NMR imaging in theory and in practice

Visualization by magnetic resonance imaging of focal cartilage lesions in the excised mini-pig knee

This study showed that magnetic resonance imaging can be used to visualize partial thickness cartilage lesions, 0.7 x 10 mm in area and 0.5 mm in depth, surgically induced in the femur (femoropatellar compartment) of a mini-pig knee joint. Formalin-fixed joints, intact as well as disarticulated, were studied by high resolution imaging using a 2.35 T, 31 cm horizontal-bore superconducting magnet. The two-dimensional and three-dimensional spatial resolutions achievable were as follows: 0.12 x 0.23 mm (two-dimensional) and 0.35 x 0.35 x 0.35 mm (three-dimensional) for the intact joint, and 0.08 x 0.08 mm (two-dimensional) and 0.14 x 0.14 x 0.27 mm (three-dimensional) for the disarticulated joint. These results demonstrate that magnetic resonance imaging, together with edge detection and volume rendering, can be used to visualize focal cartilage lesions.

Anomalous diffusion of water in biological tissues

This article deals with the characterization of biological tissues and their pathological alterations. For this purpose, diffusion is measured by NMR in the fringe field of a large superconductor with a field gradient of 50 T/m, which is rather homogenous and stable. It is due to the unprecedented properties of the gradient that we are able not only to determine the usual diffusion coefficient, but also to observe the pronounced Non-Debye feature of the relaxation function due to cellular structure. The dynamics of the probability density follow a stretched exponential or Kohlrausch-Williams-Watts function. In the long time limit the Fourier transform of the probability density follows a long-tail Lévy function, whose asymptotic is related to the fractal dimension of the underlying cellular structure. Some of the properties of Lévy walk statistics are discussed and its potential importance in understanding certain biophysical phenomena like diffusion processes in biological tissues are pointed out. We present and discuss for the first time NMR data giving evidence for Lévy processes that capture the essential features of the observed power law (scaling) dynamics of water diffusion in fresh tissue specimens: carcinomas, fibrous mastopathies, adipose and liver tissues.

Recent developments in breast imaging

A review of breast imaging has already appeared in 1982 in this journal. Consequently, the present article concentrates on a discussion of only those developments of a more recent nature. Although the emphasis is placed on the physical aspects of the different imaging methods concerned, the essential factors relating to the clinical background and the associated radiation risk are also outlined. The completeness of detail depends on the present clinical importance of the method under discussion. X-ray mammography, which is still the most important breast imaging technique and has proved to be an effective method for breast cancer screening, is therefore treated in greater detail. Since the early 1980s, ultrasound B-mode scanning has evolved to an indispensable adjunct to x-ray mammography. For Doppler sonography, diaphanography, contrast-enhanced MRI, CT and DSA, the visualization of a tumour depends essentially on the enhanced vascularity of the lesion. Whether this will prove to be a reliable indicator for malignancy remains to be shown in controlled clinical studies. Common to all imaging systems is the increasing use of digital methods for signal processing, which also offers the possibility of computer-aided diagnosis by texture analysis and pattern recognition.

Visualisation of mass transport of small organic molecules and metal ions through articular cartilage by magnetic resonance imaging

Magnetic resonance imaging of water has been used to visualise the migration of three paramagnetic species, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO, 1), Cu2+ ions, and copper ethylenediamine-tetraacetate (CuEDTA, 2) through cartilage on the femoral condyle of the chicken knee. The migration of copper ions is dominated by strong binding with the cartilage. In contrast, both 1 and 2 bind weakly, and their diffusion can be followed as a progressive wave through the cartilage and subsequently into the trabecular bone structure.

Choices in medicine: illustrations from imaging

Progress in medicine is almost always led by advances in science and technology. During the last quarter of a century, startling developments have taken place in medical imaging. X-ray techniques have been extended by improvements in film-screen technology and image intensifiers; digital radiography and computed tomography have been introduced. The gamma camera has been combined with computer processing and new radiopharmaceuticals have been devised. The resolution of real-time two-dimensional ultrasonic scanning has been improved almost to the diffraction limit and Doppler techniques provide detailed information about blood flow. Magnetic resonance imaging yields exquisite anatomical detail, tissue characterization and flow data and biochemistry can be studied through spectroscopy. Research aimed at improving all these methods is actively being pursued and there is interest in the potential of microwave, electrical impedance and light transmission techniques as medical imaging tools. In parallel with these diagnostic advances, techniques for minimally invasive surgery are being developed. Guided by appropriate imaging methods, suitable forms of radiation can be directed through the intact skin, or small instruments can be introduced through natural orifices or tiny incisions. Thus it is possible to undertake many interventional procedures that previously required open surgery. Because resources are limited, choices have to be made taking benefits, risks and costs into account. No diagnostic method is perfect; medical imaging is expensive in terms of equipment and trained personnel and its use has to be justified in competition with demands from other areas of health care. Unproductive work is having to be identified and abandoned.(ABSTRACT TRUNCATED AT 250 WORDS)