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Ultrasound Guided Equipment: Science, Technology, Clinical Applications
Transcript of Ultrasound Guided Equipment: Science, Technology, Clinical Applications
Dr. Hassan A. Chaaban
Familiarity with U/S
Understand the physics behind the equipment
Integrate ultrasound as an integral part of anesthesia education
Keep up with the common trends and advances in medicine
Understand how anatomical landmarks look on ultrasound
U/S is a great tool in well trained hands, education is hence required to understand benefits and limitations.
Sound and Ultrasound
Frequency and Wavelength
The wavelength λ, is the frequency at which the wave repeats itself. In ultrasound, the vibrating source of the wave is a vibrating piezoelectric crystals. λ = c/f; where c is speed of sound and f is frequency. Example 1540/5MHz = 0.31 mm.
Frequency of ultrasound remains constant during propagation
Frequency influences image quality
Higher frequencies have longer near field, less divergence in far field, less penetrating ability
Lower frequencies provide greater penetration depth, but less defined images
Ultrasound Transmission Properties
Sound production requires a vibrating object to produce a sound wave.
The source of sound waves in medical ultrasound is a piezoelectric transducer.
Sound waves are produced in response to an electrical impulse. Sound travels at a speed of 1540 m/sec
Piezoelectric crystals in the transducer face vibrate at a specific range of frequencies when an electric current is put through them. This vibration results in alternating compression and rarefaction of the surrounding material, which is then transmitted into the patient. These crystals act as both generator and receiver of U/S signals. As an alternating polarity is applied to the crystals, they vibrate generating sound which causes changes in the crystal when received back and thus an image is created.
Medical Uses of Ultrasound
precision-based procedures and treatments, including nerve blocks, upper extremity blocks, neuraxial blocks, femoral nerve blocks, steep angle injections, paraspinal blocks, brachial plexus blocks, epidurals, and regional pain blocks, with increased efficiency. Because ultrasound technology enables the anesthetist to visualize the needle, injections can be administered more with a higher success rate than without ultrasound guidance.
With the exception of bone, biological tissue act much like fluid for U/S propagation. Tissue with high cartilage or connective tissue content are less conductive than Fat and lungs.
The speed of sound in the human body is determined by the characteristics of the medium through which it travels
The elasticity of the medium and it’s density are most important characteristics in determining speed
The stiffer the medium, the faster the speed of transmission
The denser the medium, the slower the speed of transmission
Ultrasound Transducers or probes come in different shapes and frequency settings based on usage. The anesthetist needs to be aware of these differences when selecting a transducer.
- Linear Arrays
- Curved Arrays
- Phased Arrays
1) Sector Array
- crystals are placed parallel or in concentric rings
- transducer face is curved
- produces sector or pie-shaped image
2) Linear Array
- crystals are placed parallel
- transducer face is flat
- Produces rectangular image
1) A Mode
2) B Mode
3) M Mode
4) D Mode or Doppler
- Color Doppler imaging– displays colors based on measurement of mean Doppler shifts
- Red and Blue commonly refer to flow toward and away from transducer respectively
- Colors provide no direct indication that vessels are arteries or veins
Very specific and angle dependent.
Flow towards the transducer is displayed above the baseline
Flow away is displayed below the baseline
Helps make distinction between arterial and venous flow
Ultrasound Reflection Properties
Acoustic impedance = product of velocity of sound (v) and the physical density
Acoustic energy is reflected at interfaces between tissues with differing acoustic impedances (Z)
Strength of acoustic reflection increases as difference in Z increases
For soft-tissue/air, soft-tissue/bone and bone/air interfaces, almost total reflection occurs
Ultrasound Reflection Properties
Attenuation – progressive weakening of sound as it travels through a medium which progressively absorbs some of the energy of the U/S wave
Determined by medium, number of interfaces, wavelength of emitted sound
Reflection, scattering, absorption
Effects on Tissue
Acoustic cavitation: due to
Gas may be drawn as bubbles which may cause damage once U/S stops and bubbles collapse
High intensity Focused U/S
Conveys energy to tissue
Heat generation with temperatures of 60-80 C.
Finding The needle: failure to visualize the needle and continuous probe movement.
Most common mistake: continuing to advance needle through structure
3% LA toxicity,
2.3% pneumo PNB
Anesthesia Analgesia 2007; 104:1281
Failure to recognize limitations of U/S
Reg Anesth Pain Med. 2007 Jan-Feb;32(1):84-8.
Facilitating needle alignment in-plane to an ultrasound beam using a portable laser unit.
Normal or audible sound for human beings range between 20 Hz to 20,000 Hz. Sound frequencies < 20 Hz is referred to as infrasound; ultrasound is for frequencies > 20,000 Hz.
Ultrasound is defined as the sound of frequency above bandwidth of human hearing. Although variable, it is usually 20,000 Hz or 20,000 cycles/second in healthy adults. 1 MHz = 1 million cycles per second
Medical or diagnostic ultrasound uses frequencies in the range of 1 – 20 MHz. Therapeutic ultrasound has a frequency range of 0.75–3 MHz.
Frequency of a wave is the number of cycles/sec. if an U/S transducer produces 4 complete cycles/sec then it frequency is 4 Hz/sec, if it produces 4 million cycles/sec, then it’s frequency is 4 MHz
Is a technique that employs high frequency acoustic energy via an ultrasound transducer on skin, mucus membranes or body organs. The returning echoes are picked up by the transducer with various displays depending on tissue density.
Most modern transducers have an array of piezoelectric elements arranged in this fashion. Transducers usually have 128 elements but may go high as 4000 elements. PZT is the most common element which is a synthetic Plumbium Zerconium Titanate. Earlier version contained quartz as the main element.
Linear Array Transducer
Flat Foot Print, Rectangular image shape, high frequency 8-13 MHz, ideal for superficial structures such as IJ, Carotid, thyroid, brachial plexus. The only disadvantage is its inability to view deeper structures.
Convex Foot Print, allows for a wider field of view, preferred probe for deeper structures. Has cross-beam technology which allows for better image resolution; frequency range from 2-5 MHz, see most rib shadowing with this probe.
Image is in sector format (trapezoid). Ideal to look through acoustic windows (intercostal spaces). Frequency range from 1-4 MHz, allows to see details of the heart and abdomen. Smaller field of view.
Sector (Phased Array) Transducer
The surface of the probe that must come in contact with the skin or tissue is called the foot print. It where the Probe emits and receives U/S signals. A single marker is present on one side of the probe and usually refers to a dot or logo on one side of the screen which helps in spatial orientation of the image.
Transducer Marker and Foot Print
The gain button allows for images to be darker or lighter based on the need for the procedure. Newer machines have 3 settings low, medium and high
Posterior Acoustic Enhancement
Commonly Used Ultrasound machines in Anesthesia
Enhanced needle visualization Algorithm