CHAPTER 2 MEDICAL IMAGING WITH NON-IONIZING RADIATION 1 Ultrasound Imaging 1.1 Ultrasound Production and Detection Ultrasound is frequency vibration. To produce and detect ultrasound, we use crystals which they exhibit piezoelectric effects: When a voltage is applied across the crystal, the of it will change. This is used to ultrasound. When the crystal is deformed by an external force, a is created across it. This is used to ultrasound. 1.2 Ultrasound Probe An ultrasound probe used in hospitals has the following structures: Page 1
Piezoelectric crystal Acts as both and. Backing material the vibration of the crystal by absorbing its kinetic energy. Acoustic insulator Prevent ultrasound reaching the metal case so that ultrasound will not be. 1.3 Transmission of Ultrasound across Media Wave travels from one medium to another will undergo and. The amount of reflection and refraction depends on properties of the two media. In ultrasound transmission, relevant material properties include: To characterize ultrasound transmission, we consider acoustic impedance Z: Unit: Two media with similar Z: ultrasound readily. Two media with very different Z: ultrasound strongly. Ultrasound is readily transmitted in, but very difficult to enter. Page 2
The percentage of energy reflected at a boundary is given by intensity reflection coefficientα: Unit: α Larger αmeans more Transmission coefficient T = 1 α. This is a general formula for reflection of wave and can be derived from continuity of the wave at the boundary. Example: (a) Given the acoustic impedances for air and skin are 0.0004 MRayl and 1.63 MRayl respectively. Calculate the intensity reflection coefficient at the air skin boundary. (b) Explain why a coupling medium (gel) is applied on the skin during ultrasound imaging. Page 3
1.4 Attenuation of Ultrasound Attenuation means the in intensity when a wave propagates through medium. Attenuation of ultrasound is caused by: 1. by the body: Ultrasound energy converted to. 2. by molecules. For each medium, the attenuation of intensity follows an decay curve. Mathematically, I ( x ) = Constant µis called the. Larger µ, reduction of intensity is. Unit: µdepends on and also on of the ultrasound. In general µ with f. Hence high frequency ultrasound are penetrative. Just like any exponential decay curve, each reduction by 50% corresponds to a specific value of thickness. This is called the thickness x 1/2 and can be calculated as: Page 4
Example: 1 Given that µ = 14.5 m for fat. (a) Find the half-value thickness for fat. (b) If the intensity of the incident ultrasound is 0.02 passing through fat of thickness 1cm? -2 Wm, find its intensity after 1.5 Pulse-echo Technique To perform ultrasound scanning, a short pulse of ultrasound is sent towards a boundary. After being reflected, the round-trip time for detection of reflected pulse is measured. The first signal is usually the pulse due to partial reflection at boundary 1. The distance dcan be easily calculated as d = Page 5
Example: The diagram shows the Cathode Ray Oscilloscope (CRO) display when a pulse of ultrasound is sent towards a layer of skin and an organ underneath. If the time base of the CRO is 0.02 the thickness of the organ. 1 ms div and the speed of sound in the organ is 1500 1 m s, find Page 6
1.6 A-scan A-scan is the simplest type of scanning using pulse-echo technique. It is useful in measuring along a line. 1.7 B-scan In B-scan, each echo is represented by a on a scan line. The signal strength is represented by the of the dot. Usually multiple directions are scanned at the same time, showing a image. Usually the scanning region is in the form of, hence B-scan is often called sector scan. Page 7
1.8 Resolution Resolution is the ability of an imaging system to reveal small objects. Axial resolution: Ability to distinguish two close boundaries the scanning direction. For a thin layer of thickness dand pulse length l Well-resolved signal Poorly resolved signal Criterion for resolving a thin layer: Axial resolution can be improved by reducing. Lateral resolution: Ability to distinguish two close objects on the same boundary, which can be improved by: 1. Increase the : less 2. Increase number of. frequency is desirable for ultrasound scanning, this can give pulses hence axial resolution. beams hence lateral resolution. However, high frequency wave has the following drawbacks: 1. Higher µhence less. 2. Large effect due to absorption of energy. Common ultrasound machine allows 2 MHz to 12 MHz of frequency. Choice of frequency depends on: 1. Image resolution needed 2. of organ For organs near body surface, we usually use frequency ultrasound. For example: eye-ball, thyroid gland. For deep organs, such as liver, aorta, kidney, fetus, we usually use frequency. Page 8
1.9 Advantages and Limitations of Ultrasound Scan Advantages : does not have. : show instant image. Portable Non-painful and. Can generate images. Limitations Ultrasound cannot pass through the following interfaces due to great difference in acoustic impedances: Bone-soft tissue: Impossible to examine organs protected by bones, such as:,. Air-soft tissue: Impossible to examine internal part of air cavities, such as:,,. Difficult to image patients due to limited depth. Require high level of skill to obtain accurate image. Biological effects: Heating may lead to. Cavitation: Ultrasound may cause to form bubbles and burst quickly which may cause severe damage. 2 Optical Fibre Optical fibre is a thin filament that consists of a solid surrounded by a. The cladding should have a refractive index than the core. Generally, light from the core cannot escape to the cladding and occurs at the boundary. The function of the cladding is to prevent light signal leakage when. Page 9
2.1 Acceptance Angle The acceptance angle of an optical fibre is the from air at the core such that light is confined to inside the core. Calculate θmaxin terms of n1, n2such that the incident angle at Q is at critical angle. The acceptance angle θmaxof a straight optical fibre is given by: Page 10
2.2 Greatest Bending of an Optical Fibre The maximum bending of an optical fibre is the smallest of the fibre without of light. Refraction at Q: Length of OQ: Minimum radius of curvature: A fibre can be bent more without leakage of light. Page 11
3 Endoscope An endoscope is a medical imaging tool that allows doctors to view internal organs through natural openings or a small incision. can also be performed by modern endoscopes. Image channel Contains bundle of optical fibres to transfer image from the site. Light channel Contains bundle of optical fibres to conduct light to illuminate the site. Instrument channel Allows tools to be placed. Instrument channel can be used for: removing a piece of diseased tissue taking a sealing a site of bleeding by removing an object Water/gas channel Allows the use to water or gas to wash the interested area. Page 12
3.1 Coherent and Incoherent Bundles In a coherent bundle of optical fibres, the relative position of the fibres are. Coherent bundle are designed to transmit, with each fibre correspond to one. In an incoherent bundle, the relative position of the fibres are not fixed. This can only transmit light for illumination but not image signal. can also be performed by modern endoscopes. 3.2 Advantages and Disadvantages of using Endoscope Advantages: Allows direct observation of the organ. No harmful effects. Requires no or small surgical cut. Tissues can be taken out for further examination. Disadvantages Field of view is. Organs without a cavity, such as, cannot by examined. Risk of causing. Page 13