diagnostic radiology

Dr Masood Entezariasl

RADIATION SAFETY Ionizing radiation and radiation safety issues may

be present in remote locations Radiation intensity and exposure decrease with

the inverse square of the distance from the emitting source

It is often possible for the anesthesiologist to be immediately behind a movable lead-glass screen

Regardless of whether this is possible, the anesthesiologist should wear a lead apron and a lead thyroid shield and remain at least 1 to 2 m from the radiation source

Clear communication between the radiology and anesthesia teams is crucial for limiting radiation exposure

Monitoring the Radiation Dose Anesthesiologists, like all other health care workers who

are at risk for radiation exposure, can monitor their monthly dosage by wearing radiation exposure badges

The physics unit of measurement for a biologic radiation dose is the sievert; 100 rem = 1 Sv

Because some types of ionizing radiation are more injurious than others, the biologic radiation dose is obtained by multiplying together a "quality factor" and the ionizing energy absorbed per gram of tissue

Radiation exposure can be monitored with one or more film badges

In the United States, the average annual dose from cosmic rays and naturally occurring radioactive materials is about 3 mSv (300 mrem)

Patients undergoing a chest radiograph receive a dose of 0.04 mSv, whereas those undergoing a computed tomography (CT) scan of the head receive 2.00 mSv

Federal guidelines give a limit of 50 mSv for the maximum annual occupational dose

AllERGIC REACTIONS Contrast agents are used in more than 10 million diagnostic

radiology procedures performed each year In 1990, fatal adverse reactions after the intravenous

administration of contrast media were estimated to occur approximately once for every 100,000 procedures, whereas serious adverse reactions were estimated to occur 0.2% of the time with ionic agents and 0.4% of the time with low osmolarity agents

Radiocontrast agents can produce anaphylactoid reactions in sensitive patients, and such reactions necessitate aggressive intervention, including the administration of oxygen, intravenous fluids, and epinephrine, with epinephrine being the essential component of therapy

Adverse drug reactions are more common after the injection of iodinated contrast agents (used for x-ray examinations such as CT) than after gadolinium contrast agents (used for magnetic resonance imaging [MRI])

The signs and symptoms of anaphylactoid reactions can be mild (nausea, pruritus, diaphoresis), moderate (faintness, emesis, urticaria, laryngeal edema, bronchospasm), or severe (seizures, hypotensive shock, laryngeal edema, respiratory distress, cardiac arrest)

Prophylaxis against anaphylactoid reactions is directed against the massive vasodilatation that results from mast cell and basophil release of inflammatory cytokines such as histamine, serotonin, and bradykinin

The main approach to prophylaxis is steroid and antihistamine administration on the night before and the morning of the procedure

A typical regimen for a 70-kg adult is 40 mg prednisone, 20 mg famotidine, and 50 mg diphenhydramine

Patients undergoing contrast procedures usually have an induced diuresis from the osmotic load presented by the contrast agent

In this regard, adequate hydration of these patients is important to prevent aggravation of coexisting hypovolemia or azotemia

Chemotoxic reactions to contrast media are typically dose dependent (unlike anaphylactoid and anaphylactic reactions) and related to osmolarity and ionic strength

NONINVASIVE X-RAY PROCEDURES Sedation and General Anesthesia Radiology departments commonly use remote

locations where anesthesia services are required for patient immobility, maintenance of adequate oxygenation and perfusion, and minimization of pain and anxiety

Most adult patients, when provided with adequate instructions and preparation, do not need sedation or general anesthesia for noninvasive radiologic procedures

For many other adults, conscious sedation can be provided by qualified nurses

In contrast, sedation or general anesthesia is often required to enable children to cooperate

PHYSIOLOGIC MONITORING Normal physiologic monitoring is essential, as

is supplemental oxygen, which is usually supplied by nasal cannula attached to a capnograph

Capnography provides the respiratory rate and pattern, as well as the end-tidal CO2 concentration

Anesthesiologists commonly use specially constructed nasal cannulas that have a sample line for a capnograph

If capnography is not possible, ventilation must be assessed by continuous visual inspection or auscultation, or both

SUPPLEMENTAL OXYGEN It is preferable to have nasal cannula oxygen

come from a separate flowmeter instead of from the common gas outlet of the anesthesia machine to permit more rapid deployment of the anesthesia machine's breathing circuit for delivering facemask oxygen

For long procedures it is best to administer humidified oxygen through the nasal cannula to avoid leaving the patient with an uncomfortably dry mouth and throat

Certain patients, including infants and small children, will not tolerate a nasal cannula but will do well with an oxygen "blow-by" technique

PHARMACOLOGICALLY INDUCED SEDATION

Conscious sedation can usually be managed successfully with a continuous propofol infusion, with or without supplemental intravenous opioids or benzodiazepines (or both)

A low dose of a rapid-onset, short-acting opioid such as remifentanil or alfentanil is also an appropriate selection

Dexmedetomidine is another useful drug, primarily in procedures lasting more than an hour

This drug is especially useful for patients who cannot tolerate CO2 retention, such as those with severe pulmonary hypertension, or those who require frequent assessment of mental status

Dexmedetomidine should be used cautiously in patients who require strict blood pressure maintenance at or above their baseline levels

Because dexmedetomidine tends to lower systemic arterial pressure, its use might require pressure support or even be inappropriate in patients at risk for cardiac or cerebrovascular insufficiency

COMPUTED TOMOGRAPHY CT is most often used for intracranial imaging and for studies of

the thorax and abdomen Because CT is painless and noninvasive, adult patients

undergoing elective scans rarely require more than emotional support

CT scanning is a crucial diagnostic tool in several acute settings, including traumatic injury (head and abdominal) and stroke (hemorrhagic and nonhemorrhagic)

It is also used for rapid assessment of expanding intracranial masses when an increase in intracranial pressure may be a concern

Sedation or general anesthesia is often essential for such patients, as well as for children and adults who have difficulty remaining motionless

Airway management and adequate oxygenation are the anesthesiologist's primary concerns when providing sedation or general anesthesia to patients undergoing CT

During CT scanning the anesthesiologist steps behind radiation shielding as a controlled, mechanized table moves the patient

Airway hoses, intravenous delivery tubing, and monitors can become kinked or disconnected when the table moves the patient

MAGNETIC RESONANCE IMAGING Patient immobility is the primary indication for

sedation or general anesthesia, which is routinely needed for children, adults who are claustrophobic or in pain, and critical care patients

Although ionizing radiation is not a safety issue because no x-rays or radioactive substances are involved, other important safety issues pertain to the magnet suite. For example, missile injuries can occur if ferromagnetic objects are brought near the magnet

In addition, hearing loss may occur from high sound levels during a scan, as can electrical burns if incompatible monitoring equipment is attached to the patient

Patients with implanted devices or ferromagnetic material should never be inside a large magnetic field

Safety Considerations Objects in the magnet room need to be both MRI

safe and MRI compatible Before an MRI scan is started, the anesthesiologist

should be sure that the patient has been screened and cleared by MRI technicians responsible for knowing that the patient's body does not contain susceptible metal objects, such as incompatible orthopedic hardware, cardiac pacemakers, wire-reinforced epidural catheters, or a pulmonary artery catheter with a temperature wire

Pulse oximetry is essential during MRI scans, and only an MRI-compatible fiberoptic pulse oximeter should be used. (Patient burns can result at the point of attachment if one uses a standard pulse oximeter)

Similar concerns pertain to any other monitoring or management devices that make actual or potential patient contact

MISSILE INJURY

Missile injury in an MRI suite is a serious and life threatening risk

The superconducting electrical currents that generate an MRI scanner's large magnetic field are always "on“

Therefore, MRI scanners are also always surrounded by large magnetic field gradients (up to 6 m away)

Magnetic field gradients can pull magnetic objects into the magnet with alarming speed and force

Whereas certain metals (nickel, cobalt) are dangerous because they are magnetic, other metals (aluminum, titanium, copper, silver) do not pose a missile danger

These metals are used to make MRI-compatible intravenous poles, fixation devices, and nonmagnetic anesthesia machines

MISSILE INJURY MRI-compatible intravenous infusion pumps are

clinically available. If one must bring susceptible metal items such as infusion pumps into the MRI magnet room, they should be safely located and fixed, preferably bolted to a wall or floor, with everything being done and checked before the patient enters the MRI scanner

Anesthesiologists should know that if a missile does fly into the magnet and cause injury while pinning the patient to the inside of the scanner, there is a way that the superconducting magnet can be turned off immediately

However, this is something that should be done only by MRI technicians, and while it is being done, the anesthesiologist should initiate removal of the patient from the scanner because it can become extremely cold during magnet shutdown

Monitoring Issues Many anesthesiologists prefer to be outside

the magnet room during the scan This would seem to be acceptable if in

addition to having monitor displays of vital signs, sufficient simultaneous vigilance can also take place via video cameras and windows

INVASIVE BLOOD PRESSURE MONITORING

Critically ill patients undergoing MRI may require invasive systemic blood pressure monitoring

Long lengths of pressure tubing are added so that pressure transducers and their electrical cables can be far from the magnet, preferably outside the magnet room

All arterial catheter stopcocks should be capped so that hemorrhage is impossible in the event of accidental perturbations in the stopcock setting

Radiofrequency pulsing can sometimes cause the pressure transducer to generate artifactual spikes, which in turn cause the monitoring equipment to falsely calculate an erroneously high blood pressure and possibly mislead the anesthesiologist

Visual inspection of the waveform can lead to rapid detection of this type of artifact

Compatible Equipment The MRI-compatible equipment that goes into the

magnet room is really a second anesthesia station

Although suction, physiologic monitoring, and mechanical ventilation must be possible inside the magnet room, it is nevertheless crucial that a primary anesthesia station be located just outside the magnet room

If a potentially life threatening problem arises, it must be possible to promptly remove the patient from the scanner for transfer to the primary anesthesia station so that optimum care and additional help can be provided more efficiently

Management of Anesthesia Inhalation induction with sevoflurane plus subsequent

establishment of intravenous access for infusion of propofol is a useful technique for pediatric patients requiring anesthesia for MRI

Mechanical ventilation via an endotracheal tube may be needed (concern about aspiration risk, presence of increased intracranial pressure); alternatively, a laryngeal mask airway may be placed after sevoflurane induction for continued maintenance of anesthesia

General anesthesia for adults undergoing MRI brain scans usually requires an endotracheal tube, although a laryngeal mask airway will sometimes suffice

Upper airway obstruction during an MRI brain scan results in motion artifact that is unacceptable

Because hyperoxia can increase signal intensity in brain cerebrospinal fluid, the radiologist must consider this possibility when interpreting the MRI scan