Magnetic resonance medical imaging is becoming a very important hospital lab equipment tool in the clinical management of patients. The information density of this hospital lab equipment is that the nuclear magnetic resonance (NMR) signal is greater than that avail¬able in other hospital lab equipments like computed tomography (X-ray, CT SCAN) because the magnetic resonance signal is based on four separate com¬ponents: density of the nuclear species (usually hydrogen, which is composed of a single proton-H), two relaxation times (T1 and T2), and motion or flow. The computed tomography (X-ray, CT SCAN) radio¬graphic image, by comparison, is based mainly on one tissue char¬acteristic: electron density. The information available in nuclear mag¬netic resonance (NMR) images will in all likelihood be further en¬riched when other nuclear species such as phosphorus and carbon can be scanned in clinical NMR scanners. The signals from nuclear species other than H are low, and therefore, the likely format will be to superimpose phosphorus or carbon images or spectra on hy¬drogen density images either by color tinting of the hydrogen image or by superimposing a black dot distribution format similar to auto¬radiography. Another aspect of NMR imaging that will encourage this hospital lab equipment’s clinical utilization is that magnetic resonance imaging does not use ionizing radiation and thus is free of the potential hazards of X-ray interaction with tissue.
The practicing physician, whether an imaging specialist or primary care physician, may never have encountered NMR imaging and may have previously considered NMR as merely a chemical phenomenon useful for sophisticated chemical analysis. The principles of NMR are actually first described in 1946, and NMR imaging has been used since 1973. Although clinical medical applications of these imaging techniques have only recently been reported, NMR imaging has already been shown to be clearly superior to other competitive imaging modalities like CT scanning in certain specific situations. NMR scans are free from the artifacts produced in CT scans by sharp dense bone or metallic surgical clips. NMR is the imaging modality of choice in diagnosing mul¬tiple sclerosis, and by selecting the proper NMR imaging techniques, in determining accurately areas of edema and hemorrhage which are difficult to separate by X-ray or CT.
Whether a nodular density in the lung hilum is a normal pulmonary blood vessel or an abnormal tumor mass is a difficult clinical problem with CT scanning or conventional radiography; however, in NMR imaging no signal is returned from rapidly flowing blood, and a very strong signal is returned from be¬nign or neoplastic masses. Thus, this specific clinical problem is easily resolved using NMR. Gated NMR scans appear to be excellent for imaging the myocardium because the flowing blood in the ventricle (no NMR signal) accurately outlines the cardiac muscle. At first glance NMR scans appear very much like CT scans, but there are very definite differences. A careful examination reveals that the circular high-density region is the bony skull and calvarium on the CT scan with little of the subcutaneous fat and scalp tissue in evidence. However, in the NMR scan the high-density (white) ring structure corresponds to the subcutaneous fat and the bone is seen as a relatively dark or black ring inside the outer white ring of fat. Some white and gray areas within the black ring of the skull indicate the bone marrow, which has a relatively high fat content, and therefore a larger NMR signal than surrounding the bone.
The tempo of change in medical imaging has been accelerating, and it is continuing to accelerate at a bewildering rate even for those whose expertise is primarily in the imaging sciences. If we borrow a bit of imagery from Carl Sagan and create an imaging calendar to relate chronologically the major breakthroughs in imaging as Sagan related the major events in the development of the cosmos on a galactic calendar, spanning the events from the last big bang to the present, we find our “imaging big bang” occurs with the discovery of the roentgen ray by Wilhelm Conrad Rontgen in 1895. We can see that January in this imaging calendar year was truly a big bang with fluoroscopy, X-ray tubes and X-ray films, all developed in those early stages. Everything else through September of this year was really just a variation on a theme, which required a slightly dif¬ferent application of previously well-understood principles. However, in November things began to pick up with CT imaging, and an imaging “mini-bang” really exploded in December with clinical NMR imaging, digital imaging, digital subtraction angiography (DSA), laser imaging, and medical holography.
Nuclear Magnetic Resonance
The NMR phenomenon was originally described at Stanford by Block and at Harvard by Purcell in 1946. They later received the Nobel prize in 1952 for their work. Briefly, NMR imaging, with the use of the hospital lab equipment called an NMR scanner, is a way of making pictures of the body that look somewhat like CT scans. Atoms in the body can act like tiny bar magnets with a north and south pole. When an external magnetic field is applied across a part of the body, each little magnet lines up with the external magnetic field. If a radio wave is then broadcast into the body tissue, some of the magnets absorb some of the energy from the radio wave’s energy and tilt over. The radio wave is then turned off, and subsequently the magnets rebroadcast the signal they absorbed. An antenna can pick up this rebroad¬casted signal, and then a computer can make a picture (scan) from the signal.
In most substances, including living tissues, some of the nuclei within the specimen act as tiny magnets when the specimen is placed in a stable magnetic field. For instance, if you were to place the north and south poles of a bar magnet around your index finger, these nuclear magnets (proton nuclei of hydrogen atoms) would orient themselves either parallel or antiparallel to that magnetic field, with the antiparallel protons having a slightly higher energy level than the parallel atoms. When the antiparallel atoms give up energy and switch to a parallel relationship with the external magnetic field, the energy is lost in the form of radio waves. These radio waves exit from your finger and can be picked up by any suitable antenna similar to the one on an AM-FM radio. In a static magnetic field the energy emitted from a given type of atom or nucleus passing from the high ¬to low-energy level is always the same, thus enabling the examiner to determine the quantity and species of atoms present in the spec¬imen and where they were located.
