II. EQUIPMENT, INSTRUMENTATION AND TECHNOLOGY
IMAGING AND IMAGING SAFETY
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QUESTIONS
Ultrasound
Ultrasound imaging uses high-frequency sound waves to view inside the body based on tissue or fluid acoustic impedances.
Ultrasound images show movement of the body’s internal organs as well as blood flowing through the blood vessels.
There is no ionizing radiation exposure associated with ultrasound imaging.
- The transducer, or probe, is placed on the skin with a thin layer of gel so that the ultrasound waves are transmitted from the transducer through the gel into the body.
- The strength (amplitude) of the sound signal and the time it takes for the wave to travel through the body provide the information necessary to produce an image.
- Bones and connective tissue are high-impedance/dense structures and result in a bright (hyperechoic) image.
- Fluids appear black (anechoic) because of minimal impedance.
- Low impedance structures result in gray imaging (hypoechoic).
- One of the most common reasons for poor visualization is lack of gel.
- During regional anesthesia administration, the best view of the needle shaft and needle tip occurs when the shaft is aligned to the longitudinal axis of the ultrasound probe, also known as “in-plane”.
- The other approach provides a cross-sectional view, known as “out-of-plane”.
Fluoroscopy
Fluoroscopy uses x-ray but provides a live or active version that shows movement of the body.
- Procedures, surgeries, and/or diagnostic images utilize x-ray technology. X-rays are a static image of the body.
- X-ray images are produced by sending a current from a cathode (negative electrode) through the body to an anode (positive electrode). The radiation that passes through is converted to a visible image for viewing.
- A typical fluoroscopy machine comes equipped with a c-arm.
- The recommended annual limit of exposure to radiation is 50 mSv.
- Maximum exposure during pregnancy or lactation is 5 mSv.
- The fetus is most sensitive to radiation between 8-15 weeks gestation.
- Maximum exposure during pregnancy or lactation is 5 mSv.
- Most vulnerable tissues (because of high cellular turnover):
- stem cells
- bone marrow
- intestinal epithelium
- reproductive tissues
- Least sensitive tissues:
- bone
- muscle tissue
- Minimizing exposure risk to personnel:
- Maximize the distance between personnel and x-ray source
- Minimize the time/duration of x-ray exposure
- Physical protection from the radiation (lead aprons)
Radiography
The physical principles of magnetic resonance imaging (MRI) are based on the magnetic field.
The magnetic field of a MRI exert attractive forces on metallic objects.
Hydrogen is the atom most often used for imaging because it is present in most tissues.
- The MRI environment is hazardous due to the presence of a very strong static magnetic field, radiofrequency waves, and a pulsed magnetic field. Other hazards include high-level acoustic noise, systemic and localized heating, and accidental creation of projectiles.
- 4 zones pertaining to MRI proximity:
- Zone 1: accessible to general public
- Zone 2: patient areas to obtain history background information and ascertain MRI safety concerns; metal detector is needed here
- Zone 3: imaging control room; potential for adverse-related events
- Zone 4: MRI unit magnet room; most dangerous
- MRI safety terms:
- MR safe
- A device does not pose a threat
- No scanning restrictions
- MR conditional
- Devices may enter the MRI suite but only under specific conditions as confirmed by the label.
- MRI safety labeling should be matched with the MRI system
- MR unsafe
- Devices are unsafe and should not enter the MRI suite
- MR safe
- ANYTHING metallic must be removed from the MRI suite, this may include:
- badges
- clipboards
- piercings
- ink pens
Medical equipment like oxygen tanks, IV poles, wheelchairs, and beds can become projectiles.
- ECG monitoring must be carried out with fiberoptic or carbon fiber wiring to minimize the risk of burns.
- Average noise levels are around 90-110 dB but can be louder with increases MRI intensity.
- Implanted objects create a unique safety issue and have the potential to move or dislodge if they contain metal. These May include:
- intracranial aneurysm clips
- cochlear and stapes implants
- retained shrapnel
- orthodontic devices
- intraorbital metallic bodies
- prosthetic limbs
- Intracranial aneurysm clips made after 1995 are MR conditional (contain less ferromagnetic material).
- Cardiac and vascular stents are stable for MRI 6 weeks post-implant.
- Cochlear implants may become demagnetized and should only undergo MRI if the deemed necessary.
- Makeup and tattoos may contain small amounts of metal resulting in burns.
- Cardiac pacemakers have a reed relay switch that can convert the mode to asynchronous, which can be life-threatening if the patient is dependent.
- Patients with legacy pacemakers cannot undergo MRI scanning.
- Pacemakers and defibrillators must be investigated as to whether they are safe per guidelines and recommendations of the manufacturer.
- Children less than 5 often require sedation to remain still.
REFERENCES: Ehrenwerth J, Eisenkraft J, Berry J, eds. Anesthesia Equipment: Principles and Applications. 3rd edition, pages 453-465. Elsevier; 2020.
Cullen BF, Stock MC, Ortega R, Sharar SR, Holt NF, Connor CW, Nathan N, eds. Barash, Cullen, and Stoelting’s Clinical Anesthesia. 9th edition, pages 56, 924. Wolters Kluwer; 2024.