CEREBRAL BLOOD FLOW AND METABOLISM Part 5
Supported by: HURO/0901/069/2.3.1 HU-RO-DOCS
Laser Doppler Flowmetry
There are two different versions of the optoelectronic probe: basic and advanced. Due to its sophisticated detecting array geometry the advanced version allows for resolving between the photons coming from different depths in the tissue. This gives the possibility to distinguish between superficial blood flow and flow in deeper layers of tissue.
Laser Doppler Probes
Basic probe Advanced probe
Laser Doppler Flowmetry
Cerebral Blood Flow and Metabolism Measurement Techniques
Cerebral Blood Flow and Metabolism Measurement Techniques
¨ Ultrasound, like normal sound, is a wave. ¨ If a source of sound moves towards the listener, the
waves begin to catch up with each other. The wavelength gets shorter and so the frequency gets higher – the sound has a higher pitch.
¨ We use this principle to work out how fast blood cells move. Ultrasound reflects off the blood cells and causes a Doppler shift
Doppler ultrasound - Doppler effect
→ change in wavelength with speed
Doppler Ultrasound
• The ultrasound probe emits an ultrasound wave
• A stationary blood cell reflects the incoming wave with the same wavelength: there is no Doppler shift
• A blood cell moving away from the probe reflects the incoming wave with a longer wavelength
• In reality, there is actually two Doppler shifts. The first one occurs between the probe and the moving blood cell (not shown here) and the second one occurs as the red blood cell reflects the ultrasound.
Doppler Ultrasound
Doppler Flowmetry
Blood flow in an artery
Doppler Imaging – Carotid artery
¨ Use BOTH normal ultrasound imaging and Doppler imaging
¨ Used to image blood
flow ¨ Doppler imaging looks
at artery ¨ Get image and trace of
blood flow
This is a healthy artery. The flow is smooth and all in the same direction, like water in a large, slow river
The flow is not all in the same direction. It is turbulent, like rapids in a river This is usually due to a build-up of fatty deposits in the artery
Doppler Imaging – Carotid artery
Transcranial Doppler - TCD
¨ TCD is a non-invasive ultrasonic technique measuring local blood flow velocity and direction in the proximal portions of large intracranial arteries
¨ TCD’s principal use is in the evaluation and management of patients with cerebrovascular disease
Advantages of TCD
¤ non-invasive ¤ can be performed at the bedside ¤ easily repeated or used for continuous monitoring ¤ is generally less expensive than other techniques ¤ contrast agents are not used avoiding allergic reactions and
decreasing risk to the patient
Limitations of TCD
¤ examination of cerebral blood flow velocities in certain segments of large intracranial vessels
¤ detects indirect effects (abnormal waveform characteristics) suggesting of proximal hemodynamic or distal obstructive lesions
¤ more valuable in specific conditions
Techniques
¨ TCD ¤ Transcranial Doppler
A “blind” free hand, non-imaging technique
¨ TCI or TCCS ¤ Transcranial Imaging ¤ Transcranial Color-Coded Duplex Sonography
CT TCCS TCCS with contrast enhancement
TCD
With TCD can be visualised:
¤ Distal Internal Carotid Artery ¤ Middle Cerebral Artery ¤ Anterior Cerebral Artery ¤ Posterior Cerebral Artery ¤ Vertebral Artery ¤ Basilar Artery ¤ Posterior & Anterior
Communicating Arteries
TCD - General Prerequisites
¨ Status of the extracranial arteries has to be known
¨ The patient needs to rest comfortably to avoid major fluctuations in pCO2 and movement artifacts
TCD - General Considerations
¨ Accessibility of the ultrasonic “windows” within the skull that can be penetrated with the ultrasonic beam are often limited
¨ Arteries at the base of the skull vary greatly in respect to size, course, development & site of access
¨ The power measured behind the skull is rarely >35% of the transmitted power, i.e., the bone of the skull absorbs the major portion of the power
TCD & TCCS Devices
¨ To achieve acceptable signal-to-noise ratio systems utilize Dopplers with:
Lower bandwidth Larger & less-defined sample volume
¨ TCD systems: 2 MHz pulsed-wave Doppler
¨ TCCS systems: 1.8 – 3.6 MHz phased array sector transducer
TCD & TCCS Devices
¨ Instrument requirements: Transmitting powers between 10 and 100 mW/cm2 Adjustable Doppler gate width PRF up to 20 kHz Focusing of the US beam at a depth of 40-60 mm Online display of:
Time-averaged velocity (TAMV) Peak systolic velocity (PSV)
¨ Equipment for continuous monitoring
Ultrasonic Windows
¨ Four main windows ¤ Transtemporal ¤ Transorbital ¤ Suboccipital n Transforamenal
¤ Submandibular
The Vessels
Windows
¨ Transtemporal 3 positions Assess the: n MCA
n M1 & M2 segments n ACA
n A1 segment n Carotid Siphon
n C1 segment n AComA n PCA
n P1 & P2 segments n Basilar artery
n Top n PComA
Transtemporal Window
Transorbital Window
¨ Reduce the system power Generally to 50%
¨ Components of the anterior cerebral circulation ¤ Ophthalmic artery n 45-50 mm n Flow is towards the signal
¤ Carotid siphon n C3 segment 60-65 mm
n Flow is towards the signal n C2 segment 70-75 mm
n Flow is away from the signal n C4 segment 65-80 mm
n Flow is towards the signal
Transorbital Window
Suboccipital Window (Transforamenal)
Utilized for the evaluation of the vertebral and basilar arteries throughout their lengths ¤ Probe placement is midline between the foramen
magnum and the spinous process of the first rib ¤ The Doppler beam is aimed at the bridge of the nose ¤ Sample volume depth: n Vertebral arteries – 40-95 mm
n Aim right and left for the vertebral arteries n Flow is away from the signal
n Basilar artery – 70-115 mm n Flow is away from the signal
Suboccipital Window (Transforamenal)
Submandibular Window
¨ Distal segments of the extradural internal carotid artery (C5 & C6 segments)
¨ Useful for the detection of: ¤ ICA dissection ¤ Chronic ICA occlusion ¤ Velocities for calculating a Lindegaard ratio
Submandibular Window
Near-Infrared Spectroscopy (NIRS)
~ brain oximetry ¨ continuous noninvasive monitoring of cerebral
oxygen saturation ¨ measures arterial, venous & capillary blood
oxygen saturation ¨ accuracy (emitter-detector separation).
The History of NIRS
¨ The discovery of near-infrared energy is ascribed to Herschel in the 19th century
¨ First industrial application began in the 1950s ¨ NIRS to assess tissue oxigenation: 1977 ¨ FDA approval of INVOS cerebral oximeter for
clinical use in adults: 1996 ¨ Approved for use in children: 2000 ¨ Used in 80-90% of pediatic cardiac centers
during cardiopulmonary bypass ¨ Increasingly being used in PICU and NICU
Jobsis FF. Science. 1977; 198: 1264
Penetration of NIRS Light
The human skull is easily penetrated by near-infrared light
Longer wavelenght infrared light penetrates better than visible light
NIRS in Human Diagnostics
The INVOS System® uses two depths of light penetration to subtract out surface data, resulting in a regional oxygenation value for deeper tissues
NIRS Oximetry
NIRS Monitoring: rSO2
rSO2 ~ regional oxygen saturation ¨ Represents the balance of site-specific O2
delivery and consumption ¨ Measures both venous (75%) and arterial
blood ¨ Indicates adequacy of site-specific tissue
perfusion in real time ¨ Correlates positively with venous oxygen
saturation (SvO2)
Cerebral Oxygen Transport Balance
¨ Cerebral saturation correlates inversely with measurements of brain lactate concentration
Realtionship between NIRS-derived cerebral O2 saturations and brain tissue lactate concentration in piglets
NIRS Oximetry: Benefits
¨ Instant, noninvasive, continuous real time data ¨ Site-specific (regional) measurement ¨ Excellent indicator of regional oxygen change
associated with shock ¨ Early warning indicator ¨ Immediately reflects the impact of intervention ¨ May be deployed in virtually any setting
NIRS Oximetry: Limitations
¨ Cerebral oxygen saturation approximates mixed venous oxygen saturation (SmvO2)
¨ Sensitive to acut changes in PaCO2 ¨ May be affected by skull thickness and Hb
concentration