Page 113 - Libro 2

7 — Intracranial Cerebrovascular Examination
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SONOGRAPHIC EXAMINATION TECHNIQUES
PATIENT PREPARATION
Using simple language explain the test to the patient, instruct the patient to be quiet, and, unless neces- sary, not to speak during the exam. Take a relevant history from the patient or refer to medical records and make note of relevant indications for the test. When working in the intensive care unit (ICU) set- ting, check with the nurse before changing the level of the head. Take and record the blood pressure.
PATIENT POSITION
The patient is examined supine with the head slightly elevated during examination of the anterior circulation when using the transtemporal, transorbital, and sub- mandibular approaches. It is advisable to use a rolled hand towel or a very small pillow to allow maximum access to the head and neck. The patient should be made as comfortable as possible as his or her coop- eration and stillness is important to obtaining a good study. A semidark room facilitates relaxation as well as better image visualization on the ultrasound screen. To minimize variations in the spectral waveforms caused by fluctuating physiological variables, allow time for the heart, respiratory rate, and blood pressure to reach a steady state before beginning the study.
The VAs and BA require insonation through the fo- ramen magnum. Place the patient in the lateral decubi- tus position supporting the face and head with a small pillow or towel with the neck aligned centrally. Palpate at midline about 1.25 in down from the skull base and flex the neck slightly. If the patient cannot be turned onto his or her side, the examination can be performed with the patient supine with the head rotated to the opposite side of insonation and the transducer placed just to the left or right side of the foramen magnum. For ambulatory patients, the upright sitting position is a feasible alternative with the neck flexed slightly and the head supported by the patient’s arms and hands for stabilization. For ICU or other hospital inpatients that cannot be turned or optimally positioned, the head can be propped up using a rolled towel and turned away from the side of insonation. This will create a space large enough for the transducer to be placed at either midline or just lateral to the foramen magnum and will usually provide adequate access to the VAs and BA.
TECHNOLOGIST POSITION
The position of the sonographer may vary according to the setting. Outpatients are studied from the head or the side of the examination table, but for inpatients the
equipment and the sonographer will usually be placed at the side of the bed. Dedicated TCD instruments often have remote controls, allowing for the manipulation of the instrument at a distance from the machine. Duplex ultrasound systems are more limited and require closer proximity of the machine and the sonographer.
A nonimaging TCD transducer is smaller than most imaging probes and unique ergonomic injuries can result from gripping the transducer too tightly and bending the thumbs and wrists backward. To avoid tendonitis and other injuries of the hands, grip the transducer with minimal pressure, rest when the hand becomes tired, and seek out preventative exercises from an ergonomics specialist.
EQUIPMENT
A dedicated nonimaging TCD instrument uses a 1 to 2 MHz pulsed wave transducer, spectral analysis, and additionally, may have M-mode capabilities. Software allows for the computation of peak systolic velocity (PSV), end-diastolic velocity (EDV), time av- eraged peak velocity (TAP-V), and Goslings pulsatil- ity index (PI) at a minimum. Cursors and spectral outline tracers are available to compute values when automatic computations are erroneous. Additional software will function for specific applications such as monitoring for emboli, trending velocities, and displaying temporal changes in velocity.
The addition of power M-mode in 2002 uses simul- taneous signal acquisition from 32 gates to create a display that demonstrates flow intensity and direc- tion in bands of color (red is flow directed toward the transducer and blue is flow directed away from the transducer). The display ranges from 25 to 85 mm and corresponds to the course and depth of the arteries. Information in the power M-mode display helps the user find signals by creating a kind of visual road map much in the way color flow facilitates duplex imaging.
Standard duplex ultrasound technology allows for TCDI exams through the use of a broadband phased- array transducer with a Doppler frequency range usually of 2 to 3 MHz and imaging frequencies in the range of 4 MHz software provides computational packages simi- lar to those found on dedicated TCD systems; however, TCDI is not routinely used for monitoring applications because the transducer size is too large to attach to the head and most companies have not developed instru- ments, hardware, or software for these applications.
REQUIRED DOCUMENTATION
In general, documentation for both techniques (TCD and TCDI) will be based on spectral waveforms. The B- mode and color Doppler information yielded in TCDI studies primarily facilitates acquisition of spectral