Attenuation concepts and applications

Attenuation means a reduction in intensity and amplitude in a radio signal. Radio signals are used in medicine in the medium of ultrasound. Measured in decibels (dB) per unit length of medium, attenuation is represented by the coefficient of the medium in question (dB/cm, dB/km, and so forth) (Zagzebski 3).

Attenuation is used in ultrasound physics and is the reduction of the beam amplitude, as a function of the distance it goes through the medium being imaged. One has to account for attenuation effects because reduced signal amplitude affects the image quality. By adjusting the amplitude to account for the attenuation in the ultrasound beam going through the medium, loss of energy at the desired depth is achieved (Bushong 2).

The ISO standard on ultrasound attenuation in heterogeneous systems maintains the level of information on particle size distribution. And attenuation in ultrasounds may be used in measurement of extensional rheology by applying acoustic rheometers utilizing Stokes' law of volume viscosity and extensional viscosity. To understand attenuation is to understand that when traveling through any material, the sound waves bounce off the material and are scattered, reducing the energy by scattering and absorption. Scattering is defined as reflecting the sound in directions other than the original direction it was projected. Absorption is converting sound energy into other kinds of energy. Each one of these creates a decay in the quality and energy of the sound as it travels through the material. So the scattering and absorption can therefore be called attenuation. This decaying wave can be expressed as: A = Aoe -az

Attenuation 1).

In the above equation, Ao is the amplitude of propagating wave at a location. The Amplitude (a) is how much the wave is reduced after it has traveled Z (distance) from the initial location the quantity (a) is the coefficient of the wave traveling in z-direction. The dimensions are measured in nepers/length for (a). A neper is defined as a dimensionless quantity and (e) is Napier's constant, or 2.71828. If one wants to convert nepers/length to decibels/length, one must divide it by 0.1151. Decibels are a more constant unit when referring to amplitude of signals.

A recent study by Vlahos, Godoy and Naidich, found that single-energy CT imaging results "in an anatomic depiction of the imaged area based on depiction of differences in physical density," while dual-energy imaging differentiates structures with similar densities but differing elemental composition, based on different attenuations at different photon energies. Therefore, dual-energy moves toward elemental imaging, and possibly chemical composition (2007).

The study found that though Hounsfeld envisioned dual-energy imaging principles, the technology was not available to do so. Now, however, advances in dual-source CT scanning techniques allow MSCT-quality images of a single acquisition upon two levels simultaneously. Another advantage is that, if proper parameters are utilized, the radiation dose is almost the same as a tube/detector system and there is a similar noise level in both the 80-kVp and 140-kVp systems, which improves the accuracy of calculations.

The first system (Somatom Definition by Siemens Medical Solutions) used two acquisition devices on a rotating gantry of the scanner at 90 degrees offset, each equipped with x-ray tube and 64-channel detector. The first axial plane tube/detector covered 50 cm-diameter scan field-of-view while the second tube/detector had only a central 26-cm-diameter FOV in the z-axis position. Using both at the same energy improved temporal resolution, best used in cardiac imaging. When used at different kilovoltages resulted in material differentiation (Vlahos 2007).

This dual-energy scanner, based on different attenuations and different photon energies will likely be used for pulmonary arterial angiographic and aortic imaging with, potentially, lower contrast volume. It will also likely be used for pulmonary enhancement maps and may possibly eliminate precontrast imaging and reduce radiation exposure (Vlahos 2007).

Utilizing the distinguishing qualities of attenuation, consistency or homogeneity, number and size of tumors found in Positron Emission Tomography (PET) scans of lungs and thyroid, pancreas, kidneys, adrenals, liver, and ovaries can be evaluated with training.

Fitton, Steenbakkers, et al. worked on a study of the lymph nodes of 13 patients found to have lung cancer. The data was obtained under free breathing conditions, a protocol was determined for CT/PET registration and a comparison of the image quality of attenuation-corrected PET images using CT images and positron transmission images "in terms of signal-to-noise ratio (SNR) and lesion-to-background ratio (contrast)" (Fitton 42)