Review Article
Frontiers in optical imaging of cerebral blood flow
and metabolism
Anna Devor1,2,3, Sava Sakadzˇic´3, Vivek J Srinivasan3, Mohammad AYaseen3, Krystal Nizar1,
Payam A Saisan1, Peifang Tian1,4, Anders M Dale1,2, Sergei A Vinogradov5,
Maria Angela Franceschini3 and David A Boas3
1Department of Neurosciences, UCSD, La Jolla, California, USA; 2Department of Radiology, UCSD, La Jolla,
California, USA; 3Martinos Center for Biomedical Imaging, MGH, Harvard Medical School, Charlestown,
Massachusetts, USA; 4Department of Physics, John Carroll University, University Heights, Ohio, USA;
5Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania, USA
In vivo optical imaging of cerebral blood flow (CBF) and metabolism did not exist 50 years ago.
While point optical fluorescence and absorption measurements of cellular metabolism and
hemoglobin concentrations had already been introduced by then, point blood flow measurements
appeared only 40 years ago. The advent of digital cameras has significantly advanced two-
dimensional optical imaging of neuronal, metabolic, vascular, and hemodynamic signals. More
recently, advanced laser sources have enabled a variety of novel three-dimensional high-spatial-
resolution imaging approaches. Combined, as we discuss here, these methods are permitting a
multifaceted investigation of the local regulation of CBF and metabolism with unprecedented spatial
and temporal resolution. Through multimodal combination of these optical techniques with genetic
methods of encoding optical reporter and actuator proteins, the future is bright for solving the
mysteries of neurometabolic and neurovascular coupling and translating them to clinical utility.
Journal of Cerebral Blood Flow & Metabolism advance online publication, 18 January 2012; doi:10.1038/jcbfm.2011.195
Keywords: energy metabolism; hemodynamic; homeostasis; in vivo imaging; neurovascular
Introduction
With this Special Issue we celebrate 50 years of
dedicated symposia on Cerebral Blood Flow and
Metabolism. During the past half century we, as a
research community, have accumulated a consider-
able body of experimental and theoretical knowledge
on cellular metabolic pathways in health and
disease, identified a variety of vasoactive substances,
established correlations between vascular, metabolic,
and neuronal parameters, developed computational
models and took aboard a broad suite of methodo-
logies. Yet, a central piece of the cerebrovascular
puzzle is missing: Despite a number of hypotheses
(for recent reviews see Attwell et al, 2010; Cauli and
Hamel, 2010; Hamilton et al, 2010; Iadecola and
Nedergaard, 2007; Kleinfeld et al, 2011; Paulson
et al, 2010), we still do not have a clear mechanistic
understanding of local regulation of cerebral blood
flow (CBF) and metabolism by neuronal activity. By
‘mechanistic’ we mean determining causal relation-
ships and identifying molecular messengers, which
communicate a change in neuronal activity to the
vasculature causing dilation or constriction. What
makes the neurovascular signaling so difficult to
grasp and what is required for a breakthrough? In
this essay, we argue that further advancement in a
mechanistic understanding of neurovascular com-
munication and dynamic regulation of blood flow
critically depends on the advent of new imaging
technologies with microscopic resolution applicable
to in vivo studies.
Received 16 September 2011; revised 18 November 2011; accepted
29 November 2011
Correspondence: Dr A Devor, Departments of Neurosciences and
Radiology, University of California San Diego, San Diego, La Jolla,
CA, USA.
E-mail: adevor@ucsd.edu or
Dr DA Boas, Martinos Center for Biomedical Imaging, MGH,
Harvard Medical School, Charlestown, MA 02129, USA.
E-mail: dboas@nmr.mgh.harvard.edu
The authors gratefully acknowledge support from the National
Institute of Health: NS051188, NS057198, NS057476, NS055104,
EB00790, EB009118, EB007279, and K99NS067050; American
Heart Association: 11SDG7600037 and 11IRG5440002; and the
Glaucoma Research Foundation.
Journal of Cerebral Blood Flow & Metabolism (2012), 1–18
& 2012 ISCBFM All rights reserved 0271-678X/12 $32.00
www.jcbfm.com

The most intuitive scenario for neurovascular
coupling might be that in which consumption of
energy by neuronal tissue provides a feedback signal
to the feeding vasculature: Changes in neuronal
activity drive changes in energy metabolism, which
then drive vasodilation/constriction and the asso-
ciated changes in blood flow. This idea, usually
referred to as the ‘metabolic hypothesis,’ comes in
different flavors with relation to the putative mole-
cular mediators, including lactate, NAD+/NADH
(nicotinamide adenine dinucleotide) ratio, ATP/
ADP ratio, adenosine, and an (unidentified) O2
sensor (Paulson et al, 2010; Raichle and Mintun,
2006). As an alternative hypothesis, changes in
neuronal activity can drive vasodilation and vaso-
constriction by feed-forward mechanisms releasing
neurotransmitter and neuropeptide molecules
related to neuronal signaling (Attwell et al, 2010;
Cauli and Hamel, 2010). In this ‘neurogenic hypo-
thesis,’ blood flow and energy metabolism are driven
in parallel by neuronal activity. Astrocytes, ‘more
than a glue’ of the central nervous system (Allaman
et al, 2011; Fiacco et al, 2009; Giaume et al, 2010;
Iadecola and Nedergaard, 2007; Koehler et al, 2009),
can potentially have a role in both scenarios: via
release of vasoactive metabolic biproducts (meta-
bolic) or synthesis and release of vasoactive glio-
transmitters in response to neurotransmitters and
neuropeptides (neurogenic). Supportive evidence for
both hypotheses has been derived from experiments
in isolated tissue: brain slices, excised vessels, and
even cell cultures. Of these, brain slice preparation
produced a wealth of data in experiments with
controlled perfusion, pharmacological manipula-
tions, and excitation of single neurons with identi-
fied phenotypes (Cauli et al, 2004; Gordon et al,
2008; Zonta et al, 2003). However, homeostasis of
brain slices departs from that in vivo in many ways
(Huchzermeyer et al, 2008; Turner et al, 2007).
Significantly, many of these departures are unknown
or difficult to quantify, sometimes making extrapola-
tion of the observed phenomena to the in vivo
situation uncertain.
The ability to descend to the single-cell and single-
capillary levels in vivo and observe firing of
individual neurons, vasodilation, glucose uptake,
and infusion of O2 into the tissue—all while directly
controlling neuronal activity—has long been a dream
of scientists interested in understanding the complex
regulation of blood flow and metabolism as related
to neuronal activity. However, in contrast to the
detailed and elegant mechanistic studies in isolated
tissue, in vivo reports have, in the main, focused
simply on correlations between the ‘observables,’
limited by the available methods. This ‘too hard to
do’ status quo for mechanistic studies in vivo is
starting to change, due to rapid developments in
optical microscopy. In fact, already today, a versatile
suite of optical tools is available for high-resolution,
high-sensitivity measurements of vascular, meta-
bolic, and neuronal parameters in deep tissue and
local, cell-type specific manipulations of neuronal
activity. Below, we consider the current state of the
art of a number of key optical microscopy technol-
ogies that will be critical in the effort of graduating
from correlation driven to mechanistic approaches
for studies in vivo. The technological requirements
necessary for this endeavor include
 Resolving single cells and single blood vessels;
distinguishing local effects from global effects—
‘spatial resolution’
 Sampling fast enough to reconstruct the time course
of dynamic processes—‘temporal resolution’
 Directly measuring variables of interest taking
advantage of natural changes in the optical proper-
ties of tissue—‘intrinsic optical contrasts’
 Developing ‘optical reporters’ or ‘probes’ with high
sensitivity and specificity
 Directly manipulating neuronal, vascular, and
metabolic activity—‘optical actuators’
 Imaging deep under the cortical surface—‘depth
penetration’
The tools suitable for unraveling the mechanics of
neurovascular and neurometabolic coupling will be
complemented by other noninvasive optical technol-
ogies that will enable translation of the physiological
findings from animal to human studies and clinical
application. Importantly, these noninvasive optical
technologies can be used in both animals and
humans and thus can facilitate the connection of
microscopic to macroscopic observables from ani-
mals to humans.
The arsenal of optical tools for
neurovascular and neurometabolic
studies
The use of novel optical technologies has been
instrumental for a number of central discoveries in
both basic and clinical neuroscience. Examples from
basic neuroscience include the fine mapping of
cortical functional organization (Grinvald et al,
1986) and the discovery of glial calcium excitability
(Cornell-Bell et al, 1990; Nedergaard, 1994). Among
the clinical applications, optical tools played
an important role in the study of neurovascular
and neurometabolic disregulation in animal models
of stroke (Zhang and Murphy, 2007), epilepsy
(Schwartz and Bonhoeffer, 2001), migraine (Bolay
et al, 2002), and cancer (Barretto et al, 2011).
Likewise, noninvasive optical technologies have
started making inroads into bedside imaging of blood
flow and oxygen consumption in human patients
(Grant et al, 2009; Mesquita et al, 2011).
Below, we highlight many of the optical methods
used for vascular, hemodynamic, metabolic, and
neuronal imaging at different resolution scales—
from cellular to macroscopic—with an emphasis
on in vivo methodology (Figure 1). We apologize in
Optical imaging of blood flow and metabolism
A Devor et al
2
Journal of Cerebral Blood Flow & Metabolism (2012), 1–18