Keywords
imaging - peripheral nerve repair - CT imaging - ultrasound - peripheral vasculature
- MRI - CEMRI - surgery
Introduction
While the exact mechanism is still unknown, it has been shown that vascular supply
to peripheral nerves is crucial to both normal function and regeneration after injury.
When repairing peripheral nerves, surgeons aim to preserve intact anatomy and avoid
mobilizing a nerve in a way that separates it from perforating vessels when possible.
Knowing the location of these vessels may allow surgeons to avoid vascular damage
and improve regeneration of injured/repaired nerves. As imaging technology continues
to evolve, novel methods have emerged to create detailed images of vasculature in
vivo.[1] However, the literature is lacking in data specifically assessing the feeding and
draining vessels of nerves. In this review, we highlight the significance, historical
methods, and emerging techniques for imaging the arterial supply of peripheral nerves.
Nerve Function and Blood Supply
Nerve Function and Blood Supply
Early reports detailing the interplay between nerve injury and vascular supply were
presented by Rydevik et al, Rydevik and Lundborg, and Lundborg in the 1970s.[2]
[3]
[4] These articles evaluated structure, function, and response to injury, as well as
the extrinsic and intrinsic vasculature of nerves. More recently, researchers have
investigated the effect of nerve compression on intraneural blood flow and blood–nerve
barrier permeability.[5] Peripheral nerve dysfunction and repair continue to present challenges despite advances
in both imaging and treatment modalities. While functional testing, such as electromyography
(EMG) and nerve conduction studies (NCS), may tell us whether or not the nerve is
communicating, they do little to address the cause of diminished nerve function.
One possible explanation for persistent nervous deficiencies is a lack of vascular
perfusion. It is well documented that angiogenesis is intimately connected with reinnervation,
arborization, and growth of peripheral nerves.[5] For example, it has been shown in a model of transplanted human skin equivalents
that vascularization precedes the process of innervation, indicating that nervous
tissue proliferation is driven by nutritional and trophic factors provided by the
vascular system.[6] While some have argued that an intraneural vascularization happens concurrently
with the proliferation of fascicles, little is known about the specific role of revascularization
in nerve regeneration.[7]
[8]
Given that inflammatory changes, axon growth, and cell regeneration require a large
number of nutritive substances and oxygen, it is not surprising that damaged microcirculation
can hamper neural regeneration and functional recovery.[5] After peripheral nerve damage, the diameter of blood vessels of the intraneural
vascular (INV) system increases due to the effects of neuropeptides such as substance
P (SP) and calcitonin gene–related peptide (CGRP). Vasodilatation, associated with
the release of monocyte chemoattractant protein-1 (MCP-1) by Schwann's cells (SC),
allows macrophage recruitment.[9] Migrating and resident macrophages then secrete VEGF. This growth factor in turn
stimulates neoangiogenesis, supplying the oxygen and nutrients needed to form the
“bands of Büngner” by SCs. The molecular interactions between SCs, macrophages, and
endothelial cells of neovessels have not yet been elucidated.[9]
Given the apparently integral role of vascularization in the maintenance and regeneration
of nervous tissue, physicians and scientists have attempted to identify the vascular
connections that feed peripheral nerves. In what follows, historical methods of observation
will be evaluated, as well as current and emerging imaging techniques that may be
of use in visualizing these structures.
Cadaveric Mapping of the Blood Supply to Peripheral Nerves
Cadaveric Mapping of the Blood Supply to Peripheral Nerves
Few cadaveric studies have been performed to map the connections of peripheral arteries
to the vasa nervorum.[10]
[11]
[12]
[13] These had limitations inherent in all cadaveric studies, including possible loss
of in vivo characteristics through the storage and preparation process, inability
to account for anatomical variation, and variability of injection pressure and/or
dissection technique which may have influenced which vessels did or did not stain.[12] While they are not without limitations, these studies implicate the potential value
of mapping the arterial supply of peripheral nerves. One study of 15 forearms found
that a constant branch to the median nerve arises from the radial artery approximately
5-cm proximal to the radial styloid process. This suggests that it may be possible
to advance the median nerve at the wrist, while retaining vascular input at a common
site of nerve repair.[13] Another study identified the potential for improvements in free vascular nerve grafting.[11]
[12] Others indicated that knowing the location of these vessels could lead to superior
blood supply for neurovascular and neurocutaneous flaps.[10]
[11]
Historical Imaging of Vascular Structures
Historical Imaging of Vascular Structures
Characterization of vascular structures has historically been done using techniques
such as: digital subtraction angiography (DSA), ultrasound (US), magnetic resonance
imaging (MRI), and laser angiography. DSA provides high temporal (≤1 second frame
time) and high spatial resolution (≤1 mm) but is an invasive procedure that uses iodinated
contrast material and radiation to acquire projected images. US was later used as
a noninvasive method of visualizing blood flow to nerves but has limitations including
inability to penetrate tissue, small imaging field, and highly operator-dependent
examination quality. An MRI protocol using T1- and T2-weighted imaging is currently
the preferred method as it is able to characterize the nervous structures and surrounding
tissue noninvasively with high spatial resolution. A newer subset of MRI protocols
termed MR angiography (MRA) has shown utility in evaluating vascular structures in
vivo.[14] Optical coherence tomography (OCT) was later developed to provide high-resolution
images of superficial blood flow.[15] Recently, laser angiography has combined some advantages of US and MR techniques
to provide rapid imaging of tissue perfusion intraoperatively.[16]
Ultrasound/Doppler Angiography
Ultrasound/Doppler Angiography
Ultrasonography is a low-cost tool that has shown utility in imaging neuro vasculature.
In recent years, the spatial resolution of US has improved dramatically, and its applications
have evolved to include imaging of the peripheral nervous system.[17] In 2010, a study was performed on New Zealand white rabbits in an effort to quantitatively
assess peripheral nerve blood perfusion using contrast-enhanced US (CEUS). Using CEUS,
the supplying arteries to the femoral nerves were identified, suggesting that this
methodology could be useful for visualizing nerve perfusion.[18]
One study of human subjects attempted to observe the collateral vessels in the knee
in patients with Buerger's disease using simple Doppler US. Researchers were able
to identify collateral vessels, originating from vasa nervorum of the tibial nerve
in instances of popliteal artery occlusion.[19] Results of CEUS and Doppler US studies suggest that feeding vessels of peripheral
nerves can be adequately visualized in vivo.
One disadvantage of US techniques is the need for a skilled operator which can have
dramatic effects on image quality and precision.[17] Other disadvantages include depth of visual field and occurrence of artifact from
surrounding structures.[17]
Magnetic Resonance Imaging and Magnetic Resonance Neurography
Magnetic Resonance Imaging and Magnetic Resonance Neurography
MR neurography (MRN) protocols have been developed to provide insights into the fascicular
structure of nerves. While this has been useful in evaluating nerve injuries, fascicular
structure is not necessarily determinative of functional outcome.[20] The specificities of MR signal changes seen in histologically controlled studies
are still a contentious matter, as possible causes of signal alteration may include
regeneration, degeneration in the distal nerve, or any combination of the two.[21] Potential factors complicating analysis include high myelin turnover, presence of
inflammatory mediators, changes in the blood–nerve barrier, axoplasmic flow caused
by axon and nerve sheath degeneration, and increased water content in an enlarged
endoneurial space.[21]
[22]
[23]
Though the literature is limited, MRI has been used to map vascular supply to peripheral
nerves. One study using 3-T MRI with fat-suppressed, contrast-enhanced T1 images was
able to identify intraneural vessels in 26% of patients, and even identified some
perforating vessels along the sciatic nerve.[24] Unfortunately, the images in this study had relatively low resolution and likely
missed many smaller vascular elements.
While 7-T MRI improves resolution, it still does not clearly delineate the nerves
and vessels in close proximity.[25] The 7-T scanners provide better resolution than 3-T scanners due to their increased
signal-to-noise ratio (SNR). However, the radiofrequency (RF) field inhomogeneity
corrections are more difficult to execute in 7-T than in 3-T MRI. Image quality is
often improved in ultrahigh-resolution imaging by using more coil elements (or receiver
channels). This increases the parallel imaging acceleration factor to reduce imaging
time.[26] While MRI and MRN technologies are continually improving, current techniques are
not suitable for clinical use to visualize nervous vasculature.
Magnetic Resonance Perfusion
Magnetic Resonance Perfusion
There are two MR perfusion methods currently in use: dynamic susceptibility contrast
MRI (DSC-MRI) and arterial spin labeling (ASL). DSC requires injection of a contrast
agent and subsequent acquisition of a fast time series of images that track the contrast
through vascular structures. ASL applies a spatially selective RF excitation pulse
to a region containing large arteries and allowing this “labeled” blood to flow into
the imaging plane. As T1 decay of the label takes place, it is followed by subtraction
of labeled images from unlabeled control images, producing a measured signal that
is proportional to blood flow. Typically, multiple images must be averaged to achieve
a low SNR.[23]
MR perfusion is commonly used to study cerebrovascular disorders including: acute
stroke, subacute and chronic ischemia, stenotic-occlusive disease, moyamoya disease,
and vasospasm secondary to subarachnoid hemorrhage. MR perfusion can assess collateral
flow, duration of occlusion and hypoperfusion, and presence and/or extent of recanalization.
One early study used MR perfusion to evaluate median nerve circulation. However, their
analysis was mostly qualitative, enhanced (high signal) nerves were classified as
edemic and unenhanced (reduced signal) nerves as ischemic.[27] Soon after, a study quantifying optic nerve blood flow in rats was performed using
Gadolinium-diethylenetriamine pentaacetic acid contrast with T2-weighted MRI.[28] Subsequent calculations used signal intensity values to determine mean basal blood
flow in the anterior and posterior portions of the optic nerve.[28]
Recently, Bäumer et al used dynamic contrast-enhanced MR perfusion to evaluate human
peripheral nerve perfusion in 102 patients (43 control and 59 neuropathy).[29] Researchers measured signal intensity on multiple sequences at weights of T2 and
T1 to determine the arterial input function and mean contrast uptake. Using the Patlak
model, the authors then calculated nerve–blood volume (NBV) and blood–nerve permeability
(K-trans).[29]
While MR perfusion is advantageous for generalized perfusion of a local area, other
techniques, such as MRA, are often used as a baseline to further categorize results.[30] MR perfusion can be used to identify nerve vasculature; however, other methods offer
a more direct approach.
Magnetic Resonance Angiography
Magnetic Resonance Angiography
Time-of-Flight Magnetic Resonance Angiography
Time-of-flight (TOF) techniques distinguish between flowing blood and stationary tissues
by manipulating the magnitude of the magnetization, such that the magnitude of the
magnetization from the moving spins is large and the magnetization from the stationary
spins is small. This creates a large signal from moving blood spins and a small signal
from stationary tissue spins.[31]
TOF-MRA has been used clinically to visualize the hemodynamics of intracranial arteries,
particularly in patients suspected of having intracranial aneurisms.[32]
[33]
[34] Upon the development of phase-contrast MRA (PC-MRA), three-dimensional (3D) TOF-MRA
was shown to be an inferior diagnostic tool.[33]
More recently, Shibukawa et al developed a novel time-resolved MRA called four-dimensional
(4D) TOF-MRA using saturation pulse.[32] Another TOF-MRA protocol was developed using 7-T imaging in combination with reduced
specific absorption rate and venous suppression.[35] While both newer methods demonstrated superiority compared with 3D TOF-MRA, a direct
comparison was not performed for PC-MRA or contrast-enhanced MRA (CE-MRA).
TOF-MRA shows some benefit in mapping vasculature due to its sensitivity to blood
flow and direction. Unfortunately, TOF imaging requires a long acquisition times and
images are often obscured by artifacts.[36] These properties make TOF-MRA an unfavorable imaging option for nervous vascular
structures.
Phase-Contrast Magnetic Resonance Angiography
Phase-contrast (PC) techniques distinguish between flowing blood and stationary tissues
by manipulating the phase of the magnetization, such that the phase of the magnetization
of the stationary spins is 0 and the phase of moving spins is non-0. In phase difference
images, faster moving spins produce a larger signal, and spins moving in one direction
are assigned a bright (white) signal, whereas spins moving in the opposite direction
are assigned a dark (black) signal. Thus, the vascular anatomy can be visualized,
as well as the speed and direction of the blood flow.[31]
Recent advances in PC-MRA acquisition and reconstruction have allowed for high-resolution
images to be obtained with shorter imaging times. Nearly all variations of PC-MRA
utilize parallel imaging directly with techniques, such as SENSE or GRAPPA, but also
with indirect methods such as localized coils. In the case of Cartesian (parallel)
imaging, scan time is reduced by a factor of 2 to 4. Unfortunately, this still fails
to adequately reduce scan times for clinically relevant coverage and spatial resolution
(<1 mm).[37]
Another method for reducing scan times utilizes non-Cartesian sampling trajectories
which either collect more data per excitation (spiral trajectories) or allow greater
under sampling of the imaging volume without obscuring artifacts (radial trajectories).
Non-Cartesian imaging also reduces echo time, improving imaging of complex flow, and
producing higher spatial resolution.[38]
[39]
[40] A third time-reduction method uses a reconstruction based on the fact that the majority
of signal is from nonmoving tissues and consequently the same in all time frames.
This method accelerates acquisition time, but requires substantial computational power.[37]
In 2004, Bilecen and colleagues[41] introduced a novel MRA technique that combines subsystolic arm pressure with multiphasic
continuous acquisition of a spoiled 3D gradient echo sequence. This technique requires
only 24 seconds to record one dataset. Unfortunately, it provides no information on
hemodynamics.[42] Winterer et al addressed this issue by designing a protocol using ultra rapid time-resolved
3D imaging, with accelerated acquisition employing interleaved Stochastic Trajectories
(TWIST) and generalized autocalibrating partially parallel acquisition (GRAPPA) in
3-T CE-MRA. Using this protocol, they demonstrated that time-resolved CE-MRA is four
times faster than PC-MRA and reduces venous contamination in imaging hand arteries.[42] A recent article described the use of a minimum phase Shinnar-Le Roux excitation
and the “time‐optimal variable‐rate selective excitation” method to shorten radio
frequency (RF) and slab‐select waveforms, further reducing the time needed to acquire
images.[43]
In a clinical setting, PC-MRA can show vascular morphology and gives quantitative
measurements of blood velocity. Velocity data allows observers to derive hemodynamic
parameters such as flow volume, relative wall shear stress, streamlines, vorticity,
and pressure gradients. PC-MRA has combined anatomic vessel wall imaging, lumen visualization,
and physiologic data to augment the characterization of intracranial arterial stenosis,
aneurysms, vascular malformations, and dural sinus pathology.[37] Not all of these benefits are needed for nerve-specific applications. While PC-MRA
has demonstrated capability for visualizing small vasculature, many hospitals may
lack the resources to implement improved protocols.
Contrast-Enhanced Magnetic Resonance Angiography
CE techniques distinguish between blood and stationary tissues by manipulating the
magnitude of the magnetization (i.e., TOF techniques) and by intravenously injecting
a contrast agent into the vascular system to selectively shorten the T1 of the blood.
By implementing a T1-weighted imaging sequence during the first pass of the contrast
agent, images can be produced that show arteries with striking contrast relative to
surrounding stationary tissues and veins.[31]
CE-MRA was introduced for clinical use approximately 25 years ago, with early iterations
providing 3- to 4-mm spatial resolution in 30 seconds of acquisition timeframes. Since
then, improvements in spatial resolution and acquisition time have allowed high-resolution,
3D time-resolved studies.[44] These newer CE-MRA techniques have been utilized to provide structural details of
vasculature and flowmetry data in peripheral feeding and draining vessels of the forearm,
hand, thigh, and foot. This data helped to progress our understanding of vascular
malformation.[14]
In 2007, CE-MRA was used to determine the level and side of the suspected spinal dural
arteriovenous fistula (SDAVF) and the feeding arteries in spinal arteriovenous malformations
(SAVMs). These findings were corroborated via comparison with DSA of the same tissue.
CE-MRA was able to reliably detect or exclude various types of spinal AV abnormalities
and localize the predominant arterial feeder of most spinal AV shunts. These findings
indicate that MRA may serve as a noninvasive means of focusing subsequent DSA in these
patients.[45]
While CE-MRA requires an intravenous contrast to be administered, there are significant
benefits to this technique in mapping perfusion of peripheral nerves. High temporal
and spatial resolution can be gained from accurate timing of the contrast agent bolus,
as well as the low likelihood of flow-related artifacts.[36] Given the low acquisition times of newer protocols, CE-MRA is a promising method
of visualizing nerve perfusion in a clinical setting.
Optical Coherence Tomography
Optical Coherence Tomography
OCT is a relatively new imaging technique that uses low coherence light to produce
a 2D or 3D image. OCT is an ideal candidate for obtaining a submicrometer resolution,
and is commonly used medically to obtain high-resolution images of the retina.[15]
A study performed on rats in vivo using OCT aimed to detect cerebral edema status
post cerebral artery occlusion, and was able to detect gradual ischemic changes with
high accuracy, similar to the results of an MRI.[46] Another study compared five different systems of CTA angiography and how successfully
they can evaluate optical vasculature. All systems trialed were able to obtain accurate
depictions of vasculature but motion artifacts and acquisition times posed problems.[15]
These studies demonstrate that OCT appears reliable for imaging of superficial structures,
but few studies have attempted to visualize deeper structures. One such study attempted
to evaluate coronary arteries following stent implantation. A high amount of resolution
was achieved, but there were still shortcomings of imaging deep vessels. The study
mentions that OCT needs complete blood clearance from the lumen and that thrombi casts
large shadows of artifact.[47] Even when deeper structures are visualized, they are often compared with MRI and
MRA for validity, suggesting that other methods for mapping feeding vessels may be
more reliable.[48]
Optical Microangiography
Optical microangiography (OMAG) is a subset of the OCT technique designed specifically
for imaging blood flow in microcirculation. It is based on the principal that moving
blood scatters light in such a way that it can be captured and adjusted into a 3D
image.[49]
This technique is frequently applied to visualizing cutaneous blood flow. OMAG is
a reliable biomarker in many pathological situations such as lesions and basal cell
carcinoma.[50] In vivo experiments on mice have been performed which demonstrate a noninvasive
technique of visualizing cerebral blood flow. The 3D rendering from the experiments
showed that vessels as small as 15 μm can be observed.[49]
[51]
None of these studies have evaluated OMAG in the context of peripheral nerve vasculature;
however, the ability to visualize microcirculation could be beneficial. Most studies
using OMAG, however, view microcirculation at relatively superficial levels.[50] Few studies have been performed which would indicate deeper tissues can be visualized
in a similar manner to cutaneous vasculature.[46]
[47]
[48] These limitations may impact accuracy of observing deep feeder vessels.
Laser Angiography
Laser angiography techniques have recently been developed to combine the real-time
acquisition of Doppler angiography and precision of MRA techniques, without the highly
operator-dependent image quality (US) or long acquisition times (MRA).[52] Two common devices are the SPY Intraoperative Perfusion Assessment System (Novadaq
Technologies Inc., Richmond, BC, Canada) and SPY Elite Fluorescence imaging system
(Stryker Inc., Kalamazoo, Michigan, United States), both of which utilize indocyanine
green (ICG), a fluorescent agent that binds to plasma proteins with a half-life of
3 to 5 minutes.[52]
[53] The pharmacokinetic properties of ICG allow for rapid clearance and repeated intraoperative
visualization of tissue perforation.
Laser angiography has been used successfully to monitor tissue perfusion in a variety
of settings, including colorectal, plastic, endocrine, ophthalmologic, and vascular
surgery.[54] While the majority of literature is focused on tissue resection and/or flap design,
one case series described visualization of the sciatic nerve perivascular system in
three pediatric sarcoma patients during limb salvage surgery.[16] While depth of structures limit its application in mapping common arterial inputs
and/or venous drainage, laser angiography may be clinically useful in visualizing
these vessels intraoperatively.
Discussion
MRI and MRN provide ideal imaging of fascicular structure of nerves. However, fascicular
regeneration does not always correlate with return to function.[20] As such, additional techniques are needed to fully assess the spectrum of factors
such as blood perfusion that are known to affect recovery. There are substantial difficulties
associated with imaging the vessels that supply peripheral nerves. The vessels are
small and may be located in deep tissue layers depending on the nerve in question.
For example, vessels feeding the femoral nerve are surrounded by several dense and
vascular structures that increase susceptibility to artifact.[18]
In addition to inherent challenges of size and tissue depth, random variation can
be observed in human vascular networks, such as accessory or aberrant arterial supply.[55] While arterial variation may not be exceedingly common, it is a variable that is
difficult to control for, especially in current cadaveric models where sample sizes
are necessarily small. Given the potential anatomic variation in individuals, noninvasive
methods are essential for mapping peripheral vasculature.
Historical imaging techniques used in nervous and vascular tissue were often unable
to adequately show vascular structures at this depth with limitations including invasive
nature of the procedure, poor resolution, inability to show microscopic structures,
or the need for highly skilled operators.[17]
[26]
[36]
[37] Recent developments have provided improved resolution, less-invasive imaging, and
devices that are less prone to operator error. Emerging techniques may be divided
by their respective contributions to nerve treatment algorithms.
US/Doppler, OCT, and laser angiography are most useful in a clinical setting to visualize
superficial structures in individual patients. US is currently limited by depth of
structures and necessary operator skill. Despite these shortcomings, US is beneficial
in intraoperative settings due to its mobility and rapid function. CEUS has been used
successfully to measure intraneural vascularity in a variety of nerve pathologies,
most commonly compressive neuropathy.[56] OCT and OMAG excel at visualizing morphology of microcirculation but may also be
limited by depth of structures.[49]
[50]
[51] Laser angiography provides fast, accurate imaging with minimal required skill and
can be used intraoperatively to identify and preserve blood flow to nerves.[16]
Various MR techniques are most useful for mapping common vessels to develop a knowledge
base for cases in which direct visualization is untenable. These may also be beneficial
for nonacute visualization in individual cases, either pre- or postoperatively. While
the MR perfusion has been used to detect intraneural edemic/ischemic conditions in
peripheral nerves, this is an indirect method of determining the location of feeder
vessels.[27] TOF-MRA and PC-MRA provide high-resolution images by utilizing the contrast between
flowing blood and stationary tissue.[37] Studies assessing peripheral vascular disease indicate that these may adequately
image neurovascular structures.[57] Given the long acquisition times associated with TOF and PC-MRA,[37] they will likely be of greater use in studies mapping common feeder vessels than
in acute or subacute clinical cases.
CE-MRA produces high-resolution images of deep structures with little to no artifact
and reduced acquisition time compared with TOF or PC-MRA.[36] Studies have demonstrated high accuracy of CE-MRA in visualizing feeding and draining
vessels with excellent contrast between arteries and surrounding tissue.[14]
[31] Given its success as a noninvasive alternative to DSA in patients with vascular
malformations, CE-MRA may be a valuable tool for mapping nerve vasculature and nonacute
visualization in individual patients.[44]
Currently, US imaging has demonstrated the greatest clinical utility. When a skilled
operator is available, US allows surgeons to evaluate nervous vascularization to corroborate
clinical symptoms and direct treatment. Multiple studies have shown that US has improved
diagnostic accuracy compared with MRI.[58]
[59] Future diagnostic and/or monitoring algorithms should incorporate both ultrasonography
and CE-MRA to address the broad spectrum of patient dynamics and available hospital
resources.
Conclusion
While imaging technologies are rapidly advancing, studies evaluating their efficacy
in the context peripheral nerve vasculature are limited. Accurate imaging and mapping
of these vessels may augment surgeons' ability to perform repairs without compromising
nerve perfusion, ultimately resulting in improved postoperative outcomes. The published
data, while limited, support further investigation to determine which modality(s)
might be best suited to the task of locating arterial inflow to peripheral nerves.
Whether by direct visualization in individual cases or by mapping structures conserved
across patients, the techniques presented in this review may translate to improve
patient outcomes following nerve injury.
