Clinical Neurophysiology 138 (2022) 153–163

Contents lists available at ScienceDirect

Clinical Neurophysiology
journal homepage: www.elsevier.com/locate/clinph

Magnetoneurography as a novel functional imaging technique for theulnar nerve at the elbow
Yuko Hoshino a, Shigenori Kawabata a,b,⇑, Yoshiaki Adachi c, Taishi Watanabe d, Kensuke Sekihara a,
Toru Sasaki b, Jun Hashimoto a, Koji Fujita b, Akimoto Nimura b, Atsushi Okawa baDepartment of Advanced Technology in Medicine, Graduate School of Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8510, Japan
Department of Orthopedic Surgery, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8510, Japan
Applied Electronics Laboratory, Kanazawa Institute of Technology, Kanazawa-shi, Ishikawa 920-1331, Japand
Healthcare Business Group, Ricoh Company, Ltd., 2-3-10 Kandasurugadai, Chiyoda-ku, Tokyo 101-0062, Japanbca r t i c l e

i n f oArticle history:
Accepted 18 March 2022
Available online 30 March 2022
Keywords:
Evoked potential
Evoked magnetic field
Magnetoneurography
Magnetospinography
Recursively applied null-steering
Superconducting quantum interference
deviceh i g h l i g h t s
 Evoked magnetic fields of the ulnar nerve at the elbow were recorded by magnetoneurography.
 The neural activity of the ulnar nerve was visualized as reconstructed currents.
 Magnetoneurography is a novel functional imaging technique for the ulnar nerve.

a b s t r a c t
Objective: To visualize the neural activity of the ulnar nerve at the elbow using magnetoneurography(MNG).
Methods: Subjects were asymptomatic volunteers (eight men and one woman; age, 26–53 years) and amale patient with cubital tunnel syndrome (age, 54 years). The ulnar nerve was electrically stimulated
at the left wrist and evoked magnetic fields were recorded by a 132-channel biomagnetometer system witha superconducting quantum interference device at the elbow. Evoked potentials were also recorded andtheir correspondence to the evoked magnetic fields was evaluated in healthy participants.
Results: Evoked magnetic fields were successfully recorded by MNG, and computationally reconstructedcurrents were able to visualize the neural activity of the ulnar nerve at the elbow. In the affected arm of
the patient, reconstructed intra-axonal and inflow currents attenuated and decelerated around the elbow.
Latencies of reconstructed currents and evoked potentials were correspondent within an error of 0.4 ms inasymptomatic participants.
Conclusions: Neural activity in the ulnar nerve can be visualized by MNG, which may be a novel functional
imaging technique for ulnar neuropathy at the elbow, including cubital tunnel syndrome.
Significance: MNG permits visualization of evoked currents in the ulnar nerve at the cubital tunnel.
Ó 2022 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. This is an open
access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction
Abbreviations: CV, conduction velocity; MEG, magnetoencephalography; CMAP,
compound muscle action potential; MNG, magnetoneurography; MSG, magnetospinography; RENS, recursively applied null-steering; SNAP, sensory nerve action
potential; SQUID, superconducting quantum interference device; SD, standard
deviation.
⇑ Corresponding author at: Department of Advanced Technology in Medicine,
Graduate School of Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyoku, Tokyo 113-8510, Japan.
E-mail addresses: hoshino.atm@tmd.ac.jp (Y. Hoshino), kawabata.orth@tmd.ac.
jp (S. Kawabata), taishi.watanabe@jp.ricoh.com (T. Watanabe), k-sekihara@nifty.
com (K. Sekihara), fujiorth@tmd.ac.jp (K. Fujita), nimura.orj@tmd.ac.jp (A. Nimura),
okawa.orth@tmd.ac.jp (A. Okawa).

Cubital tunnel syndrome is a common peripheral nerve compression disorder at the elbow. It is generally diagnosed by neurological examination, an electrophysiological study, and imaging.
Some authors have reported that cubital tunnel syndrome is progressive, although others consider it to have a more favorable outcome (Eberlin et al., 2017). However, untreated chronic cubital
tunnel syndrome can lead to a permanent change in sensory andmotor function and joint contracture (Boone et al., 2015).
Although a differential diagnosis including cervical nerve
radiculopathy and brachial plexus pathology must be considered,https://doi.org/10.1016/j.clinph.2022.03.017
1388-2457/Ó 2022 International Federation of Clinical Neurophysiology. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Clinical Neurophysiology 138 (2022) 153–163

developed a magnetospinography (MSG) system that enables an
examination of the spinal cord and peripheral nerves (Fukuoka
et al., 2002, 2004; Kawabata et al., 2002; Hoshino et al., 2005;
Tomizawa et al., 2008; Ishii et al., 2012; Sumiya et al., 2017;
Sasaki et al., 2020; Ushio et al., 2019; Watanabe et al., 2019).
In this article, we show that MNG can visualize the neural activity of the ulnar nerve around the medial epicondyle. In addition,
we report that it has the potential to be a novel functional imaging
technique that can identify subclinical electrophysiological abnormalities and localized lesions in unconfirmed cases of ulnar neuropathy at the elbow.

the diagnosis of cubital tunnel syndrome is typically not difficult.
Nonetheless, some patients with cubital tunnel syndrome present
normal electrophysiological findings. Greenwald et al. reported
that electromyography and nerve conduction velocity (CV) studies
may have false-negative rates exceeding 10% in patients presumed
to have clinical cubital tunnel syndrome. They considered that thismight be because very few axons need to be functional for the
study to be interpreted as normal (Greenwald et al., 2006). Beekman et al. pointed out that electrodiagnostic studies of ulnar neuropathy at the elbow may obtain false-negative results,
particularly in patients with milder lesions. In addition, theyreported that nerve enlargement in sonography was related to anabnormal electrophysiological test and that the sensitivity of theexamination increased using both modalities (Beekman et al.,2004). Pelosi et al. (2018) reported that ulnar neuropathy withabnormal non-localizing electrophysiology was not uncommon.
They showed that nerve ultrasound could identify the local lesion
at the elbow in the majority of such cases.
Furthermore, there has been controversy regarding the treatment option for cubital tunnel syndrome. A more precise functional imaging technique may contribute to new insights into thepathology and prognosis of cubital tunnel syndrome.
Magnetoneurography (MNG) is a new functional imaging
method for the nervous system that records magnetic fields generated according to the Biot–Savart law or Ampère’s law after electrical nerve stimulation. One of the most valuable features of MNG isthat it can reconstruct and visualize currents inside and outside of
the nerve. Given that traditional electrophysiological studies evaluate only evoked potentials, MNG could contribute not only toclinical examination, but also to novel physiological findings.
The development of MNG for the spinal cord and peripheral
nerves has been gradual compared with magnetoencephalography
and magnetocardiography, mainly owing to the anatomical features of these nerves (Mackert, 2004; Afra, 2020). We previously

2. Methods
2.1. MSG system
Biomagnetic signals were recorded in a magnetically shieldedroom with a 132-channel superconducting quantum interference
device (SQUID) biomagnetometer system developed by the Kanazawa Institute of Technology and Ricoh Company, Ltd. (Adachi
et al. 2017). The system has 44 vector-type SQUID magnetic sensors arranged in a 180-mm  130-mm area along a cylindrical surface with a radius of 200 mm (Fig. 1). Each sensor has three pick-up
coils orthogonally facing each other, which record magnetic fields
from three orthogonal directions (X-, Y-, and Z-coils).
2.2. Positional information
Before the recording, two marker coils were placed under the
subject’s upper extremity and a lateral X-ray image was obtained(Fig. 2). The relative positions of the marker coils to the MSG sensors were estimated from magnetic fields generated by the marker
coils. Subsequently, the subject’s position on the recording area
was obtained.

22 22 22 22 22 (mm)b)

180 (mm)

Top view

25 23 22 20 22 23 25 (mm)a)

Sensor unit

z

y
x

(a)
130 (mm)X-coil12 (mm)r = 200 (mm)Y-coilZ-coilSide viewFig. 1. Structures of the magnetoneurography (MNG) system and sensor units. (a) Side and top views of the MNG sensor array. It consists of eight columns, which each have
five or six sensor units. The four sensor units at the four corners have only one channel for positional information and each of the other 40 sensor units have three channels forrecording three-directional magnetic fields (X-, Y-, and Z-coils in Fig. 1b). This Figure is reproduced with permission from the reports of Adachi et al. (2017) (Ó2017 IEEE.
Reprinted with permission from https://doi.org/10.1088/1361-6668/aa66b3) and Sasaki et al. (2020).
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c)

a)

150

100

50

Y [mm]

0

-50

-100

-150

b)

-100

-50

0

Z [mm]

Y [mm]

X [mm]
Fig. 2. Recording setting and acquisition of positional information. (a) Although there are five marker coils on the sensor wall, two were used to acquire positional
information. The black box shows the area of the X-ray image. (b) X-ray image and sensor array. White circles represent sensors and there are three electrodes for recording
evoked potentials and one for a reference near the elbow joint. Two marker coils marked with yellow circles were used for positional information. (c) The yellow line
represents the depth of the region of interest (ROI). The line passes through the cubital tunnel and is parallel to the ulnar margin of the humerus and the ulna. Blue circles
represent the positions of sensors. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

condyle (10-kHz sampling rate; 20–2000-Hz bandpass filter). The
CV of the nerve action potential was calculated from the differencein latencies at two points: 3 cm proximal and 3 cm distal to themedial epicondyle.
Because the electrophysiological examination of the patient
was performed at a hospital, his recording setting was different
from that of the healthy participants. For the patient, the little finger of each side was electrically stimulated, and SNAPs wererecorded at the wrist, distal and proximal points of the medial epicondyle, and axilla. Compound muscle action potentials (CMAPs)of the abductor digiti minimi were also recorded in response toelectrical stimulation of the ulnar nerve at the wrist, distal andproximal points of the medial epicondyle, and axilla. Before surgery, a neuromagnetic recording of the patient was conducted in
the same manner as in the healthy participants. Electrical stimulation and recordings were performed by the MEB2200 electromyogram/evoked potential measuring system (Nihon Koden, Tokyo,
Japan).
All procedures in this study were approved by the Ethics Committee of Tokyo Medical and Dental University and carried out inaccordance with the Declaration of Helsinki. We obtained written
informed consent and releases for images and photographs fromall participants.

2.3. Participants and recordings of evoked potentials and magnetic
fields
The subjects were nine asymptomatic volunteers without any
history of neurological disease (eight men and one woman; age,
26–53 years) and one male patient with left cubital tunnel syndrome (age, 54 years). The patient presented with a 1-year historyof numbness of the left little finger but did not show muscle atrophy. He underwent subcutaneous anterior transposition of theulnar nerve, and the numbness in the left little finger had almost
disappeared 9 months after the surgery. For the recordings of neuromagnetic fields and nerve action potentials in the healthy participants, the ulnar nerve was electrically stimulated at the left wristwith a supramaximal intensity that evoked maximal sensory nerve
action potential (SNAP) at the little finger (5 Hz; monophasicsquare waves, 0.3-ms width). Evoked magnetic fields wererecorded by the MSG system with the subject seated using a 40kHz sampling rate and a 100–5000-Hz bandpass filter (Fig. 3).
About 2000 responses (2000–2126) were averaged. The MNG
examination time was about 15 min per individual.
For the healthy participants, nerve action potentials wererecorded at three points at the left elbow: just above the medial
epicondyle and 3 cm proximal and 3 cm distal to the medial epi155

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Clinical Neurophysiology 138 (2022) 153–163

a)Reference electrodes

Recording electrodes

Belt for ﬁxing the stimulationelectrode in placeb)

Recording electrodes for SNAP

Grounding wire

Fig. 3. Recording setting of evoked potentials. (a) Evoked potentials were recorded at three points along the left cubital tunnel (10-kHz sampling rate; 20–2000-Hz bandpass
filter) in response to electrical stimulation of the ulnar nerve at the left wrist (5 Hz; monophasic square waves, 0.3-ms width). (b) Electrical stimulation at the wrist was at a
supramaximal intensity that evoked maximal sensory nerve action potential at the little finger.

arbitrary point on the plane at the ROI as if a virtual electrode wereplaced there.
To evaluate the CVs of reconstructed currents and the correspondence between them and evoked potentials, the assumed conduction pathway was set at the peak of reconstructed currents
selected computationally. CVs of intra-axonal currents and currents perpendicularly flowing toward the conduction pathway
were respectively calculated from the latencies at the points
3 cm proximal and 3 cm distal to the tip of the medial epicondyle.
Regarding the influent currents, waveforms from the virtual electrodes 10, 15, and 20 mm away from the conduction pathway were
evaluated. The peak latencies of evoked potentials and reconstructed currents nearest to the medial epicondyle were evaluatedto assess correspondence.

2.4. Signal processing
Recorded magnetic fields were first processed using an artifact
reduction method, the dual signal subspace projection (DSSP) algorithm (Sekihara et al., 2016, Sekihara and Nagarajan, 2017). Then,the positions and intensities of current sources were reconstructedfrom the processed data using a spatial filter algorithm, the recursively applied null-steering (RENS) beamformer (Kumihashi and
Sekihara, 2010; Sekihara and Nagarajan, 2015). The depth of theregion of interest (ROI) was set at the pathway of the ulnar nerve,
which was assumed to run through the posterior of the medial epicondyle and to be parallel to the ulnar margin of the humerus andthe ulna in an anteroposterior X-ray image (yellow line in Fig. 2c).
This procedure allowed reconstruction of the evoked current at an

A
C
B

A

B

C
20 μV
3 ms

Fig. 4. Evoked potentials in response to electrical stimulation of the ulnar nerve at the left wrist. Evoked potentials were recorded at three electrodes around the cubital
nerve. B is at the cubital tunnel. A and C are 3 cm distal and 3 cm proximal to B. Evoked potentials propagated from distal to proximal.
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In the patient with left cubital tunnel syndrome, the CVs of the
SNAPs between the distal and proximal points of the medial epicondyle were 59.5/30.1 m/s (right/left) and 58.1/55.6 m/s (right/left)
before and after the surgery, respectively. Similarly, the CVs of the
motor nerve were 52.6/31.7 m/s and 57.1/50.0 m/s, respectively.
The SNAP in the left arm was attenuated and decelerated aroundthe cubital tunnel (pink arrowheads in Fig. 5a). Note that the scaleof the amplitude in the left arm before surgery is much smaller thanin other recordings. Similarly, the motor nerve conduction decelerated near the cubital tunnel (pink arrowheads in Fig. 5b).

3. Results
3.1. Recording of evoked potentials
In all participants, evoked potentials were recorded after stimulation of the left ulnar nerve (Fig. 4). The upward peak of the
waveform propagated from the distal to proximal electrodes. The
mean CV of the evoked potentials was 61.2 ± 9.8 m/s(mean ± standard deviation [SD], n = 9). The same healthy subject
and the patient are shown in Figs. 4–10.

Sensory nerve action potentials
Left arm: before surgery

Sensory nerve action potentials
Right arm: before surgery

Wrist
57.8 m/s

Wrist
62.9 m/s
Distalelbow

Distalelbow
Proximalelbow

30.1 m/s

59.5 m/s

Proximalelbow
59.6 m/s

Axilla68.7 m/s
Axilla
20 μV

5 μV

2 ms

2 ms

Sensory nerve action potentials
Right arm: after surgery

Sensory nerve action potentials
Left arm: after surgery

Wrist
Wrist

63.3 m/s
64.5 m/s
Distalelbow

Distalelbow

55.6 m/s

58.1 m/s
Proximalelbow

Proximalelbow
63.2 m/s

63.2 m/s
AxillaAxilla

50 μV

50 μV

2 ms

2 ms

Fig. 5. Electrophysiological studies of a patient with left cubital tunnel syndrome. (a) Sensory nerve action potentials (SNAPs) in response to electrical stimulation of thedigital nerve at the little finger. SNAPs of the right and left arms were recorded at the wrist, distal and proximal points of the medial epicondyle, and axilla. Distances between
each recording point are not constant. Conduction velocities are shown on the right side of the charts. Upper row: SNAPs before surgery. On the affected side (left arm), thepropagation of SNAPs attenuated and decelerated around the elbow (pink arrowheads). Lower row: The deceleration of SNAPs in the left arm improved after surgery. (b)Compound muscle action potentials (CMAPs) of the abductor digiti minimi in response to electrical stimulation of the ulnar nerve recorded at the wrist, distal and proximal
points of the medial epicondyle, and axilla. Upper row: CMAPs of the right and left arms before surgery. Conduction velocities between two recording points are shown on theright side of the charts. On the affected side (left arm), the propagation of CMAPs attenuated and decelerated beyond the medial epicondyle (pink arrowheads). Lower row:
CMAPs after surgery show that the conduction in the left arm recovered compared with before surgery. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)157

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Clinical Neurophysiology 138 (2022) 153–163

Fig. 5 (continued)

potentials and conducted proximally (pink stars in Fig. 7). In addition, there were currents flowing toward the end of the abovedescribed currents and also conducting proximally (yellow arrows
in Fig. 7). From 4.9 ms to 7.0 ms, currents directing caudally
appeared and conducted proximally (yellow stars in Fig. 7).
The CV of the intra-axonal reconstructed currents was 60.8 ± 5.
6 m/s (mean ± SD, n = 9). As described in Section 2.4, perpendicularly flowing currents 10, 15, and 20 mm away from the conduction pathway of the intra-axonal currents were reconstructed
and their suitability evaluated. The CVs of these currents were62.5 ± 16.6 m/s, 60.3 ± 9.1 m/s, and 60.1 ± 9.0 m/s (mean ± SD,
n = 9), respectively.
We evaluated the differences between the peak latencies of
evoked potentials and reconstructed inflow currents near the cubital tunnel. For the reconstructed currents 10, 15, and 20 mm away

3.2. Recording of evoked magnetic fields
In all subjects, evoked magnetic fields were recorded in
response to the electrical stimulation. In Fig. 6, the evoked magnetic fields are superimposed on the X-ray image. In the recording
setting, the elbow and the ulnar nerve are above the sensor. Polarity is reversed between the two sides of the assumed position of
the ulnar nerve (pairs of colored stars in Fig. 6).
3.3. Reconstructed current distribution
The equivalent currents reconstructed by the RENS beamformer
are illustrated in a pseudocolor map and superimposed on the Xray image (Fig. 7). From 3.5 ms to 6.3 ms, high-intensity currents
flowing toward the head appeared near the electrodes for evoked
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away from the conduction pathway are shown in Fig. 8b. The correspondence between the waveforms of the evoked potentials andreconstructed inflow currents is shown in Fig. 8c. In the representative subject shown in Fig. 8, the differences in the peak latencies
at three points around the medial epicondyle (A & 2, B & 4, and C &6 in Fig. 8c) were 0.005, 0.045, and 0.165 ms, respectively.
In the patient with cubital tunnel syndrome, reconstructed
intra-axonal and inflow currents propagated along the ulnar nerveon the right intact side (yellow and pink arrowheads in Fig. 9a). In
contrast, these currents disappeared after 5.0 ms in the left arm(yellow arrowheads in Fig. 9b).
The reconstructed intra-axonal and inflow currents propagatedcranially in the right elbow (Fig. 10a) and the CVs were 50.1 m/s
and 48.2 m/s, respectively. However, the upward pointed waves
seen in the intra-axonal currents were not clear in the left arm (affected side), and the inflow currents decelerated and attenuatednear the medial epicondyle (electrodes 5–7 in Fig. 10b). Note thatthe scale of the amplitude on the left side is one-fifth that on theright side. The CVs of the intra-axonal reconstructed currents could
not be calculated. The inflow currents propagated up to the distal
point of the medial epicondyle at a CV of 48.3 m/s (calculated fromelectrodes 3–5).

Fig. 6. Evoked magnetic fields of a representative case. Evoked magnetic fields
recorded from each sensor are superimposed on the X-ray image. White circles
represent sensors. Red waveforms show magnetic fields recorded from X coils, withupward positive in the X-axis; green waveforms are from the Y coil, with upward
positive in the Y-axis; and black waveforms are from the Z coil, with upward
positive in the direction toward the floor. Polarity is reversed between either side of
the assumed position of the ulnar nerve (marked with colored stars). (Forinterpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

4. Discussion
The development of the MNG technique has recently been progressing due to refinements of the recording system. One of theadvantages of MNG is that it can visualize neural activity as reconstructed currents. In the healthy participants, the CVs of evoked
potentials recorded above the medial epicondyle and of reconstructed currents flowing toward the conduction pathway from15 mm away were similar: 61.2 ± 9.8 m/s (mean ± SD, n = 9) andfrom the conduction pathway, the differences were 0.058 ± 0.34
ms, 0.011 ± 0.29 ms, and 0.031 ± 0.25 ms, respectively ([evoked
potential][reconstructed current]; mean ± SD, n = 9). We thus
chose data at the position of 15 mm away for the consistency evaluation (described below) because it showed the lowest difference.
Time courses of intra-axonal currents and inflow currents 15 mm

4.2 ms

4.9 msY [mm]

Y [mm]

Y [mm]

3.5 ms

8 nAm

x [mm]

x [mm]

x [mm]

5.6 ms

6.3 ms

7.0 msx [mm]

Y [mm]

Y [mm]

Y [mm]

0x [mm]

x [mm]

Fig. 7. Spatiotemporal changes in equivalent currents reconstructed with the recursively applied null-steering (RENS) beam former. From 3.5 ms to 6.3 ms, high-intensity
currents flowing toward the head appeared and conducted proximally (pink stars). Currents flowing toward the end of the currents marked with pink stars appeared (yellow
arrows) and also propagated proximally. From 4.9 ms to 7.9 ms, currents directing caudally appeared and conducted proximally (yellow stars). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)159

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Clinical Neurophysiology 138 (2022) 153–163b)Inflow currents

Intra-axonal currents

[nAm]

[nAm]
Proximal direction

10

a)

C
B

9

8

8

7

7

6

6

5

5

4

4

3

3

2

2

Outward direction

1

5 nAm
Latency [ms]Inward direction

10

9

1

A

Y [mm]

Distal direction

3 nAm

2 ms

2 ms

Latency [ms]

c)
A&2

B&4

[nAm・μv]

C&6[nAm・μv][nAm・μv]

X [mm]

0

2

4

6

8

10

0

2

Latency [ms]Evoked potential [μv] (A, B, C)
Inﬂow current [nAm] (2, 4, 6)

46

Latency [ms]

8

10

0

2

4

6

8

10

Latency [ms]1.5 nAm 20 μv
2 ms

Fig. 8. A time course of the reconstructed currents and the correspondence between evoked potentials and reconstructed currents. (a) The yellow line represents the pathwayof the peak intensity of the reconstructed currents marked with pink stars in Fig. 6. Red circles show virtual electrodes on the yellow line placed 15 mm apart. Green circles
show virtual electrodes 15 mm away from the yellow line. A, B, and C represent the electrodes used to record evoked potentials. (b) Intra-axonal currents and inflow currents
reconstructed at virtual electrodes in (a). Intra-axonal currents are reconstructed at red-circled virtual electrodes and conducted cranially. Inflow currents are reconstructed
at virtual electrodes with green crosses and also conducted cranially. Waveforms at the electrodes opposite the green crosses are not shown because some of them are outside
the arm. Numbers represent the electrode number shown in (a). (c) Correspondence between evoked potentials and reconstructed inflow currents. Evoked potentials wererecorded at the electrodes indicated as A, B, and C (gray waveforms). Inflow currents were picked up from the virtual electrodes labeled with 2, 4, and 6 in (a) and (b) because
they are the nearest to the evoked potential electrodes A, B, and C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web versionof this article.)

As stated in the Introduction, some patients with cubital tunnel
syndrome show false-negative results in the electrophysiologicalexamination. A functional imaging technique that can localize
lesions that electrophysiological examinations are unable to detectis desired. Given that MNG can visualize intra-axonal currents andvolume currents even at curved sites, MNG has the potential to be
a complementary functional imaging method to evaluate ulnar
nerve entrapment.
Nonetheless, there are some limitations to this study. One isthat the depth of the ROI for signal processing was fixed because
positional information was acquired from an X-ray image that
does not contain information on the exact position of the ulnar
nerve. In further studies, we would like to use a magnetic resonance image for positional information, which should improve
the accuracy of neural activity localization. Second, dysfunctional
ulnar nerve was evaluated in only one patient in this study. We
need to examine a greater number of pathological cases before
and after the surgical treatment to develop a detailed diagnostic
method in MNG.

60.3 ± 9.1 m/s (mean ± SD, n = 9), respectively. In addition, the difference in their peak latencies at the medial epicondyle was as
small as 0.011 ± 0.29 ms (mean ± SD, n = 9). As we have previously
reported, reconstructed inflow currents are believed to represent
volume currents flowing into the depolarization site (Fukuoka
et al., 2002; Sasaki et al., 2020). Consequently, it is reasonable thatthe position of the inflow current would be consistent with that of
the negative peak of the evoked potential.
The electrophysiological study of the patient with left cubital
tunnel syndrome revealed decelerated CVs of SNAPs and CMAPs,
which recovered after the surgery. On the intact side of the patient,
MNG before surgery showed that both intra-axonal and inflow
reconstructed currents propagated at similar CVs. On the affectedside, the intra-axonal currents were not distinct, and the inflow
currents decelerated and attenuated at the distal point of the medial epicondyle (after 5.0 ms in Fig. 9b; electrodes 5–7 in Fig. 10b).
The inflow currents were able to visualize the propagation of thepathological nerve, although the intra-axonal currents could not
be evaluated.

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a)

6 nAm

0

b)

1 nAm

0

Fig. 9. Spatiotemporal changes in equivalent currents reconstructed with the recursively applied null-steering (RENS) beamformer. A patient with left cubital tunnel
syndrome before surgery. (a) The right arm (intact side). Reconstructed intra-axonal (pink arrowheads) and inflow (yellow arrowheads) currents propagated proximally. (b)The left arm (affected side). Reconstructed intra-axonal currents are indistinct and inflow currents (yellow arrowheads) disappeared near the medical epicondyle at 5.0 ms.
(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)161

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Intra-axonal currentsa)Right elbow (intact side)

Inflow currents
[nAm]

[nAm]

7

Proximal direction 7
Distal direction

6

6

5

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3

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1

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5 nAm

5 nAm
2 ms

Latency [ms]b)Left elbow (affected side)

2 msLatency [ms]Intra-axonal currents

Inflow currents

[nAm]

[nAm]

7

Proximal direction

6

Distal direction

7
6

5

5

4

4

3

3

2

2

1

1
1 nAm
Latency [ms]

1 nAm
2 ms

Latency [ms]2 msFig. 10. A time course of the reconstructed currents. A patient with left cubital tunnel syndrome. (a) The right arm (intact side). Intra-axonal and inflow currents propagatedproximally. (b) The left arm (affected side). Upward pointing waves of intra-axonal currents are not as clear as in (a). Inflow currents diminished and decelerated at electrodes
5 to 7. Note that the scale of the amplitude is one-fifth that of the right arm.
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5. Conclusions
MNG can visualize the neural activity of the ulnar nerve at the
elbow as reconstructed currents. Our results show that MNG has
the potential be a novel and universal functional imaging technique for evaluating ulnar nerve entrapment, which may boost
its diagnostic ability and its use in surgical decision-making.
Funding
This research was supported by Ricoh Company, Ltd. The funding body played no role in study design, data collection and analysis, or preparation of the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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