Ultrasound Imaging.pdf

Ultrasound Imaging By: Abdullah Munqith History of Ultrasound Imaging History of Ultrasound Imaging 1942: First application of Ultrasound in medical imaging. End of 1960's: Boom of Ultrasound in medical imaging. Early 1970s: Gray scale static images of internal organs. Mid 1970s: Real-time imaging....

Ultrasound Imaging By: Abdullah Munqith History of Ultrasound Imaging History of Ultrasound Imaging 1942: First application of Ultrasound in medical imaging. End of 1960's: Boom of Ultrasound in medical imaging. Early 1970s: Gray scale static images of internal organs. Mid 1970s: Real-time imaging. Early 1980s: Doppler effect. Sound Wave Sound is a mechanical and pressure wave. Sound is a Longitudinal Wave. Mechanical vibration transmitted through an elastic medium. Sound waves when propagate through air at appropriate audible frequency produce sensation of hearing. Velocity of Sound ✓ Velocity = frequency x Wavelength V=f λ ✓ Frequency – Number of wavelengths per unit time. Unit of measurement: 1 cycle/sec = 1 Hz. Frequency is inversely related to wavelength ✓ Velocity – Speed at which waves propagate through a medium. Depends on the physical properties of the medium through which it travels. Directly proportional to the stiffness of the material. Velocity of Sound in different materials Material Velocity (m/s) Air 330 Water 1497 Fat 1440 Blood 1570 Soft tissue 1540 Bone 4060 Metal 3000-6000 Frequency of Sound Ultrasound Ultrasound is sound with a frequency over 20,000 Hz. The frequencies of medical Ultrasound waves are several magnitudes higher than the upper limit of → human hearing. Frequencies used for diagnostic ultrasound are between 2 to 20 MHz. The basic principles and properties are same as that of audible sound. Acoustic Impedance The resistance that a material offers to the passage of a sound wave. The difference between the acoustic impedances of the different tissue types are responsible for the echoes on which ultrasound imaging is based. ✓ Unit of measurement: Rayl = kg/m2.s Reflection at boundaries Reflection at boundaries Specular Reflection (large, regularly shaped objects with smooth surfaces): Perpendicular Incidence to get —> strong signal. Diffuse Reflection (Small structures): Reflected waves travel in various directions away from the interface —> weak signal. Rayleigh Scattering (Particles size Very weak signal Attenuation Attenuation: Loss of intensity and amplitude of ultrasound wave as it travels through the tissues, due to reflection, scattering and absorption (dissipation as heat). Proportional to Frequency and the distance the wave front travels. Higher frequency, more attenuation. Longer the distance (Depth), more the attenuation. Ultrasound generation 1- Equipment and principle Ultrasound scanners consist of a console containing a computer, a screen, and a transducer that is used to do the scanning. The transducer, which can convert one form of energy into another, is a small hand-held device that resembles a microphone, attached to the scanner by a cord. The transducer converts electrical impulses into ultrasound waves and sends them into the body. Then listens for the returning echoes from the tissues in the body and converts these echoes into electrical impulses. The ultrasound image is immediately visible on a video display screen that looks like a computer or television monitor Ultrasound generation Ultrasound generation Ultrasound generation 2- Piezoelectric effect piezoelectric crystal or material has the ability to generate an electric charge in response to applied mechanical stress and vise versa. Image formation As the ultrasonic beam passes through or interacts with tissues of different acoustic impedance, it is attenuated by a combination of absorption, reflection, refraction, and diffusion. Sonic waves that are reflected back (echoed) toward the transducer cause a change in the thickness of the piezoelectric crystal, which in turn produces an electrical signal that is amplified, processed, and ultimately displayed as an image on a monitor. The fraction of the beam that is reflected back to the transducer depends on the acoustic impedance of the tissue How is the US procedure performed? How is the US procedure performed? For most ultrasound exams, the patient is positioned lying face-up on an examination table. A clear water-based gel is applied to the area of the body being examined to eliminate air pockets between the transducer and the skin that can block the sound waves from passing into your body. The sonographer (ultrasound technologist) or radiologist then presses the transducer firmly against the skin in various locations to better see an area of concern. Benefits vs. Risks Benefits : Most ultrasound scanning is noninvasive (no needles injections)and is usually painless. Ultrasound imaging does not use any ionizing radiation. or Ultrasound scanning gives a clear picture of soft tissues that do not show up well on x-ray images. Ultrasound is the preferred imaging modality for the diagnosis and monitoring of pregnant women and their unborn babies. Ultrasound provides real-time imaging, making it a good tool for guiding minimally invasive procedures such as needle biopsies and needle aspiration. Benefits vs. Risks Risks : For standard diagnostic ultrasound there are no known harmful effects on humans. Sound waves can increase body temperature (This is known as cavitation). Significant only for long exposure time. Limitations of General Ultrasound Imaging Ultrasound waves are disrupted by air or gas; therefore, ultrasound is not an ideal imaging technique for air-filled bowel or organs obscured by the bowel. In most cases, CT scanning, and MRI are used. Ultrasound has difficulty penetrating bone and, therefore, can only see the outer surface of bony structures and not what lies within. For visualizing internal structure of bones or certain joints, other imaging modalities such as MRI and X-rays are typically used. Large patients are more difficult to image by ultrasound because a greater amount of tissue attenuates (weakens) the sound waves as they pass deeper into the body.