Vadim Kuperman

Granular convection observed by magnetic resonance imaging

Vibrations in a granular material can spontaneously produce convection rolls reminiscent of those seen in fluids. Magnetic resonance imaging provides a sensitive and noninvasive probe for the detection of these convection currents, which have otherwise been difficult to observe. A magnetic resonance imaging study of convection in a column of poppy seeds yielded data about the detailed shape of the convection rolls and the depth dependence of the convection velocity. The velocity was found to decrease exponentially with depth; a simple model for this behavior is presented here.

Differentiation between the effects of T1 and T2* shortening in contrast-enhanced MRI of the breast

The purpose of this study is to describe a technique for magnetic resonance imaging (MRI) that can potentially improve identification of malignant tissue in the human breast. The suggested MRI technique is based on the differentiation between two competing effects leading to opposite changes in image intensity, namely, T1 and T2* shortening caused by administration of gadolinium chelate. The proposed approach also allows calculation of changes in the R2* relaxation rate in breast tissue. The feasibility of the technique for in vivo MRI and increased lesion contrast is demonstrated. The results indicate that this technique may improve detection of malignant breast tissue. J. Magn. Reson. Imaging 1999;9:172–176.

Magnetic resonance measurement of response to hyperoxia differentiates tumors from normal tissue and may be sensitive to oxygen consumption.

Karczmar GS, Kupertnan VY, River JN, Lewis MZ, Lipton MJ. MR measurement of response to hyperoxia differentiates tumors from normal tissue and may be sensitive to oxygen consumption. Invest Radiol 1994;29:S161–S163.

A new analytical model for Varian enhanced dynamic wedge factors

Dynamic and physical (hard) wedges are used in 3D conformal radiotherapy in order to improve dose distribution in patients. Unlike wedge factors for physical wedges that depend on wedge material and thickness, wedge factors for Varian dynamic wedges depend on the relationship between the position of the moving jaw and the number of delivered monitor units. In this study, we describe a new analytical model for dynamic wedge factors. We also review the existing analytical models and compare calculated and measured wedge factors. The comparison is performed for different wedge angles, symmetric and asymmetric fields and two different photon energies. The obtained results indicate that the new dynamic wedge model provides the best overall agreement (within 1%) with the measured wedge factors.

Cell kill by megavoltage protons with high LET.

The aim of the current study is to develop a radiobiological model which describes the effect of linear energy transfer (LET) on cell survival and relative biological effectiveness (RBE) of megavoltage protons. By assuming the existence of critical sites within a cell, analytical expression for cell survival S as a function of LET is derived. The obtained results indicate that in cases where dose per fraction is small, [Formula: see text] is a linear-quadratic (LQ) function of dose while both alpha and beta radio-sensitivities are non-linearly dependent on LET. In particular, in the current model alpha increases with increasing LET while beta decreases. Conversely, in the case of large dose per fraction, the LQ dependence of [Formula: see text] on dose is invalid. The proposed radiobiological model predicts cell survival probability and RBE which, in general, deviate from the results obtained by using conventional LQ formalism. The differences between the LQ model and that described in the current study are reflected in the calculated RBE of protons.

On the usefulness of portal monitor unit subtraction in radiation therapy

In order to avoid additional dose to patients caused by portal imaging with megavoltage x-rays, portal monitor units (MUs) are frequently subtracted from the actual treatment MUs. This study examines the usefulness of portal MU subtraction in radiation therapy. For 11 prostate cancer patients treated with 23 MV photons, dose to prostate due to portal filming with 6 MV photons was determined. In all 11 patients subtraction of portal MU values from the actual treatment MUs resulted in a small underdosing of the prostate with an average treatment error of -0.5%. Portal filming without MU subtraction would cause small overdosing of the prostate with an average treatment error of 1.2%. The results of this study indicate that the benefits of portal MU subtraction are in doubt if (a) the energy of treatment x-rays is much higher than that of the portal x-rays and/or (b) when radiotherapy is performed with physical wedges. Based on the obtained results, we argue against unconditional use of the portal MU subtraction method to eliminate the dose from portal imaging.

Contrast in MR Imaging

Magnetic resonance imaging (MRI) has become an important diagnostic tool in medical imaging because it provides the necessary contrast between various soft tissues required to identify pathologic processes. The observers ability to differentiate between different structures in images depends on an image contrast. This chapter considers different mechanisms of contrast in MRI as well as techniques used to improve contrast in Magnetic resonance (MR) images. The emphasis is on the relationship between image contrast and NMR relevant tissue parameters, such as inherent relaxation times and proton density. The dependence of MRI contrast on intrinsic relaxation times and proton density can be studied by using the equation describing steady-state transverse magnetization in a given pulse sequence. Certain materials, known as contrast agents, can enhance MR image contrast by altering relaxation times. Contrast agents are frequently used in diagnostic MRI in order to achieve better assessment of local physiologic and anatomic conditions, or to improve detection of malignancy. Because contrast agents are usually administered internally, they must possess low toxicity and be easily excreted from the body. Unlike contrast agents used in nuclear medicine and clinical radiography, MRI contrast agents affect the signal indirectly via interaction with the hydrogen nuclei. Contrast agents that significantly alter the local magnetic field in the specimen are frequently referred to as susceptibility agents.

Signal-to-Noise Ratio in MRI

This chapter focuses on the relationship between signal-to-noise ratio (SNR) and imaging parameters as well as the field strength. The signal used for reconstruction of magnetic resonance (MR) images is always corrupted by noise caused by randomly fluctuating currents in the receiver coil and the imaged object. The quality of MR images can be severely degraded by noise, making it difficult to distinguish between different structures in the object. The three most important parameters defining image quality are spatial resolution, image contrast, and signal-to-noise ratio (SNR). SNR, is defined as the ratio of the mean image intensity in a chosen region of interest (ROI) to the square root of the noise variance. SNR in MRI depends upon a number of factors including strength of the static magnetic field, type and characteristics of r.f. coils, imaging parameters (e.g., image resolution and matrix size), and chosen pulse sequence. In this chapter, SNR dependence is considered on the basic imaging parameters, such as voxel volume, matrix size, and acquisition time in 2D and 3D imaging. The obtained results demonstrate that 3D imaging in general provides higher SNR as compared to 2D imaging. The SNR dependence on voxel volume, number of phase-encoding steps, and acquisition time should be kept in mind when choosing imaging parameters. The SNR in 2D and 3D imaging is proportional to the voxel volume and the square root of the acquisition time. Experience shows that SNR in MR images varies depending on the strength of the static magnetic field.

Basic Principles of Nuclear Magnetic Resonance

This chapter discusses the basic principles of nuclear magnetic resonance (NMR). The Bloch equations are discussed and used as the main tool for analysis of the NMR phenomenon and associated effects important for magnetic resonance imaging (MRI). The basic physical effect at work in NMR is the interaction between nuclei with a nonzero magnetic moment and an external magnetic field. The NMR phenomenon is observed when a system of nuclei in a static magnetic field experiences a perturbation by an oscillating magnetic field. Although only quantum mechanics can completely describe the NMR phenomenon, some of its features can be explained by the classical theory of electromagnetism. The classical model of motion of a free magnetic moment, although useful for understanding of the NMR phenomenon, cannot explain many of its important features defined by the interactions between the nuclei. To overcome some of the limitations of the classical model, F. Bloch introduced the phenomenological equations describing the dynamics of nuclear magnetization that have become an extremely useful tool for theoretical analysis in NMR imaging and spectroscopy. Fast dephasing of nuclear spins due to magnetic field heterogeneity causes a significant loss of signal and thereby presents a serious problem for NMR imaging and spectroscopy. It is therefore extremely important that dephasing of spins is significantly reduced by using spin echo.

Basic Techniques for 2D and 3D MRI

This chapter describes principles of spatial encoding and the conventional acquisition scheme used in nuclear magnetic resonance (NMR). Because image reconstruction in magnetic resonance imaging (MRI) typically involves the use of Fourier transform (FT), the basic principles of Fourier analysis are described in the chapter. Kumars technique to magnetic resonance (MR) imaging employs a sequence of pulsed magnetic field gradients, which are applied during free induction decay or a spin echo to ensure that the signal is given by Fourier transform of the transverse magnetization in the sample. At this stage, Two basic imaging techniques for Fourier encoding: frequency encoding and phase encoding, are discussed. Frequency encoding is implemented by acquiring signal in the presence of an external magnetic field gradient. The purpose of the gradient, known as the frequency-encoding or readout gradient, is to make the Larmor frequency of nuclei spatially dependent during signal acquisition. Two-dimensional spatial encoding in MRI is normally achieved through the use of an additional gradient, known as the phase-encoding gradient, which is perpendicular to the frequency-encoding gradient. A gradient-echo pulse sequence begins with the excitation of the transverse magnetization in a slice of material. The main difference between gradient-echo and spin-echo pulse sequences is that the latter also includes a 180-degree pulse that causes formation of a spin echo during signal acquisition.