CRT04210 Ultrasound Imaging

DIAGNOSTIC RADIOGRAPHY · NTA LEVEL 4 · SEMESTER TWO Ultrasound Safety and Biological Effects CRT04210 · Ultrasound Imaging START READING NOTES Study Ultrasound Safety and Biological Effects using the sections below. Use the topic navigation to continue through Ultrasound Imaging. Contents of This Topic ULTRASOUND SAFETY IS ULTRASOUND SAFE? OPERATING MODES AND THEIR RISKS PULSED DOPPLER AND POWER DOPPLER BIOLOGICAL EFFECTS SAFE USE SOME GUIDELINES CONCLUSION ULTRASOUND SAFETY ULTRASOUND SAFETY BASIC RULES AND PRINCIPLES IS ULTRASOUND SAFE? So far ultrasound has not been proven to be unsafe in humans. However, ultrasound is now being very widely used and advances in technology are leading to increasing power levels being used to obtain diagnostic information. This can lead to measurable biological effects The interaction of ultrasound with biological tissue can result in effects which may cause heating, interfere with normal functioning of cells, and cause structural damage. It is important for people using ultrasound to do all they can to minimize the exposure of the patient to ultrasound to reduce these risks. OPERATING MODES AND THEIR RISKS B Mode ultrasound imaging records returning echoes strength as a brightness display on the screen. This is the normal grey scale imaging used for real time ultrasound. This uses the lowest output power and intensities and is generally considered safe in all applications, PULSED DOPPLER AND POWER DOPPLER Pulsed Doppler uses higher intensity of ultrasound than B Mode, and is therefore more likely to cause biological effects, particularly from heating. Pulsed Doppler ultrasound focuses the beam in a small area for longer periods increasing the dose to those tissues. Power Doppler also uses intensities higher than B Mode, but less than pulsed Doppler BIOLOGICAL EFFECTS There are two important effects which can be caused by ultrasound: 1/ Thermal (heating) 2/ Non thermal effects (cavitation) 1/ As the ultrasound beam travels through the tissue some of its energy is lost through absorption. This absorbed energy is converted into heat which causes a temperature rise of the tissues. The heat rise will depend upon: * the attenuation coefficient of the tissue * the operating frequency of the transducer * power/ intensity of the ultrasound beam * the length of operating exposure time BIOLOGICAL EFFECTS 2/ Non thermal effects ( cavitation ) The oscillating pressure wave caused by an ultrasound beam creates areas of compression and rarefaction. This can cause small bubbles to form in tissues. These can grow, oscillate in size, an at high intensity and pressure these can collapse. This causes high pressures and temperatures in the localized area with the potential to cause biological damage. While this risk is very small in general ultrasound, it is wise to be careful in early pregnancy where the tissues are more vulnerable. CAVITATION SAFE USE Changes in power output and increased use of Doppler ultrasound have led to awareness of the need to take every measure to ensure safe practice. The accepted guideline for safe use is known as the ALARA principle ( “As Low As Reasonably Achievable” ) This is a guide to minimizing exposure times and exposure intensity. ALARA SOME GUIDELINES 1/ Examinations should use B mode real-time in the first instance and employ color and spectral Doppler only when and if required. 2/ The overall examination should be kept as short as possible to minimize exposure times. 3/ The transducer should not be held in a fixed position any longer than necessary. 4/ The transducer should never be left resting on the patients body when not in use. 5/ The output power setting should be as low as possible consistent with obtaining a satisfactory image. 6/ Ultrasound examinations should only be performed for a specific medical indication to obtain useful diagnostic information. 7/ Only suitably trained people should be allowed to operate ultrasound machines. CONCLUSION Although harmful effects have not been shown to result from the use of ultrasound at normal diagnostic levels, the potential for harm does exist. More research still needs to be done. For example, we do not know if the effects from ultrasound exposure are cumulative. As in the use of x-rays, a very small risk is justified when the risk is minimal and the benefit high. ← PREVIOUS TOPICNEXT TOPIC →VIEW ALL MODULE NOTESVIEW SEMESTER NOTESVIEW ALL LEVEL NOTESALL DIAGNOSTIC RADIOLOGY NOTES Need These Notes as PDF? Need a clean, well-formatted PDF copy for offline study, revision or printing? Request the complete notes directly through WhatsApp. GET WELL-FORMATTED PDF NOTES

DIAGNOSTIC RADIOGRAPHY · NTA LEVEL 4 · SEMESTER TWO Ultrasound Artifacts CRT04210 · Ultrasound Imaging START READING NOTES Study Ultrasound Artifacts using the sections below. Use the topic navigation to continue through Ultrasound Imaging. Contents of This Topic ULTRASOUND PHYSICS LECTURE 4 DEFINITION COMMON ARTIFACTS REVERBERATION ACOUSTIC SHADOWING ACOUSTIC ENHANCEMENT EDGE SHADOWING BEAM WIDTH ARTIFACT SLICE THICKNESS ARTIFACT SIDE LOBE ARTIFACT MIRROR IMAGE ARTIFACT DOUBLE IMAGE ARTIFACT EQUIPMENT GENERATED ARTIFACTS ULTRASOUND ARTIFACT ULTRASOUND PHYSICS LECTURE 4 ARTIFACTS DEFINITION In ultrasound imaging, an artifact is an appearance on the image which does not faithfully represent the structures present in the subject being scanned. Therefore the echoes are a false or misleading image which need to be properly understood and interpreted. Artifacts may appear as structures seen in the image which do not exist in the subject, or structures which should be seen in the image but are not. Structures may also be represented in the wrong location in the image ( mis -registration ). Artifacts may be caused by improper function or use of equipment, or they may simply be a characteristic of the physical properties of ultrasound waves and their interaction with tissues. COMMON ARTIFACTS Reverberation Acoustic shadowing Acoustic enhancement Edge shadowing Beam width artifact Slice thickness artifact Side lobe artifacts Mirror image artifacts Double image Equipment generated artifacts REVERBERATION Reverberation is the production of false echoes due to repeated reflections back and forth between the transducer face and a strong reflector in the subject. The first returning echo is displayed correctly, but some of the returning sound bounces off the face of the transducer back into the subject like a weak transmitted pulse. This returning pulse meets the strong reflector again causing another returning echo from the same interface. As the time taken for the second echo to arrive is twice that taken by the first echo, the image will display it at twice the depth. This results in a series of equally spaced false echoes to be displayed on the ultrasound image. REVERBERRATION REVERBERATION The bouncing of the sound back and forth (reverberation) often occurs between the transducer face and the subcutaneous fat / muscle layer. It also commonly occurs when the sound waves reverberate between two anatomic reflecting surfaces in the subject, such as between the near and far surface of an area of gas. In this case the echoes are placed much closer together in the image, and the appearance is termed comet tail. Small gas bubbles create an almost continuous band of reverberation called Ring down FORMATION OF COMET TAIL ARTIFACT COMET TAIL ARTIFACT RING DOWN ARTIFACT ACOUSTIC SHADOWING Acoustic shadowing appears as an area of low level echoes or no echoes at all distal to an interface with a large acoustic mismatch. It is caused by severe attenuation of the beam at the interface. This will occur where the beam passes from soft tissue to gas (99.9 % of the beam is reflected) It also occurs where the beam passes from soft tissue to bone (40% reflection and 60% absorption by the bone) ACOUSTIC SHADOWING GAS OR STONES? ACOUSTIC ENHANCEMENT This artifact appears as an area of increased echo brightness distal to an area of low attenuation. Enhancement is normally seen distal to structures containing fluid such as the urinary bladder, gall bladder, or cyst. Time –gain compensation is applied equally to areas of the subject with minimal attenuation such as a cyst, and to adjacent areas of tissue. Therefore echoes from within the cyst are increased by the TGC unnecessarily resulting in a distal brightness. ACOUSTIC ENHANCEMENT EDGE SHADOWING When the sound beam meets a curved interface both refraction and reflection occur which results in an edge shadowing artifact. As the ultrasound beam meets the curved surface, some is reflected away from its true course and some continues into the second medium, but is refracted or bent from its original course This results in an acoustic shadow distal to the curved edges of the mass. EDGE SHADOWING BEAM WIDTH ARTIFACT The beam width can vary depending upon focusing, divergence etc. Since the width of the beam is not always as small as the objects being imaged, echoes arising from the edge of the beam will be displayed as having arisen from the beam centre. A small reflector will generate an echo for as long as it remains in the beam and so will be represented as a line in the display rather than a dot. Some echoes from outside the assumed plane will be displayed as if from within the plane. We may for example, see some echoes arising from outside the bladder but displayed within the bladder BEAM WIDTH ARTIFACT SLICE THICKNESS ARTIFACT This is similar because it is related to the size of the ultrasound bream, and creates false echoes in areas that should be echo free. The ultrasound machine assumes the echoes received by the transducer are from a very thin slice of tissue. However, an image is actually composed from several slices, and therefore echoes from slices either side of the intended slice may be displayed in the image. This is often seen in transverse views of the urinary bladder. Structures adjacent to the slice through the bladder will be included within the bladder. SLICE THICKNESS ARTIFACT SIDE LOBE ARTIFACT As well as the main beam used to form the ultrasound image, the transducer also produces many smaller and weaker side lobes. These are not normally used to form the primary image, but they can cause artifacts These also cause echoes to be displayed in areas which should be echo free. This is more apparent when there are very strong reflectors , such as gas, adjacent to an echo free area. This can give a false impression of a septation within the urinary bladder or gall bladder. SIDE LOBE ARTIFACT MIRROR IMAGE ARTIFACT Some large smooth reflectors, such as the diaphragm, return such a strong echo to the transducer that they are read by the machine as part

DIAGNOSTIC RADIOGRAPHY · NTA LEVEL 4 · SEMESTER TWO Ultrasound Transducers and Scanning Modes CRT04210 · Ultrasound Imaging START READING NOTES Study Ultrasound Transducers and Scanning Modes using the sections below. Use the topic navigation to continue through Ultrasound Imaging. Contents of This Topic THE ULTRASOUND BEAM BEAM SHAPE PULSED ULTRASOUND Beam intensity BEAM WIDTH AND SLICE THICKNESS FOCUSING THE BEAM IMAGE RESOLUTION AXIAL AND LATERAL RESOLUTION AXIAL RESOLUTION SPATIAL PULSE LENGTH LATERAL RESOLUTION Ultrasound Transducers and Scanning Modes FACTORS AFFECTING BEAM WIDTH INTRODUCTION OF USS THE ULTRASOUND BEAM CHARACTERISTICS BEAM SHAPE The path which the ultrasound waves travel from the transducer is called the ultrasound beam. It is divided into two regions. The near field (or Fresnel zone) which is shaped like a cylinder. The far field (or Fraunhofer zone) where it becomes cone shaped. NOTE: Increasing the frequency will give a longer near field and less far field divergence. PULSED ULTRASOUND Instead of a continuous output of waves from the transducer face, most transducers use a series of short pulses of sound energy. These short pulses form the ultrasound beam. This is important, as we will see later, in giving good axial resolution. Beam intensity The intensity of the beam is the power (measured in watts) flowing through a unit area of the ultrasound beam. Increasing the output power to the transducer produces high intensity sound pulses, which make returning echo signals from all reflectors appear brighter. BEAM WIDTH AND SLICE THICKNESS Beam width means the measurements of the beam in the scan plane. This will vary according to distance from the transducer and what focusing is applied to the beam. The width of each pulse of sound will therefore change with depth. The beam width affects the spatial resolution of the image (the ability to differentiate small structures). The narrower the beam width the better the spatial resolution. Slice thickness is the measurement of the beam at 90 degrees to the scan plane FOCUSING THE BEAM The ultrasound beam can be focused to improve image quality. This is mostly done electronically. The depth of the focal zone and the length of the focal zone can be changed by the operator on modern ultrasound machines. IMAGE RESOLUTION Spatial resolution is the ability of an ultrasound system to display two closely spaced reflectors as separate structures. This depends upon the wavelength of the sound used to produce the image. For example, the wavelength of a 5MHz ultrasound beam is about 0.3mm, so it could not resolve (display separately) objects less than 0.3mm apart AXIAL AND LATERAL RESOLUTION Axial resolution is resolution along the axis of the direction of the ultrasound beam. Lateral resolution is resolution at right angles to the direction of the ultrasound beam. Axial resolution depends on the spatial pulse length. Lateral resolution depends on the ultrasound beam width. AXIAL RESOLUTION A short pulse length gives good axial resolution and the best resolution that can be achieved is half the spatial pulse length Therefore if the pulse length is 1mm, then structures along the axis which are less than 0.5 mm will not be resolved. Axial resolution is affected by the frequency of the ultrasound beam and damping applied to the piezoelectric crystal SPATIAL PULSE LENGTH Each ultrasound pulse is about two wavelengths long, and therefore a shorter wavelength will reduce the pulse length. Since wavelength is related to frequency, we can see that a high frequency beam will result in a shorter pulse length and better axial resolution. Pulse length also depends upon the amount of damping applied to the piezoelectric crystal. Increasing the damping shortens the pulse length and improves axial resolution. LATERAL RESOLUTION A narrow ultrasound beam gives best spatial resolution and the best resolution that is achieved is equal to beam width at the focus of the beam. The beam width determines the size of the echoes displayed and the structures must be separated by a distance greater than the beam width for them to be resolved. That is, the beam must fit into the gap between the two structures so that is can return “no echoes” from that position. Ultrasound Transducers and Scanning Modes Reflector separation is less than beam width in fig A and B but greater than beam width in Fig C and D. Fig A The beam first encounters the left reflector, then both reflectors, then the right reflector. Fig B This results in continuous reflection from one or both reflectors. Fig C The beam encounters the left reflector, then fits between both, and then meets the right reflector. Fig D Separate echoes are shown and the reflectors are resolved. FACTORS AFFECTING BEAM WIDTH As the diameter of the piezoelectric crystal increases so does the beam width near the crystal face. Higher frequency beams have a longer near field and less divergent far field. The amount of focusing and the length of the focal zone. Divergence of the beam increases beam width further from the crystal face. The best lateral resolution is at the focal point of the beam. ← PREVIOUS TOPICNEXT TOPIC →VIEW ALL MODULE NOTESVIEW SEMESTER NOTESVIEW ALL LEVEL NOTESALL DIAGNOSTIC RADIOLOGY NOTES Need These Notes as PDF? Need a clean, well-formatted PDF copy for offline study, revision or printing? Request the complete notes directly through WhatsApp. GET WELL-FORMATTED PDF NOTES