TRANSDUCERS FOR REAL- TIME IMAGING BY Dr/ Dina Metwaly

real-time ultrasound The concept of real-time ultrasound Essentially involves the generation of images of the same cross-section repetitively and at a rate exceeding about 25 frames per second which is high enough to create the impression of continuity of events in time. This facilitates the observation of motion in any part of the subject within the cross-section being scanned.

When a permanent record is desired, the image at the particular moment is held still, or "frozen, and a hard copy of it recorded (freeze frame). Capturing a record without freezing would cause motional blurring of the captured image.

Transducer design Transducers for real-time imaging may be classified broadly into two categories: mechanical transducers and electronic transducers. In mechanical transducers, the beam sweep is achieved through physical movement of some part of the transducer, usually the crystal element(s), whereas in electronic transducers the beam is swept by electronic activation of crystal elements, without causing the transducer to move physically.

Transducer Types Mechanical Oscillating Rotating Electronic Linear Arrays Curved Arrays Phased Arrays

Mechanical transducers Mechanical transducers are made using either a single piezoelectric crystal or a small group of crystals. A. A single crystal element: 1. may be rocked to perform pendulum motion through a suitable angle using an electric motor. The angle of swing will define the field of view. The image is triangular in shape and is referred to as a sector scan. Each swing of the crystal produces one image frame, and the frame rate is equal to the number of swings per second.

2. Alternatively, the crystal can be driven linearly along the scan section to perform rapid to-and-fro motion. The field of view in this case will be rectangular. 3. Another strategy that has been used with single crystal transducers is to employ an oscillating mirror to swing the beam from a stationary crystal by reflection.

B. small group of crystal elements: The most common method,employs a (3 or 4 crystals) mounted symmetrically on a rotating wheel. The wheel is driven by an electric motor to perform circular motion in one direction only. The crystal elements are excited one at a time to provide the ultrasound beam.

Each crystal is activated to transmit and receive only as it moves through a predetermined arc which may be referred to as the active sector. Outside this arc, the crystals remain inactive. Only one crystal may be active at any given moment For each complete rotation of the wheel, the number of image frames generated will be equal to the number of crystal elements constituting the transducer.

The frame rate will further be influenced by the speed of wheel rotation. For example a transducer with 4 crystals performing 10 revolutions per second will produce real-time images at a rate of40 frames per second. As Beam shape can be influenced by focusing. Mechanical transducers employ fixed focusing methods.

At the high speeds used to move the crystal elements, direct contact between the crystals and the patient's skin would be impractical and uncomfortable to both the patient and the operator. To avoid contact scanning, the crystals are housed in a small oil-filled bath. This measure introduces other advantages as well: the oil lubricates the moving parts, the field of view near the skin is improved, and some near field artefacts are transferred from regions of anatomical interest (on the image) to the region of the liquid path.

Electronic transducers Electronic transducers are made from a large number of small, identical crystal elements which are acoustically insulated from each other. The crystals are arranged in a suitable geometrical configuration, or an array, to provide the desired field of view. Movement of the beam is produced by exciting the crystal elements in an orderly fashion without having to move the transducer physically. The crystal elements may be pulsed individually, one at a time, to provide the instantaneous beam, a pulsing procedure known as sequential pulsing.

Alternatively, the instantaneous beam may be provided by a small group of crystals excited together. The group is a segment of the array, and such group pulsing is called segmental pulsing. The choice between sequential and segmental pulsing is identified by the need to provide a suitable beam shape. In a multi-crystal transducer with very many crystal elements, the crystals will be small in size, otherwise the transducer would be too large and bulky.

Sequential pulsing of very small crystal elements would therefore give a poor beam pattern. To avoid this problem, segmental pulsing may be employed: a small number of adjacent crystals are excited together as a group to provide the instantaneous beam. Each pulse of ultrasound from this group results in one scan line. The number of crystal elements constituting the group is chosen such that the resulting beam shape will be similar to that from a transducer in which crystals are pulsed individually.

Electronic Arrays Groups of piezoelectric material working singly or in groups Sector ArrayLinear Array crystals are placed parallel or in concentric rings crystals are placed parallel transducer face is curvedtransducer face is flat produces sector imageProduce rectangular image

Field Of View--the display of the echo amplitudes shape dependent on transducer type and function Field of View Shapes LINEAR FOV SECTOR FOV produced by linear arrays produced by oscillating rotating curved arrays phased arrays typically used in superficial application typically used in abdominal application

Sector

ULTRASOUND ARTIFACT

Definition An ultrasound artifact is a structure in an image which does not directly similiar with actual tissue being scanned. Artifact assumes different forms including : Structures in the image that are not actually present Objects that should be represented but are missing from the image. Structures which are misregistered on the image.

1. REVERBERATION 2. ACOUSTIC SHADOWING 3. ACOUSTIC ENHANCEMENT 4. EDGE SHADOWING 5. BEAM WIDTH ARTIFACT 6. SLICE THICKNESS ARTIFACT 7. SIDE LOBE ARTIFACT 8. MIROR IMAGE 9. DOUBLE IMAGE 10. EQUIPMENT-GENERATED ARTIFACT 11. REFRACTION ARTIFACT COMMON ARTIFACTS

This is the production of false echoes due to repeated reflections between two interfaces with a high acoustic impedance mismatch. The echo from the interface is received by the transducer and displayed on the image. Some of the energy in the returned echo is reflected at the transducer face, and return to the reflecting interface as if it was a weak transmitted pulse, returning as a second echo. As the time taken for the second echo to arrive is twice that taken by the depth. REVERBERATION

This sequence of reflection and transmission can occur many times, with the third echo taking three times as long to return to the transducer and being displayed at three times the depth, and so on. The reverberation echoes will be equally spaced because the time for each additional echo is multiple of the time of return of the first echo. This artifact will be seen at the skin-transducer interface and behind bowel gas. Rectification: Increase the amount of gel used. Reduce the gain. Move the position of the transducer.

This appears as an area of low amplitude echoes behind an area of strongly attenuating tissue. It is caused by severe attenuation of the beam at an interface, resulting in very little sound being transmitted beyond. The attenuation can be due to either absorption or reflection of the sound waves, or a combination of the two. Acoustic shadowing will occur at interfaces with large acoustic mismatch such as: Soft tissue and gas Soft tissue and bone or calculus ACOUSTIC SHADOWING

This artifact appears as a localized area of increased echo amplitude behind an area of low attenuation. On a scan it will appears as an area of increased brightness, and can commonly be seen distal to fluid-filled structures such as the urinary bladder, GB or a cyst. It is caused by the low level of attenuation of the beam as it passes through fluid relative to the greater attenuation of the beam in the adjacent more solid tissue. This artifact can often be an useful diagnostic aid, particularly when scanning a soft-tissue mass or cyst containing low level echoes. ACOUSTIC ENHANCEMENT

A combination of refraction and reflection occurring at the edges of rounded structures will result in edge shadowing artifact. When the ultrasound beam reaches the rounded edge of a structure, reflection will occur, with an angle of incidence equal to the angle of reflection. The outer part of the beam will be totally reflected, but the reminder of the beam passes through the rounded structure and is refracted. This combination results in a thin strip of tissue behind the edge not being insonated and causes a shadow. EDGE SHADOWING

Caused due to the widening of the main beam after the focal spot. Correct positioning of the focal zone will help to reduce this artifact. The focal zone is controlled by electronically narrowing the beam BEAM WIDTH ARTIFACT

These occurs due to the thickness of the beam. These artifacts will typically be seen in transverse views of the urinary bladder when structures adjacent to the slice through the bladder being scanned will be incorporated into the image. These echoes are then displayed as if they were arising from within the bladder. Although the appearance of this artifact is similar to the beam width artifact, the differentiating factors is that the reflector causing the slice thickness artifact will not be seen on the display. SLICE THICKNESS ARTIFACT

Side lobes are multiple beams of low-amplitude ultrasound energy that project radially from the main beam axis, mainly seen in linear-array transducers. Side lobe echoes will therefore be misregistered in the display. This artifact can often be seen in area such as the urinary bladder and may also arise within a cyst. SIDE LOBE ARTIFACT

These artifacts results in a mirror image of a structure occurring in an ultrasound display. They arise due to specular reflection of the beam at a large smooth interface. An area close to a specular reflector will be imaged twice, once by the original ultrasound beam and once by the beam after it has reflected off the specular reflector. Mirror image artifacts are most commonly seen where there is a large acoustic mismatch, such as a fluid-air interface. MIRROR IMAGE ARTIFACT

Typically this artifact can occur during the scanning of a full bladder, when air in the rectum behind the bladder act as specular reflector and mirror image of the bladder is displayed posteriorly. It will then have the appearance of a large cyst behind the bladder. It can also be seen when scanning the liver, and the diaphragm act as a specular reflector.

This image is caused by refraction of the beam and may occur in areas such as the rectus abdominis muscle on the anterior abdominal wall. In the transverse plane the edges of the muscle act as a lens and the ultrasound beam to be refracted and this causes the single structure to be interrogated by two separate refracted beams. Two sets of echoes will therefore be returned and these will cause display of two structures in the image. DOUBLE IMAGE ARTIFACT

This results in, for example, two images of the transverse aorta side by side in the abdomen.

Incorrect use of the equipment controls can lead to artifact appearing. Misuse of controls such as the gain or TGC can result in echoes being recorded as too bright or too dark. Care must be taken when setting these controls, to ensure an even brightness throughout the image. If the dynamic range control is incorrectly set, this can lead to an image which has too much contrast, and result in the loss of subtle echo information. EQUIPMENT-GENERATED ARTIFACT

REFRACTION ARTIFCT The refraction is the change of the sound direction on passing from one medium to another. In ultrasound, refraction is due to sound velocity mismatches combines with oblique angles of incidence, most commonly with convex scanheads. When the ultrasound wave crosses at an oblique angle the interface of two materials, through which the waves propagate at different velocities, refraction occurs, caused by bending of the wave beam. Refraction artifact cause spatial distortion and loss of resolution in the image.

CONTRAST ENHANCED ULTRASOUND (CEUS)

Ultrasound contrast Composed of microbubbles. Consists of a microbubble shell usually made of albumin, galactose, lipid or polymers and a gas core composed of air or heavy gases like perfluorocarbon or nitrogen. Microbubbles have a high degree of echogenicity. The echogenicity difference between the gas in the microbubbles and the soft tissue surroundings of the body is immense. Thus the microbubble contrast agents enhances the reflection of the ultrasound waves, to produce a unique sonogram with increased contrast due to the high echogenicity difference.

CEUS requires contrast-specific software on the ultrasound equipment that suppresses the signal from the background tissue leaving only the signal from the microbubbles. Pulse inversion harmonic imaging is used whereby two signals are sent down a single scan line and the second is a mirror image of the first. Echoes from both pulses are collected by the transducer and summed.

Linear reflectors, such as normal tissue, produce no net signal. However, nonlinear reflectors, such as microbubbles, produce echoes that are asymmetric and do not sum to zero. The result is that echoes from bubbles are detected preferentially using this method, improving image contrast between tissue and microbubbles.

Targeted CEUS Microbubbles are attached with ligands that bind certain molecular markers that are expressed by the area of imaging interest are then injected systemically in a small bolus. Microbubbles theoretically travel through the circulatory system, eventually finding their respective targets and binding specifically. The ultrasound system converts the strong echogenicity into a contrast-enhanced image of the area of interest, revealing the location of the bound microbubbles.

Potential applications Inflammation: Contrast agents may be designed to bind to certain proteins that become expressed in inflammatory diseases such as Crohn's disease and atherosclerosis. Thrombosis and thrombolysis: Activated platelets are major components of blood clots (thrombi). Microbubbles can be conjugated with a ligand specific for activated glycoprotein IIb/IIIa (GPIIb/IIIa), which is the most abundant platelet surface receptor. The microbubbles will specifically bind to activated platelets and allow real-time molecular imaging of thrombosis, such as in myocardial infarction. Can also be used to image malignant tissues, as a way to deliver genes and drugs to the tissues.

Untargeted CEUS Benefits: Organ Edge Delineation: Microbubbles can enhance the contrast at the interface between the tissue and blood. Blood Volume and Perfusion: contrast-enhanced ultrasound holds the promise for (1) evaluating the degree of blood perfusion in an organ or area of interest and (2) evaluating the blood volume in an organ or area of interest. Lesion Characterization: contrast-enhanced ultrasound plays a role in the differentiation between benign and malignant focal liver lesions.

hemangioma hemangioma

LIVER Metastasis

Some of the current uses of CEUS. 1. Contrast enhanced voiding urosonography. 2. Monitoring of tumor response to anti-angiogenic therapy. 3. Monitoring of local ablative therapy in HCC and metastasis. 4. Characterization of kidney lesions. 5. Charecterization of liver lesions. 6. To characterize pancreatic lesions. 7. Blunt abdominal trauma. 8. Transcranial vascular imaging. 9. Assessment of atherosclerotic plaques.

Bioeffects of ultrasound

Introduction Ultrasound is a high frequency mechanical waves that are above the human hearing range(>20,000 Hz). They are produced by converting the electrical energy into mechanical energy. When transmission is through biological tissues & under certain conditions, they may cause biological effects.

Biological Effects of US Mechanisms Thermal Non- thermal MechanicalCavitation

Thermal effects Def: temperature within a medium (locally). How? As the sonic energy is absorbed & converted into heat. Thermal effect depends on: Beam intensity, tissue absorption coefficient, blood flow, exposure parameters(e.g. Duration of exposure, frequency, )

Transducer Self-heating Electrical energy is converted to thermal energy instead of sonic energy. More likely to occur with endocavity probes where the probe is enclosed within the body & can be almost stationary for several minutes. Clearly express thermal injury e.g. trans- esophageal exams. Endocavity probe

A. Mechanical(direct) effects e.g. Particle displacement & fluid streaming: Target particles are pushed away from the transducer acoustic streaming in uids, cell distortion* and lysis.

Non-thermal effects B. Cavitation Regions of compression & rarefaction are created in the medium. increases & decreases in pressure alternatively. Gas bubbles form (how?) & grow until critical size then collapse. generates the energy for mechanical effects.

Cavitation Types Cavitation may be transient or stable. Transient cavitation : very rapid expansion & violent collapse. Causing high temp. & pressure, release of free radicals May cause genetic damage in vitro. Stable cavitation : bubbles oscillating with sound beam. Cause mechanical damage, membrane rupture & sometimes cell lysis.

Safety Indices Thermal Index (TI) & mechanical Index (MI) Not perfect; but they are the most common & practical measurements available at present. Indicate the probability of thermal & non thermal effects. Assist the sonographer in patient exposure. How? > By keeping these indices as low as possible while obtaining the best possible diagnostic images.

Thermal Index(TI) An indicator of the temp. elevation possible at a particular equipment setting. TI has 3 subdivisions : Soft tissues (TIS); bone (TIB); and adult cranial exposure (TIC).

An indicator of the probability of cavitation events. Generally, MI should be < 1.9 Mechanical Index(MI)

MI & TIS are displayed on screen.

Thermal effects Temperature rise of less than 1.5 degrees C no hazard to human (including fetus). Temp. rise of 4 degrees C, lasting for 5 min or more hazardous specially to a fetus.

Imaging modes and thermal effects Doppler image M-mode image In routine practice : B-mode, M-mode and 3D imaging are less likely to give rise to thermal injury. >> figures Doppler US can cause signicant temp. rises. B-mode image

Mechanical Effect No mechanical bioeffects have been reported in humans from currently used exposure in diagnostic US.

Cavitation Currently, No significant cavitation damage in vivo caused by diagnostic or physiotherapy beams.

How did these mechanisms help in developing other techniques The development of new imaging techniques e.g. IV Injection of gas-filled micro bubbles as contrast agents to enhance the echogenicity. New therapeutic applications. e.g. (next slides) Maintaining the safe use of diagnostic US.

Therapeutic U/S Usually continues US wave or pulses of much higher intensities than in diagnostic. Examples of applications: Lithotripsy, (mechanical) Tumor therapy by high intensity focused ultrasound (HIFU): heat tissue (thermal) & produce necrosis.

Diagnosis vs. Therapy Diagnostic exposures are designed to the interaction of US with tissue to avoid potential bioeffects. Therapeutic application depends on the direct interaction of US with tissue to produce the desired beneficial bioeffect. Exposure parameters are often different. Therapeutic intensities, pulse durations far exceed the diagnostic devices output.

US Exposure During Pregnancy US Exposure During Pregnancy Almost 100% of fetuses in the developed world receives one or more US scans. Risks & benefits are different depending on: Types of US, stages of pregnancy, machines, centers, & sonographers Each situation must be judged in its own merit. Major centers are preferable for better trained sonographers & powerful machines No long or repeated scans.

Long Term Adverse Effect During Pregnancy Some reported fetal effects of US exposure: Delayed speech, dyslexia*, growth restriction, & non-right-handedness. BUT up to date (7/2009), there is insufficient justication to conclude that there is a causal relationship between diagnostic US & long-term adverse fetal effects.

Recommendation for clinical practice of diagnostic US No routine US with no clear indications for use. Should only be used when benefits outweigh risks. Users should know the exposure parameters of US equipment they employ. Users must know how to alter machine settings so as to reduce exposure. Instruments must be checked routinely to maintain the capability of obtaining reliable diagnostic information at ALARA exposures.

Recommendation for clinical practice of diagnostic US For US scans for operator training, memorial pictures & videos of the fetus, or research, a lower threshold is recommended ) TI 0.5, MI 0.3 ( It is not recommended to use color Doppler mode of the 1 st trimester embryo routinely; as this mode has a potential to produce signicant temp. rises. Acoustic output from B-mode, M-mode, 3D imaging is safe during all pregnancy stages(if used as needed).

Quality control

The generation of satisfactory ultrasound images depends on the skills of the operator as well as the performance of the equipment. The former is addressed through personnel training, and the latter through proper selection, care and maintenance of the equipment to ensure optimum performance. Ultrasound equipment contains some delicate components which can easily be damaged physically or electrically. These include the transducers and the computer. Conscious effort must be made to protect the equipment during routine usage.

For example, dropping a transducer, or knocking it against other objects, may lead to damage. The accumulation of dust in the electrical components, especially the computer, causes danger to the equipment. Regular servicing of the equipment by qualified technical personnel should be catered for in the operating plans, and implemented. For ultrasound equipment, routine servicing is required about four times a year.

Where resources are available, quality control measures can include performance testing of the equipment. Various test objects have been developed for testing ultrasound equipment. However, their use in practice has not gained the same level of acceptance as the quality assurance kits used in other areas of medical imaging. Particularly useful would be phantoms which can be used to assess image resolution, beam penetration, the accuracy of measurement callipers, system sensitivity, and dynamic range.

Documents

TRANSDUCERS FOR REAL- TIME IMAGING BY Dr/ Dina Metwaly