Keywords
high-resolution - magnetic resonance - neurography - submillimeter - ultrasound
Key Message
High-resolution ultrasound (HRUS) may be a better means of evaluation of peripheral
nerves with submillimeter caliber than magnetic resonance imaging (MRI). MRI evaluation
should be done when clinical suspicion is high and HRUS fails to diagnose any lesion,
because of its superior accuracy and sensitivity in detecting nerve or muscle edema
and perineural changes.
Introduction
Peripheral nerve pathologies are commonly encountered by clinicians in practice. They
rely primarily on the information gained by nonanatomical tests like clinical examination,
neurophysiological assessment, and on clinical history for the evaluation and management
of these cases. With the use of imaging, it is possible to get spatial information,
regarding the exact site and nature of pathology as well as the surrounding structures,
which is crucial for further management.[1] HRUS and magnetic resonance neurography (MRN) are now considered complementary to
clinical and neurophysiological assessment for neuropathies and depending on the clinical
question, appropriate choice needs to be made.[2]
[3] Both the modalities are unique in their respective ways, with HRUS being more comfortable
for the patient, cheap, easily available, provides higher image resolution than MR
but has a steep learning curve and is highly operator dependent.[4]
[5] MRI is expensive, sometimes not comfortable for the patient, not dependent on the
operator, and has a high spatial resolution. The aim of our study was to compare accuracy
of HRUS and MRN for detecting various types of peripheral nerve pathologies, to choose
the correct investigation to facilitate prompt patient management.
Materials and Methods
A prospective study was performed using HRUS imaging with 14 MHz linear transducer
(Siemens S2000) and Siemens MAGNETOM 3 or 1.5T MR (Siemens Healthcare, Erlangen, Germany)
in cases referred for peripheral nerve pathologies. Ethical committee approval and
prior patient consent were obtained. Image interpretation was done using a scoring
system (score 0–3 confidence level) to assess for nerve continuity/discontinuity,
increased nerve signal/edema, fascicular change, caliber change, and neuroma/mass
lesion. Highest confidence level was denoted by score 3 and lowest by score 1. The
confidence level for each of these findings compared for both the modalities using
the z-test and p-value was calculated. Patients with polyneuropathies, MR contraindication, claustrophobia,
and imaging of brachial/lumbar plexus were excluded. MRN was performed using body
coil (3T: 6-channel; 1.5T: 9-channel) or large flex coil (3T: 4-channel; 1.5T: 4-channel)
depending upon the size of the region scanned and various sequences (T2-weighted imaging
[T2WI] fat-sat, T1WI with or without contrast, three-dimensional [3D] T2WI/short-time
inversion recovery Sampling Perfection with Application optimized Contrasts using
different flip angle Evolutions for submillimeter resolution of nerves, and 3D diffusion-weighted
PSIF) were obtained. MRs were reported by two radiologists each with more than 10
years’ experience in MSK MRI. US was subsequently performed by a single fellowship-trained
MSK consultant (JPS with more than 10 years’ experience), who was unaware of MR findings.
Radiologists were blinded to nerve conduction velocity (NCV), electromyography (EMG),
and clinical details. Studies were conducted in close time intervals to exclude any
error or difference in findings due to interval change in lesion characteristics.
We determined the accuracy, sensitivity, and specificity of these modalities against
the diagnostic standard determined by surgical and/or histopathological evaluation,
if not performed then clinical and/or electrodiagnostic evaluation.
Results
Thirty-eight patients (25 males, 13 females), with the mean age of 43 years and a
total of 47 nerves were evaluated using HRUS and MRN, against diagnostic standard
determined by surgical (29%), histopathological (10.5%), clinical (13.1%), and/or
electrodiagnostic evaluation (47.3%). The nerves involved included median (9), ulnar
(8), radial (7), anterior interosseous (2), posterior interosseous (2), sciatic (3)
common peroneal (6), sural (3), tibial (4), and others (3) like one each of spinal
accessory, posterior, and medial cutaneous nerve of the forearm. Specificity of HRUS
was higher (100%) than that of MRI (66.6%). Sensitivity of MR was higher at 92.6%
versus 80.4% for HRUS (p = 0.104). The overall accuracy of MRN ([Table 1]) was 89.3% (negative predictive value [NPV]: 57.1%, positive predictive value [PPV]:
95%) and that of HRUS was 82.9% (NPV: 42.8, PPV: 100). Confidence level ([Table 2]) for detecting nerve discontinuity and change in nerve caliber ([Fig. 1]) was found to be higher on US than MRI (100 vs. 70% [p = 0.009] and 100 vs. 50% [p = 0.007], respectively). MRI detected nerve/muscle edema in cases ([Fig. 2]), where US had less confidence level (p = 0.033), leading to higher sensitivity of MRI. Confidence for diagnosing neuroma
formation ([Fig. 3]) was high for both modalities (100% for US vs. 88.8% for MRI) with no statistically
significant difference (p = 0.317). Pathology of submillimeter caliber nerves (n = 3) was accurately detected by HRUS ([Figs. 4 ]and [5]), and these could not be diagnosed on MRI.
Table 1
The overall accuracy of MRN
|
MRI
|
US
|
|
Statistic
|
Value
|
95% CI
|
Value
|
95% CI
|
|
Abbreviations: CI, confidence interval; MRN, magnetic resonance neurography; US, ultrasound.
|
|
Sensitivity
|
92.68%
|
80.08–98.46%
|
80.49%
|
65.13–91.18%
|
|
Specificity
|
66.67%
|
22.28–95.67%
|
100.00%
|
54.07–100.00%
|
|
Positive predictive value
|
95.00%
|
85.93–98.34%
|
100.00%
|
|
|
Negative predictive value
|
57.14%
|
28.09–81.98%
|
42.86%
|
28.71–58.27%
|
|
Accuracy
|
89.36%
|
76.90–96.45%
|
82.98%
|
69.19–92.35%
|
Table 2
Confidence level for various parameters on MRI and US
|
n
|
MRI
|
US
|
p-Value
|
|
Abbreviations: MRI, magnetic resonance imaging; US, ultrasound.
a
p-Value <0.05, statistically significant.
|
|
Nerve discontinuity
|
10
|
7 (70.0%)
|
10 (100.0%)
|
0.009a
|
|
Increased nerve signal
|
13
|
13 (100.0%)
|
9 (69.2%)
|
0.033a
|
|
Fascicular change
|
18
|
16 (88.9%)
|
18 (100.0%)
|
0.151
|
|
Caliber change
|
14
|
8 (57.1%)
|
14 (100.0%)
|
0.007a
|
|
Neuroma/mass lesion
|
9
|
8 (88.9%)
|
9 (100.0%)
|
0.317
|
Fig. 1 Cubital tunnel syndrome (right elbow) (A, B) proximal to ulnar nerve compression, there is swelling with loss of normal fascicular
pattern (A). Hyperintensity (B) is seen (arrows) on axial proton-density fat saturated images. (C, D) Reformatted three-dimensional short-time inversion recovery Sampling Perfection
with Application optimized Contrasts using different flip angle Evolutions image clearly
shows site of nerve compression by thickened retinaculum (blue arrow C, D, and F)
with proximal increase in signal intensity of nerve. (E, F) Ultrasound images show compressed ulnar nerve with sudden increase in caliber of
nerve proximal to it (yellow arrow).
Fig. 2 Posterior interosseous nerve syndrome. Ultrasound images (A) in a 60-year-old female patient with pain in anterolateral aspect of right elbow
show hypoechoic mildly thickened posterior interosseous branch of radial nerve (yellow
arrow in A) with a thin echogenic constricting band (arrow in B) seen at proximal edge of supinator
muscle (arcade of Frohse). Sagittal short-time inversion recovery (STIR) (C) magnetic resonance imaging (MRI) depicts the thickened hyperintense nerve (arrow
head) with abrupt narrowing (yellow arrow). Axial STIR MRI (D) shows edema within supinator and extensor digitorum muscles.
Fig. 3 Musculocutaneous nerve neurogenic tumor. Sagittal ultrasound image (A) at distal arm shows a well-defined hypoechoic lesion (asterisk) along the musculocutaneous
nerve. Axial T2-weighted (B) and sagittal postcontrast T1-weighted (C) magnetic resonance imagings show the well-defined intensely enhancing oval lesion
(arrow in B and C) along the course of musculocutaneous nerve between the brachialis
and biceps brachii muscles.
Fig. 4 Spinal accessory nerve neuroma. A 37-year-old male with weakness of left trapezius
muscle following biopsy of a neck lymph node. Ultrasound image (A) shows an irregular (1.8 mm) neuroma (arrow) within the submillimeter caliber spinal
accessory nerve (block arrow), which was not identifiable on magnetic resonance imaging
(coronal T2 and short-time inversion recovery MIP images) with capsule at the site
of lymph node biopsy (arrow).
Fig. 5 End neuroma of medial cutaneous nerve of forearm ultrasound image (A: short axis, B: long axis) at the level of distal forearm in a 28-year-old male patient shows a
1.5 mm end neuroma (arrow) in the submillimeter caliber medial cutaneous nerve of
forearm (block arrow). Magnetic resonance imaging of the same patient (images not
shown) could not conclusively depict the neuroma formation.
Discussion
Imaging in peripheral nerve pathologies complements clinical history/examination,
EMG, NCV findings by giving the spatial and morphological information of the pathology
and thus influences patient management.[1]
[2]
[3] Also, peripheral nerve imaging is helpful in patients with indeterminate findings
on electrodiagnostic studies (especially postoperative patients) and in patients in
whom electrodiagnostic studies are not feasible due to inaccessible nerves or with
dermatological conditions.[6]
[7] With limited or indirect indications of computed tomography or radiography, peripheral
nerve imaging mainly relies on HRUS or MRN and appropriate choice of imaging modality
needs to be made to facilitate prompt patient management. In our study, we compared
accuracy of HRUS and MRN for detecting various types of peripheral nerve pathologies.
Characters like the continuity of the nerve, change in caliber, focal lesion or neuroma
formation, and focal nerve edema were evaluated.
Our study has shown MRN to be more accurate in detecting the peripheral nerve pathologies
(89.3 vs. 82.9%) with higher negative predictive values for diagnosing the lesions.
Similar findings were noted in a comparative study published by Agarwal et al,[8] who also reported higher accuracy of MRI over HRUS (93.89 vs. 86.11%) with higher
negative predictive value of MRI. The confidence levels to detect pathological characters
like caliber change (p = 0.007) and nerve discontinuity (p = 0.009) were higher with HRUS than MRI (100 vs. 50% and 100 vs. 70%, respectively)
and found to be statistically significant (p<0.05). Confidence level for detection of focal neuroma formation was high with both
(100% for US vs 88.8% for MRI) with no statistically significant difference. However,
MRI detected nerve edema with more confidence in cases, whereas US depicted no abnormality
leading to higher rates of pathological diagnosis (p = 0.033). Garg et al[9] in their study also evaluated confidence levels for these characteristics and they
had a similar impression for detecting nerve discontinuity, neuroma detection, and
detection of nerve edema on MRN. However, in their study confidence level was higher
for MRN in detecting change in caliber, which can probably be explained due to difference
in the frequency of the US probe used in both studies. We used a 14-MHz high-resolution
probe and they used 7 to 10 MHz linear array probe.
We found MRN to be more sensitive than HRUS (92.6 vs. 80.4%) in detecting the pathology,
which is similar to what Garg et al[9] found in their study. High sensitivity of MR has also been reported by Andreisek
et al[10] However, this was not found to be statistically significant (p = 0.107) in our study.
These observations in our study were in contradiction to the findings in study by
Zaidman et al[11] who reported higher sensitivity and accuracy of US over MR. This might be attributed
to their study being retrospective and the authors were comparing MR reports done
at various centers, in which they had no control in the acquisition of images in most
of the cases, to the US performed by a single operator (the author). Also, they had
access to only a few studies for review and that too were performed on 1.5T MR scanner
with no known details of the sequences and protocols used.
Agarwal et al[8]also reported higher specificity with MRI (86.67 vs. 80%) with higher positive predictive
values; however, in our study HRUS showed higher positive predictive value (100 vs.
95%) with higher specificity (100 vs. 66.6%), which can be attributed to poor image
quality for MRN in few of our patients with metallic implants or movement artifacts
and use of high-resolution sonographic probe. Zaidman et al[11] found specificity to be similar with both modalities, while Garg et al[9] have not evaluated both for the same.
Pathology of submillimeter caliber nerves (i.e., spinal accessory, posterior and medial
cutaneous nerve of forearm) was accurately detected on US because of a high-frequency
probe that gives submillimeter resolution. These could not be diagnosed on MRI, probably
because of wider field of view imaging that made it difficult to evaluate submillimeter
caliber nerves.
The study limitations were small sample size, referral bias with no control group,
and the low reliability of the diagnostic standards (as only 39.5% cases are confirmed
with surgery or histopathology). Statistical difference between 3 and 1.5T MRI with
US neurography was not assessed, considering small sample size and it needs future
research.
Conclusion
Imaging plays a crucial role in the evaluation, prognostication, and management of
patients with peripheral nerve pathologies. HRUS is a powerful tool that may be used
as the first-line imaging modality for the evaluation of peripheral nerve pathologies,
as it is dynamic, economical, comfortable for the patients and has high confidence
levels to detect pathology with a trained operator. We also conclude that for peripheral
nerves with submillimeter caliber, HRUS may be a better means of evaluation than MRI
and that the confidence level for detecting nerve discontinuity and change in nerve
caliber is higher on HRUS. MRI evaluation should, however, be done when clinical suspicion
is high and HRUS fails to diagnose any lesion, because of its superior accuracy and
sensitivity in detecting nerve or muscle edema and perineural changes.
