CSB 046 Complementary Imaging Techniques

Transcription

1 CSB 046 Complementary Imaging Techniques - Quizzes are only ultrasound, final includes nuc med and ultrasound Week 1 Intro to Ultrasound Physics - Uses 1 to 20 MHz frequencies, which is way above the sound wave range for human ears - Higher frequency is better detail but less penetration - Images created by interpreting sound reflections - Each dot in the image represents an echo that is returned to the probe - Ultrasound is performed in real time and some images are taken throughout - Images are poorly understood, can t compare to the real time images so sonographers usually fill out a sheet of what they saw - High degree of decisional latitude with sonographers, radiologist relies heavily on the sonographer s observations - Advantages: no radiation, mobile equipment that is quick and cost effective, real time can see motion, accurate measurement of structures, good soft tissue information and can be used for intervention (blood studies) - Disadvantages: not suitable for bone, lungs, abdominal structures covered by bowel, adult cranium and the GIT (most of these surfaces can t be penetrated). Hand held transducer leads to high degree of scan plane variability between sonographers making comparison images difficult. Interpretation is difficult. - Scan planes must understand the orientation of the image of the screen. Remember it is a two dimensional slices of anatomy in of the scan planes - Transverse plane: divides the body into superior and inferior segments. Transducer on the anterior surface of the patient with the bottom being the posterior section. Left and right are reversed. Posterior is not necessarily the very back of the patient but just as far as the beam goes. - Note that the transducer is ALWAYS at the top of the screen - You can image in the transverse plane from a multitude of angles, it doesn t have to be midline. Might do this to avoid artefacts - Sagittal plane: divides the body into left and right segments. Probe located up and down the patient. Image is always viewed with the patient s head (superior) to the left of the screen - Can go from a midsagittal plane to a para-sagittal plane (parallel to the plane), some image orientation - Coronal plane: probe coming in from the side with lateral aspect at the top of the image and the medial at the bottom. Superior is still on the left and inferior on the right. A coronal can be obtained from either side of the patient. - Longitudinal: plane that divides the body into section along its axis. Sagittal and coronal planes are both longitudinal planes. About aligning to the long axis of the anatomy - Oblique: any plane through the body or organ other than the planes already described - These last two relate to the anatomy present rather than the approach to the scan like the first three - Echogenicity: relates to the brightness of the structure, how intense the echoes are

2 - Anechoic: Under the banner of echogenicity. Used to describe an area that has no echoes and appears completely black on the image. Structures such as blood, bile or urine usually appear anechoic because there is no interface within the fluid to return an echo. May also hear the word echolucent or sonolucent which mean the same. - Hypoechoic: an area where the echo is of low intensity, where it is dark on the screen. Not anechoic as some echos are returning. Echopenic is also used to mean the same - Echogenic: an area where the echoses are more intense and bright on the screen, high number of echos returning to the probe. For example, gas will do this. Also called hyperechoic - Echogenicity is a relatively subjective term, may also use terms such as mid/ moderate level echogenicity - Is useful to use these terms to describe one structure relative to another - Echotexture: relates to the pattern of the image, a description of the pattern of the echoes - It could be described as fine (can t identify individual pixels), coarse, homogeneous (even pattern/gray scales) or heterogeneous - Acoustic Windows: an anatomical structure or configuration that allows anatomy to be visualized deeper because they naturally cause little attenuation. Suitable windows are used to view anatomy from as many angles as possible - Examples include a full urinary bladder to help see the prostate or uterus and ovaries - Liver and spleen are also acoustic windows - Amniotic fluids in pregnancy act as an acoustic window to help visualize the developing foetus - Aqueous and vitreous humour help visualize eye structures - Intercostal spaces and sternal notch allows visualization of the heart and thorax as the bones will attenuate the beam too much - Fontaelles of neonatal heads also act as an acoustic window PHYSICS - Lower frequencies penetrate deeper into the tissue - Ultrasound started by having patient emerged in water (which doesn t attenuate the beam) - Used to be considered more therapeutically rather than diagnosis - Somascope was the original, followed by water bath scanner - Contact scanners was the first precursor to modern technology but not in real time - Modern equipment consists of: transducer, monitor for display, housing with electronics/controls and a recording device - Wave physics determine how ultrasound works - Electromagetic waves are xray and require no medium for propagation - Mechanical wave are ultrasound, defined as the propagation of energy trhough a medium which often needs to be a deformable elastic medium - Transverse waves (e.g. water) the waves move longitudinal and particles up and down - Longitudinal waves both move longitudinal - Sound propagates through a medium in the form of compression waves - As you compress the wave, the area directly behind it gets a bit more relax and lengthened

3 - Velocity variation with heavy molecules the sound transmits at a lower velocity because it take more energy to move - With more compressible materials (e.g. air) the sounds transmits with lower velocity. Weaker intermolecular forces and more compressible - Less compressible and stiffer materials such as bone transmit with higher velocity - Gain is the overall brightness of an image, can be overall or TGC - Overall gain = overall brightness of the entire image - TGC = time gain compensation which increases gain (attenuation) as you get deeper. So deep structures may appear slightly different normally than the superficial structures. Used to compensate for this loss of attenuation by amplifying structures that are deeper - The electrical power from machine passed to transducer crystal and converted to the sound wave - Can manipulate the beam, which is divergent or shaped and not necessarily constant to help focus and show the depth of anatomy better - Narrow and tighter beam has greater intensity as its focused into a small area, beams intensity effects echo strength and therefore brightness - Intensity is the preferred unit to measure the power of the ultrasound beam - Dynamic range is the range of grey scales that can be shown in the image - The range of echoes on the image range from the high level before saturation to the lowest level - Soft tissue interactions are important to image formaition, gain and production of artifacts - Refractions or attenuation may occur (may be absorption, divergence, scattering or reflecting) - Refraction: change in direction of ultrasound beam when it crosses as tissue boundary at an angle. Depends on the difference in sound velocity of the two structures - Reflection and refraction interaction are the primary causes of artefact in ultrasound - Attenuation: reduction of intensity of the beam as it passes through medium. Degree depends on the material, the distance travelled from the source and the frequency of the beam - Attenuation co-efficients in deciebel, water is very low and so can t attenuate the beam whereas lung has a very high co-efficient and reflects back a lot of echoes - Absorption result from internal frictional forces that directly remove energy from the beam that is converted to heat. Decreases the penetration of the beam - This depends on the viscosity of the material itself (more viscous means less can penetrate) - Depends on materials molecule relaxation time, if they are slow they are still returning when the next compression wave hits - Also depends on the depth of the tissue because obviously more leads to more beam absorption - Divergence (diffraction) spreading of the ultrasound beam as it moves further from the source. Increased diffraction causes increased attenuation - Occurs in far field (non focused) or beyond the focal zone (focussed) of a transducer - Also occurs after reflection from a convert or curved interface, creating artefact - Scattering the dispersion of the ultrasound beam in many directions when the wave strike a very small object. Scatter pattern depends on size of interface

4 - Only a very small portion of the scatter actually returns to the transducers to creater the image - Reflection can be non-specular (diffuse, small portion of beam only returns due to irregular surface) or specular (smooth surface where amount of return depends on angle of incidence. Greatest when probe and surface are at 90 degrees) - Therefore try and get probe and area of interest at a 90 degree angle to get the maximum return of echoes - % of beam returned depends on: angle of incidence, surface texture and acoustic property difference - Near gain represents the amount of gain applied to the closest echoes. Shape of this graph depends on the tissue type you re passing through - Use delay control to regulate the time at therefore depth at which the TGC begins to be applied Revision 1. What is a sonographer and the role they play? A sonographer is a trained professional that perform ultrasound, which uses sound waves from 1 to 20 MHz (outside of human hearing) to create images as they propagate through the medium. Commonly 3 to 10 MHz is used. They have any important role in making written observations and have a high degree of decisional latitude because they are seeing the images in real time. 2. Advantages and Disadvantages of Ultrasound Advantages: does not use ionizing radiation, produces real time images, mobile equipment that is cost effective and quick to use, produces good soft tissue information, is generally well tolerated by patients and can also be used for intervention such as blood studies. Disadvantages: many areas not suitable for imaging with US especially gas filled areas such as the GIT, abdominal structures or lungs that will return very little signal, hand held transducer that makes it highly operator independent and leads to scan plane variability, can be hard to interpret. 3. Imaging planes Transverse plane a scan plane that divides the body into superior and inferior portions. On the image presented, the top will be the anterior portion where the transducer sits, the bottom will be posterior and left and right will be reversed (like a CT slice). You can achieve this plane from a multitude of angles. Sagittal plane divides the body into left and right segments, with the transducer still placed on the anterior portion of the body. The anterior portion is at the top of the screen, the posterior at the bottom, the superior at the left and the inferior at the right. When this is in the midline it is the sagittal plane, anywhere else it is the parasagittal plane. Coronal plane the transducer is placed on the side of the patient to divide the body into anterior and posterior sections. The top of the image will be the lateral aspect and the bottom will be the medial aspect, this is because the beam will never penetrate fully

5 through to the other side of the patient. Again the superior side must be on the left and the inferior on the right. Longitudinal planes these are planes that include those above such as sagittal and coronal, which cut the body along its long axis. This relates to the anatomy present rather than the scan approach. Oblique plane may also be used in similar fashion e.g. a coronal scan approach producing an oblique plane of the kidney. 4. Acoustic windows Using a structure or anatomical configuration to allow deeper penetration of the ultrasound beam. This is done by using a structure of little attenuation to view the anatomy from as many angles of possible. An example is using a full urinary bladder to visualize the prostate in male or the uterus/ovaries in females. Another method is filling the stomach with water to see the pancreas as this helps displace bowel gas that would appear echogenic and obscure visualization. Amniotic fluid is perhaps the best known example of an acoustic window in ultrasound. Anatomical configurations used as acoustic windows include using intercostal spaces or the sternal notch to see thoracic structures and the fontanels of neonates heads to see deeper brain structures. 5. Ultrasound terminology Anechoic area of total blackness on the image that returns no echoes. This may be structures such as blood, urine or bile. Any region of the body where there is no tissue interface within to produce the echo. Hypoechoic an area of grey on the image that returns some echoes. Still dark on the screen but there is the presence of some echoes. Echogenic an area of bright whiteness on the screen showing a high number of returning echoes e.g. air. Fine/Course used to describe the echotexture or pattern of the echoes. Determined by looking at the size of each individual pixel and if they are visible Homogeneous/heterogeneous also used to describe the echotexture, looking at how uniform the echogenicity is within a structure. 6. Ultrasound Physics / Wave Physics A highly compressible medium such as air has a low speed of sound as it takes more energy to overcome the molecules inertia and weak intermolecular forces. A less compressible medium transmits sounds at a higher velocity. This is called velocity variation, in air sound travels around 330 m/s where in steel it is much faster at 5800m/s. This difference in speed of sound at tissue boundaries is a fundamental property that creates echoes. High frequency and small wavelength beam provides better resolution and detail that a low frequency beam but the depth of the beam is significantly reduced. Sound waves are longitudinal waves where the wave motion and particle motion is in the same direction. As sound propagates through a medium, it compresses much like a spring and the area of rarefaction occurs immediately behind the wave. This is a type of mechanical wave, meaning it needs a deformable, elastic medium to propagate.

6 The ultrasound beam is measured in intensity where the electrical power of the beam is turned into sonic power at the transducer crystal. This beam can be focused to increase the intensity, as where the beam is most compressed it will be strongest. The intensity of the beam in turn effects the intensity of the returning echoes. 7. History of Ultrasound In 1880 the piezoelectric effect was first discovered in which it was found that certain crystals produced electrical potential when subject to mechanical pressure. This would become the basis of transducer crystals. Originally, ultrasound was used therapeutically rather than diagnosis. Early forms of scanning such as the somascope and water-bath scanner immersed the patient in water because this would not attenuate the beam. From here there was evolution to contact and real time scanners. In 1975 some Australian scientists produced the UI Octoson, which had several transducers underneath the patient that was suspended above a large water bath. This produced great detail of the breast, abdomen and uterus. 8. Ultrasound equipment and controls Equipment will always include these 4 components: a transducer, monitor for display, housing for electronics/controls and a recording device. 9. Sound Tissue Interactions The main two interactions are refraction and attenuation. Under the subheading of attenuation there are 4 further types of interaction: absorption, divergence, scattering and reflection. Refraction A change in the direction of the ultrasound beam when it crosses the tissue boundary at an angle, that is to say the beam is not perpendicular. This is one of the primary causes of artefact in an ultrasound image. This change in direction depends on the velocity of the two media. Attenuation a general term form the reduction of intensity of the ultrasound beam, that may be from one of four possible processes. Degree of attenuation depends on the material involved, the distance travelled and the frequency of the beam. Attenuation coefficients are measured in decibels per centimeter per MHz and is the relative intensity loss per centimeter of the medium. Lung has a high co-efficient (returns lots of echoes, echogenic) and water has a very low one (no echoes returned, anechoic) o Absorption: directly removes energy from the beam due to internal friction forces. Absorbed energy turned into heat. Depends on the materials viscosity (more viscous means more energy to move molecules so more attenuation) and relaxation time (if they are slow, molecules are still returning with the next wave and takes more energy to reverse than if it is at rest). Also depends on frequency of the beam as higher frequency moves molecules faster so expends more energy and shorter wave lengths reduces the relaxation time before the next wave. The depth of the tissue also affects this as the beam travels further it will obviously absorb more. o Divergence (diffraction): the spreading of the ultrasound beam as it moves further from the source which causes increased attenuation. Occurs in the farfield of a non focused, after the focal point in a focused and after reflection at convex or curved interfaces.

7 o Scattering: dispersion of ultrasound beam in several directions usually from striking a very small object. The amount of scatter depends on the size of the wave length in relation to the size of the interface. Rayleigh scattering occurs equally in all directions and is the type that occurs from blood cells. Only a very small portion of this scatter returns to the transducer to effect the image. The scattered waves interact with each other to create backscatter patterns that contribute to echotexture. o Reflection: This is the major contributor to producing images and there are two types, specular and non specular. Non-specular (or diffuse) is when the soundwave strikes an irregular surface, the portion that returns to the transducer is small but the beam can strike the surface at a large number of angles and some will still return. Specular is when it strikes a smooth surface and detection of this echo is highly dependent on the angle of incidence. If the beam does not strike at 90 degrees to the interface it will not return to the transducer. The percentage reflected back to the transducer therefore depends on the angle of incidence, the surface texture and the acoustic property differences of the two tissues. REFLECTION AND SCATTERING = PRIMARY INTERACTIONS FOR IMAGE PRODUCTION REFRACTION AND SCATTERING = PRIMARY INTERACTIONS FOR IMAGE ARTEFACT 10. What is TGC? What is gain the overall brightness of an image. It is affected by the interactions between sound and the tissue it passes through. Also affected by basic machine parameters. May be overall gain or TGC. This stands for total gain compensation and is used to compensate for the loss of energy that occurs within tissue at increased depths. As the beam travels deeper more attenuation occurs and therefore the returning echoes become progressively weaker. These weaker signals may create the appearance of pathology as deeper sections of the same organ may appear different due to the depth even though they are attenuating the same degree. Algorithms are used to compensate for this by amplifying the deeper signals more and the more superficial signals less. This helps produces equal brightness echoes from similar interfaces regardless of their depth within the patient. Near gain is the amplification applied the closest echoes and far gain to the furthest echoes. The slope of the graph is far gain and the flat section is near gain. Delay control can be used to determine at what time the signals will be amplified, helps avoid things such as the skin surface. Week 1 Intro to Nuclear Medicine - Taking images of the body post injection/oral consumption/inhalation of a radioactive material (usually technetium) - Functional imaging of physiology rather than just anatomy - A branch of medical radiation science - Uses unsealed radioactive sources (radiopharmaceuticals) to diagnose, treat and assess pathologies - Radiopharmaceutical depends on the organ or anatomical function to assess - Hybrid imaging with CT and MRI combine functional and anatomical information

8 - Scans may take anywhere from 60 seconds up to 5 days but most take a few hours to complete - Used for diagnostic and therapeutic procedures - Radionuclide produced with a generator system (allows production on site), nuclear reactor or cyclotron (used for PET) - NMS/NMT s will prepare patients, prepare and administer pharmaceuticals (in hot lab), perform imaging and then process/analyse images - Radionuclides have no effect on the kidneys, it is nothing like CT contrast - If they patient has been released from nuclear medicine, the radiation level they are emitting is safe - Gamma camera/scanning room = location of imaging equipment - Radioactive store = assigned location to store radioactive materials until they reach safe levels for disposal - Generator = used to produce the radionuclides needed to create radiopharmaceuticals - Techy = technetium 99 which is the radionuclide used for a large number of studies - Kit = vials containing pharmaceuticals which are combined with the nuclides to make radiopharmaceuticals - Hot means radioactive/high activity on image and cold means not radioactive/low activity on image - Generator uses saline passing through molybdenum column in a milking process that creates techy (6 hour half life) - Patient is injected radioactivity emits gamma rays gamma camera detects it - Challenges with radiation safety: containing spills, minimising radiation exposure to all, handling and transporting radioactivity - Patient compliance can be an issue such as claustrophobic patients, inability to stay still and refusing the test due to radiation fear - Very expensive for cost of scanners, radioactivity and other equipment - Can be difficult to get radionuclides that are unavailable (may be unavailable for years at a time) - Most radiopharmaceutical suppliers are located overseas - Not widely available outside of metropolitan areas and not known about amongst clinicians - First gamma camera known then as the Anger Camera developed in Modern gamma cameras have a dual head that rotates and has a CT gantry aligned directly behind it - SPECT single photon emission computed tomography. Gamma heads do a slow full 360 degrees. Ideal for examinations not requiring CT - SPECT/CT also has a CT unit combined. Usually a low dose CT done with a standalone nuclear medicine scan. Not a diagnostic CT. Most versatile camera in the modern era - Dedicated scanners for specific organs in the body, most common is the one used to image the heart. Small field of view that gets very close to the patient and gets more counts. - Portable gamma cameras not very common with no CT capability. Usually used in emergency cases where patient is unstable - Intraoperative gamma cameras used frequently in breast imaging during surgery but not widely available