by moving blood when compared to the signals from stationary blood. These changes can be used to measure the open area of vessels. In addition, specific relaxation time signatures of lesions may help in identifying the nature of the obstructions.

Whole body nuclear magnetic resonance (NMR) imaging of the cardiovascular system was carried out in early clinical trials in 244 volunteers and patients using a 3.5 KGauss (0.35 T) unit. The spin echo technique with multiple imaging parameters was used. Blood vessels were clearly discriminated from solid organs and lesions because little or no intraluminal signal is seen with laminar blood flow at normal velocities, whereas a more intense image is generated by solid organs. Characteristic flow signals were observed in normal patients and were accentuated by varying the imaging parameters. Cardiac chambers were well delineated in some patients on nongated images. In one case, internal topography of the ventricles was exquisitely displayed on a gated image. Intraluminal pathology, such as dissection of the aorta, aneurysms of the aorta and left ventricle, and aortic atheroma, was clearly demonstrated. Patency of coronary arterial bypass grafts was shown. Abnormal flow patterns due to slow or turbulent flow were accentuated on images using the second spin echo. This preliminary experience indicates the considerable potential of NMR imaging in the evaluation of cardiovascular diseases.

The availability of semiconductor detectors, principally High Purity Germanium, permits consideration of new generations of nuclear medicine imaging devices. These are characterized by excellent spatial resolution and by simultaneous multiple radioisotope imaging capabilities. The technology and some of its resultant benefits are presented here.

This paper describes an MRI technique which can be used to acquire images at short TR values while maintaining the sensitivity to disease found in longer TR images. For spin echo imaging there are three acquisition parameters that can be set in the imaging protocol; TR, the repetition interval; TE, the time of echo and THETA, the excitation flip angle. Standard imaging techniques set THETA to 90 degrees regardless of the TR value. With THETA fixed, imaging systems have been optimized by varying the value for TE and TR with the results in general indicating the need for long TR values. However, if the flip angle is included as a variable acquisition parameter the optimal operating point can be changed. The solution to the Bloch equation shows a functional relationship between the flip angle and the ratio TR/T1. This functionality was first observed by Ernst and Anderson as a method to increase the signal generated in fourier transform magnetic resonance spectroscopy. When TR/T1

The role of nuclear magnetic resonance (NMR) imaging of the kidney was analyzed in 18 persons (6 normal volunteers, 3 patients with pelvocaliectasis, 2 with peripelvic cysts, 1 with renal sinus lipomatosis, 3 with renal failure, 1 with glycogen storage disease, and 2 with polycystic kidney disease). Ultrasound and/or computed tomography (CT) studies were available for comparison in every case. In the normal kidney distinct anatomical structures were clearly differentiated by NMR. The best anatomical detail was obtained with spin echo (SE) imaging, using a pulse sequence interval of 1,000 msec and an echo delay time of 28 msec. However, in the evaluation of normal and pathological conditions, all four intensity images (SE 500/28, SE 500/56, SE 1,000/28, and SE 1,000/56) have to be analyzed. No definite advantage was found in using SE imaging with a pulse sequence interval of 1,500 msec. Inversion recovery imaging enhanced the differences between the cortex and medulla, but it had a low signal-to-noise level and, therefore, a suboptimal overall resolution. The advantages of NMR compared with CT and ultrasound are discussed, and it is concluded that NMR imaging will prove to be a useful modality in the evaluation of renal disease.

Although the basic physics of the NMR process dates back about 40 years, and the demonstration that images could be formed from NMR signals is over a decade old, only recently has NMR imaging come to receive wide attention. Analysis of the hardware of a system shows that success depends less on stressing the state‐of‐the‐art of individual components than on careful engineering design and thoughtful integration of existing technologies.

Nuclear magnetic resonance (NMR) is used to generate cross-sectional images of the human body that show excellent anatomic and functional definition. The NMR imaging process involves interactions between electromagnetic fields and the hydrogen nuclei being imaged. These interactions occur on time scales of milliseconds to seconds. Consequently, the motion of these nuclei, for instance, when carried by blood, produces distinct signatures that are used to assess flow in major vessels. Myocardial dyskinesis also produces visible effects. Because of these effects, NMR imaging may be a safe and effective tool in the diagnosis and assessment of cardiovascular disease.

NMR imaging is based on the ability to induce and monitor resonance of the magnetic moment of nuclei in the presence of magnetic fields. By the use of magnetic fields whose strength varies with position, it is possible to define both the location and concentration of resonant nuclei, and, thereby to create images that reflect their distribution in tissue. Hydrogen, because it is the most sensitive of the stable nuclei to NMR and because it is also the most abundant nucleus in the body, is ideally suited for NMR imaging.

Conjugation can be used to synthesize half of the data acquired during a conventional two-dimensional Fourier transform imaging procedure, thus reducing imaging time by nearly half. The images acquired by this process have the same object contrast and spatial resolution as conventional images do, but with a 40% reduction in the signal-to-noise ratio (S/N). Conjugation can be used to advantage in magnetic resonance imaging units in which S/N levels are higher than needed to permit imaging with a single acquisition of each projection.

The use of non-radioactive tracers for measuring certain clinically significant parameters by x-ray fluorescence analysis (XRFA) is well established at our institution (1,2). Among the technique's advantages are high accuracy, simplicity, cost effectiveness and reduced or eliminated radiation exposure to patients. One of the more versatile tracers is iodine, which has a low body toxicity and desirable chemical properties, e.g. it is easily bound to organic molecules. Other elements of interest are cesium (3), a potassium analogue of low toxicity; and xenon, the highest atomic number, stable, noble gas.

The signal-to-noise ratio (S/N) in magnetic resonance imagining is one of the variables that must be measured when comparing the relative performance of different techniques. Although various investigators and official groups have proposed different methods for measuring S/N, these are generally not practical for use by a physician working in a clinical situation. The authors present a simple method that should serve for estimating S/N in most cases.

Phase-sensitive imaging was used to correlate signal distribution with phase shift and velocity distribution in spin-echo magnetic resonance imaging (MRI). Flow-dependent, changing intensity patterns that were seen in a constant-flow phantom study were explained by the simultaneous effects of inflow signal enhancement, first-echo dephasing, and outflow signal loss occurring during laminar flow. In clinical studies, first-echo dephasing was shown during laminar flow in the inferior vena cava. Turbulent flow was demonstrated in the descending thoracic aorta during late systolic flow, and turbulent dephasing-rephasing was shown in the abdominal aorta.

The NMR image is dependent on multiple tissue parameters: hydrogen density, relaxation times T1 and T2, and flow. From the acquired intensity images, T1, T2, and hydrogen images may be produced. The signal/noise of these derived images is critically dependent on the choice of acquisition parameters. Using the calculated T1, T2, and hydrogen images, intensity images may be calculated at arbitrary values of TE and TR, some of which (for example, short TE or TR) may be physically unacquirable.

Nuclear magnetic resonance (NMR) imaging characteristics of the normal and abnormal adrenal gland were evaluated and compared with findings on computed tomography (CT). Forty-two patients were examined: 36 had normal adrenal glands and 6 had adrenal disease (3 metastatic lesions, 1 pheochromocytoma, and 2 cortical hyperplasia). NMR clearly showed all 42 left adrenals (100%) and 36 right adrenals (86%). In some patients, it appeared to differentiate the adrenal cortex from the medulla. The ability of NMR to detect adrenal disease was similar to that of CT in 6 cases examined. CT demonstrated superior spatial resolution in most cases, but NMR provided superior soft-tissue contrast. Since NMR does not involve ionizing radiation and provides excellent soft-tissue differentiation without contrast material, it has advantages over CT and appears to be a promising modality for imaging of the adrenal gland.

A preliminary study of the relation between food intake and intensity of gallbladder bile on nuclear magnetic resonance (NMR) images was made. Twelve subjects (seven volunteers, five patients) were imaged following a minimum of 14 hours of fasting. Six of seven volunteers were reimaged one hour after stimulation by either a fatty meal or an alcoholic beverage. An additional seven patients were imaged two hours after a hospital breakfast. It was found that concentrated bile emits a high-intensity spin echo signal (SE), while hepatic bile in the gallbladder produces a low-intensity SE signal. Following ingestion of cholecystogogue, dilute hepatic bile settles on top of the concentrated bile, each emitting SE signals of different intensity. The average T1 value of concentrated bile was 594 msec, while the T1 value of dilute hepatic bile was 2,646 msec. The average T2 values were 104 msec for concentrated bile and 126 msec for dilute bile. The most likely cause for the different SE intensities of bile is the higher water content, and therefore longer T1 and T2 relaxation times, of hepatic bile. It is suggested that NMR imaging has the ability to provide physiological information about the gallbladder and that it may prove to be a simple and safe clinical test of gallbladder function.

The pressurized xenon-filled multi-wire proportional chamber (MWPC) is an imaging detector with high intrinsic spatial resolution and good uniformity of response. We describe the design and operation of a 20 x 20 cm2 MWPC with a mass of 0.14 g/cm2 of xenon, maintained at 60 psi absolute. Electromagnetic delay-lines are used to obtain position information. Imaging studies have been performed using a Nuclear Chicago low-energy high-resolution collimator. The intrinsic resolution of the system was determined for 60 keV and 140 keV photons using bar phantoms, and was found to be 1 mm and 2 mm respectively. The Modulation Transfer Function for 140 keV is down to 70 percent response at 1.4 cycles per cm. Stopping-power is about 65 percent for 1-125 and 10 percent for Tc-99m. The distribution of thyroid hormones in the rat was clearly demonstrated. Picker thyroid phantoms filled with 1-125 and TC-99m were imaged with good detail. Preliminary 1-125 thyroid studies in humans have shown a promising potential. Other studies in rats have included liver/spleen imaging with Tc-99m-sulfur colloid and skeletal imaging with Tc-99m-polyphosphate. The excellent spatial resolution of the detector, together with the addition of higher efficiency collimators and operation at higher pressures realize a system with characteristics that are adequate for clinical use. These components are now being incorporated in a 30 x 30 cm2 MWPC.