)
and velocity (V). Other relevant properties of ultrasound waves include
amplitude, power, and propagating wave type.Nuclear Magnetic Resonance (NMR)
Based on deriving a signal from nuclear magnetic moments by exciting them with an applied radiofrequency (RF) pulse.
NMR System
A sample containing hydrogen is placed between the poles of a magnet that produces a magnet field B. The sample is surrounded by a coil of wire that serves as both a transmitting and recieving antenna. RF oscillator is tuned in frequency until it is in rensonance with the protons.
Radio waves are emitted by the sample, inducing small currents in the coil. Amplitude of the signal is a function of the number of protons in resonance, therefore the hydrogen density or spin density (SD) can be obtained SD differs among various tissues.
In NMR nucleus magnetic moments can be aligned in very strong magnetic fields and resonance transfer of energy can occur when energy is supplied in the radiofrequency range (about 100MHz).
Nucleus acts as a magnetic dipole if placed in a strong magnetic field.
A small fraction (10-5) of the nuclei become aligned due to the magnetic force on the dipole.
Larger the number of aligned nuclei -----> Greater the NMR signal.
The magnetic moment does not align itself with the magnetic field.
The energy of the magnetic dipole which is represented by the spinning nucleus depends on its orientation relative to the magnetic field.
From a quantum mechanics perspective:
The spin may be either Parallel to the magnetic field vector (point north) or Antiparallel (Pointing toward the south pole) in the field BO.
In NMR there is no signal from a flowing fluid such as blood or moving tissue.
NMR provides better differentiation between soft tissue since it presents 20-40% difference in SD, T1 and T2 in comparison to 1% difference inherent to X-ray.
Magnetic Resonance Imaging (MRI)
Consists a magnet assembly - high strength for BO field, cods to improve homogeneity
Problems Encountered in measuring a Living System:
Inaccessibilty of variables to measurement
Impossible to place a suitable transducer in a place.
Need to make indirect measurement.
(Must be aware of the limitations of the substitute variable)
Variability of the data
Few variables are travels deterministic.
Physiological variables always must be represented by a statistical distribution.
Lack of knowledge about Interrelationships
Better understanding permits more effective use of indirect measurement.
Interactions among physiological systems
Larger number of feedback loops involved
Severe degree of interaction exists.
Effect of Transducer on the Measurement
Any measurement is affected in some way.
Physical presence of the transducer charges the reading significantly.
Flow transducer may partially block the flow.
Energy Limitations
Many physiological measurement techniques require that a certain amount of energy be applied to the living system in order to obtain a measurement, as not to damage the cells.
Safety Considerations
Extra caution must be taken in the design of my measurement.
Should not cause undue pain, trauma, or discomfort.
Medical Ultrasound
The term ultrasound refers to acoustical waves above the range of human hearing (frequencies higher than 20,000 Hz). Medical ultrasound systems operate at frequencies of up to 10MHz or more. An ultrasonic wave is acoustical; i.e., it is a mechanical wave in a gaseous, liquid, or solid medium. Such mechanical waves consist of alternating areas of higher and lower pressures, called compression and rarefaction zones, respectively. Ultrasonic imaging is used in medicine, engineering and other scientific areas.
Radio signals are other electromagnetic waves, while medical ultrasound signals are acoustical. The acoustical signal requires a medium in which to propagate, while the electromagnetic signal can propagate in outer space, where no known medium exists.
Physics of Sound and Ultrasonic Waves
Some animals are able to hear frequencies above the average range of human hearing, which is about 30 Hz to 20 kHz, none are believed capable of hearing waves in the range above 500kHz, at which medial ultrasound operate.
All waves, including both acoustical and electromagnetic possess three related attributes:
frequency (F), wavelength ()
and velocity (V). Other relevant properties of ultrasound waves include
amplitude, power, and propagating wave type.
 |
| Figure 17-1 from Carr shows frequency, amplitude, and wavelength. |
Frequency is defined as the number of complete cycles per unit of time.
The basic unit of cycles is the hertz (Hz), which equals one cycle per second (1 Hz = 1cps), while the superunits are kilohertz and megahertz (Mhz).
Wavelength is the distance traveled by one cycle propagating away from the source and is expressed in meters or centimeters or millimeters.
Velocity is the speed of propagation of the wave. In radio signals, the velocity is the speed of light (c), or 300,000,000 meters per second (m/s).
In human tissue, ultrasound propagates at a much slower rate, i.e., around 1500 m/s.
For all forms of wave, the relationship between frequency, wavelength, and velocity is:
V = F
Where:
V is the velocity of the wave is the time required to complete
one cycle and can be measured in terms of either time (T) or angle (where one
cycle = 2
radians). (T = 1/F)
V =
/ T
The figure 1500 m/s as the velocity of ultrasound in human tissue is accepted as an average for devices operated in the 2- 3-MHz region. Actual velocity depends on other factors that are properties of the transmission media as well as the frequency.
The amplitude (A) of the wave is the difference between the zero baseline and either peak. The amplitude of the ultrasonic signal is directly related to its power, which is expressed in watts (W), and is related to the electrical signal applied to the transducer, efficiency of the transducer, and efficiency of the coupling between the transducer and tissue. The energy level of the signal is measures in joules in which 1 J = 1 W-s.
The wave type refers to the method of propagation. The two forms are longitudinal propagation and transverse propagation. In the longitudinal form, the waves propagate in the same direction as the zones of compression and refraction. In transverse propagation, the waves propagate in a direction orthogonal (at right angles) to the direction of the zones of compression and rarefaction.
Reflection, Refraction, diffraction, and scattering phenomena
These all define wave behavior. These phenomena occur when the waves impinge on a surface or boundary between zones of materials of different density. For example, reflection is observed when we see ourselves in the mirror.
 |
| Figure 17-2 from Carr showing reflection and refraction of waves. |
By convention, refraction and reflection angles are measured
as an acute angle to a line normal (perpendicular) to the boundary surface at
the point where the incident wave strikes the boundary. In reflection, we know
that the angle of incidence (
i)
and the angle of reflection (r)
are equal to each other:
i
= r
If the incident wave impinges on the surface or boundary at an angle of 90 degrees (i.e., it is coincident with the normal line), it will be reflected back on itself. But if the angle is other than 90 degrees, than the reflected wave will travel away from the surface at the same angle.
Refraction phenomena affect the portion of the incident wave that enters the second medium.
The index of refraction (n) is defined as the ratio of velocity of the wave in air (or in a vacuum for electromagnetic waves) to the wave velocity in the medium, that is:
n = Vair / Vmed
Absorption and Attenuation of Ultrasonic energy
A number of mechanisms cause loss of energy in the transmitted ultrasound signal. We have seen the loss due to beam spreading. Because the beam is not coherent (as in a laser), the signal level attenuates at a rate determined by the inverse square law. There is also attenuation due to interaction with biological tissue, and is related to the acoustical impedance of the tissue. Finally, there is also some loss due to scattering of the signal.
The total attenuation is:
= BW + S + A
Where:
:
attenuation coefficient in (dB/cm)The value of attenuation by absorption depends on both the acoustical impedance and the frequency used.
Biological effects of Ultrasound
The increased use of ultrasound monitoring, therapeutic, and diagnostic equipment has led to some concern over the possible hazard levels. Thus far, no definite information exists concerning safe levels. Most clinical and research instruments are designed to produce to produce power output levels between 5 and 50 mW/cm2 at the transducer. Several factors are believed to have importance in the biological interaction of tissue and ultrasound waves: frequency, irradiation time, beam intensity, and the duty cycle. The principle biophysical effects of ultrasound are thermal, cavitation, shearing action, and intracellular motion.
Echoencephalography
Echo ranging can also be used to probe the brain, and it forms the basis for an ultrasonic device called the echoencephalograph. The echoencephalograph will fire 1-µs bursts of 2 to 3 Mhz ultrasound energy at a repetition rate of 500 burst per second. An oscilloscope connected for A-scan will show traces. The location of tumors, aneurysms, and other lesions can be determined by multiple scans to find the distance from the skull walls at different angles.
Note: The solid and soft (fluid-filled) tumors produce radically different echo responses.
Models and Physiology
The biomedical engineer uses physical and mathematical models to predict or control the behavior of an organ, or system of organs, and to design devices for diagnosis, therapy or rehabilitation. To construct models, it is necessary to have a base of empirical data. Collection of data and testing of models are often performed on animals.
Biomaterials
Biomaterials are manufactured substitutes for natural tissues. Standard design considerations such as strength and deformation, fatigue and creep, friction and wear resistance, flow resistance and pressure drop, thermal stability and expansion, electrical conductivity, optical transparency and refractive index. A material is biocompatible if it evokes a minimal adverse biological response.
The materials should be resistant to corrosion.
Ex. 316L stainless steel, cobalt alloys, titanium and alloys.
Ceramics are resistant to corrosion in the tissue environment. They have good strength in compression and are hard, and thus wear resistant, but brittle.
Ex. High-density polyethylene, rigid polyvinyl chloride, polytetrafluoroethylene (PTFE) (Teflon), and Polymethylmethacrylate (PMMA).
However, a ceramic coated metal should respond like a ceramic. Composites of fibers strong in tension, with a matrix strong in compression, combine strength, toughness, and stiffness and may permit parts to be made lighter or stronger.
Effects on tissue Thrombosis, hemolysis, inflammation, adaptation, infection, carcinogenesis, and hypersensitivity.
Thrombosis Coagulation of blood. The product of coagulation is a clot.
In some cases, heparin may have to be added to the blood.
Hemolysis Disruption of erythrocytes through mechanical stress. Shear stresses above 1500 to 3000 dynes/cm2.
Red blood cells are immersed in a hypotonic solution, distilled water.
Inflammation is a generalized response in tissue injury or destruction.
Adaptation Neutrophils migrate to the wound site within minutes to hours and persist for days. The inflammatory response ends with encapsulation of the wound by mature scar tissue, which is relatively acellular.
Infection and Sterilization There is a race between regenerating tissue and bacteria for sites on or near an implant. In addition to being clean and pyrogen free, an implant must be sterile.
Carcinogenesis Cancer may be induced by exposure to chemical carcinogens or foreign bodies (FB), or by mechanical irritation. Other metallic carcinogens are cadmium, lead and beryllium. The latent period may be 20 years.
Systemic Effects Fine particles may be transported in the lymph and blood and thus have systemic effects.
Mechanical properties of soft tissues are characterized by a nonlinear stress-strain relationship. The stress-strain loop is repeatable. That is there is hysteresis. Soft tissues exhibit viscoelasticity: After sudden stretching, the stress gradually decreases with time (stress relaxation).
Bone is an inhomogeneous, anisotropic, viscoelastic material. For more purposes, however, it is sufficient to assume that bone is linearly elastic so that:
Tij = Cijkl Ekl
An example of the use of biomaterials is replacement of a joint by metal and polymer.
