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