About “axial” and “radial” diffusivities

This article presents the potential problems arising from the use of “axial” and “radial” diffusivities, derived from the eigenvalues of the diffusion tensor, and their interpretation in terms of the underlying biophysical properties, such as myelin and axonal density. Simulated and in vivo data are shown. The simulations demonstrate that a change in “radial” diffusivity can cause a fictitious change in “axial” diffusivity and vice versa in voxels characterized by crossing fibers. The in vivo data compare the direction of the principle eigenvector in four different subjects, two healthy and two affected by multiple sclerosis, and show that the angle, α, between the principal eigenvectors of corresponding voxels of registered datasets is greater than 45° in areas of low anisotropy, severe pathology, and partial volume. Also, there are areas of white matter pathology where the “radial” diffusivity is 10% greater than that of the corresponding normal tissue and where the direction of the principal eigenvector is altered by more than 45° compared to the healthy case. This should strongly discourage researchers from interpreting changes of the “axial” and “radial” diffusivities on the basis of the underlying tissue structure, unless accompanied by a thorough investigation of their mathematical and geometrical properties in each dataset studied. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

“All-inside” arthroscopic meniscus repair

The arthroscopic “all-inside” meniscus suturing technique offers the arthroscopist a way of placing vertically oriented sutures through peripheral posterior horn tears located posterocentral without the risks of nerve, vessel, or posterior capsular entrapment inherent in both the “outside-in” and the “inside-out” arthroscopic methods. This technique introduces new instrumentation that allows the surgeon to both place sutures and tie suture knots intra-articularly under arthroscopic control.     

Don Lawrence and the “k-capture” revolution

PurposeThe practice of brachytherapy was in steep decline in the mid-20th century, largely because of safety issues. This article explores the innovations that revitalized brachytherapy with special attention to the introduction of low-energy seeds for permanent implantation.Methods and MaterialsLiterature review; interviews; and the memos, records, and correspondence of Donald C. Lawrence.ResultsPaul Harper first proposed the use of radionuclides that decay by k-capture in the 1950s. But it was the vision and tenacity of health physicist Donald Lawrence that led to the successful implementation of I-125 (in the 1960s) and Cs-131 (40 years later).     

Arthroscopic meniscal repair: “Inside-out” technique

Good results with the “inside-out” technique of meniscal repair depend on a number of factors: selection of the proper tears for repair, identification of the appropriate patients in whom to carry out repair, stimulation of the tear to generate a vigorous healing response, and stabilization by proper suturing methods. A number of different cannula systems have been developed, but close adherence to technical details are critical to success with any system.Vertical or oblique placement of sutures provide for a more anatomic and secure repair, and enough sutures on both the femoral and tibial surfaces are required for adequate stabilization. Serious complications can occur, most significantly damage to the popliteal neurovascular structures, the peroneal nerve, and the saphenous nerve and its branches. Thorough knowledge of knee anatomy is critical to avoid complications as is the use of accessory posterior corner incisions, the exposure of the posteromedial and posterolateral capsule, and the use of a popliteal retractor for safe retrieval of the repair needles.     

“Silent” MRI with soft gradient pulses

A method to reduce the acoustic noise generated by gradient systems in magnetic resonance imaging (MRI) is proposed based on the linear response theory. Since the acoustic frequency response function of typical gradient coils is low in the range below 200 Hz, the noise level can be significantly reduced by using gradient pulse sequences whose spectra are limited to this frequency range. Such “soft,” i.e., band-limited, pulse shapes can be designed using sinusoidal ramps individually adjusted to available delays. “Silent” versions of three basic MRI sequences [gradient-echo (GE), spin-echo (SE), and rapid acquisition with relaxation enhancement (RARE)] were programmed on 2 and 3 T whole-body scanners. High-quality images could be acquired at noise levels as low as 40 dBA (GE and SE) and 60 dBA (RARE). Magn Reson Med 42:6–10, 1999. © 1999 Wiley-Liss, Inc.     

Is the “biexponential diffusion” biexponential?

Diffusion-weighted signal from the brain, S, deviates from monoexponential dependence on the b-factor. This property is often referred to as biexponential diffusion, since the corresponding model fits data well. The aim of this study is to examine the necessity of using the biexponential model in homogeneous voxels under isotropic diffusion weighting up to b = 2.5 ms/microm(2). The model is compared to the cumulant expansion of ln S in a power series in b, which takes its origin in fundamental properties of the diffusion-weighted signal, but diverges at large b. The absence of statistically significant evidences for the biexponential diffusion is demonstrated in gray matter. The cumulant expansion terminated after the term b(2) describes data equally well with fewer adjustable parameters. The biexponential model is preferable in voxels with a partial volume of CSF.