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. Author manuscript; available in PMC: 2011 Jun 1.
Published in final edited form as: J Magn Reson Imaging. 2010 Jun;31(6):1395–1401. doi: 10.1002/jmri.22183

Vibration Imaging for Localization of Functional Compartments of the Extrinsic Flexor Muscles of the Hand

Yogesh K Mariappan 1, Armando Manduca 1, Kevin J Glaser 1, Jun Chen 1, Kimberly K Amrami 1, Richard L Ehman 1,*
PMCID: PMC2911947  NIHMSID: NIHMS218299  PMID: 20512892

Abstract

Purpose

To develop and test an MRI-based imaging technique for the localization of the functional compartments of the functionally finger-specific, yet anatomically indistinct, flexor muscles of the hand.

Materials and Methods

A total of 6 normal healthy volunteers were involved in five studies in which individual fingers were vibrated with mechanical actuators and the resultant motion within the corresponding functional compartments of the flexor muscles, mechanically transferred through the structurally connected tendons, was imaged with a phase-contrast MR imaging technique that is highly sensitive to cyclic motion. The motion amplitude and relative phase relationship between the functional compartments of various muscles and fingers were obtained and analyzed from these images as a means to differentiate the various subcompartments.

Results

The results show that this technique provides a detailed mapping of the regions of the complex flexor muscle compartments that correspond to each digit for both the flexor digitorum profundus and the flexor digitorum superficialis. The results also demonstrate the presence of mechanical interdependence between the flexor muscles.

Conclusion

It is concluded from the results that localization of the finger-specific subcompartments of the forearm flexor muscles can be performed with this technique.

Keywords: Vibration imaging, forearm flexor muscles, flexor digitorum profundus, flexor digitorum superficialis, functional forearm compartments, elastography

INTRODUCTION

The amazing dexterity of the human hand is made possible, in part, by the flexion and extension of the interphalangeal joints of the fingers, which are partly controlled by the extrinsic forearm muscles, such as the flexor digitorum profundus (FDP). Even though these muscles can achieve independent mechanical actions for each of the fingers through finger-specific tendons, they do not possess distinct anatomical compartments specific for each finger (1). The individuated finger movements are possible due to selective activation of functional compartments within each muscle specific for each finger (2,3). The knowledge of the location and boundaries of these compartments within each of the muscles could be valuable for studies involving techniques such as MR spectroscopy and electromyography. These techniques are widely used in exercise physiology and other studies involving individual finger manipulative tasks, but it is difficult to obtain this finger-specific information with standard imaging techniques like conventional MRI and CT since these are functional rather than structural compartments.

Magnetic Resonance Elastography (MRE, (4)) is a novel phase-contrast elasticity imaging technique developed to quantitatively map the mechanical properties of tissues. It has emerged as a diagnostic tool for hepatic fibrosis (5,6) and is also being investigated for applications in the breast, brain and prostate (7–9). In conventional MRE, time-harmonic external vibrations of a single frequency are introduced into an object and the resultant shear wave propagation is mapped into the phase of MR images with the use of motion-encoding gradients inserted into standard MR pulse sequences. From these images, the shear stiffness (the mechanical property elicited by palpation) is estimated throughout the object using mathematical inversion algorithms (10). The MRE motion-encoding process is very sensitive to motion and can detect displacements on the order of 100’s of nanometers (11).

In this work, we propose that the motion-encoding principle used in MRE can be utilized for the localization of the functional compartments of the forearm flexor muscles. Previous evidence suggests that skeletal muscle motion and velocity can be measured with the help of phase-contrast MR imaging (12,13). We hypothesized that by vibrating individual fingers and encoding the motion of the corresponding muscle compartment that is transferred through the structurally connected tendons with an MRE pulse sequence, functional compartment localization can be achieved. The purpose of this work is to test this hypothesis in a series of human volunteer experiments and to investigate the advantages and limitations offered by this technique.

MATERIALS AND METHODS

A 1.5-T whole-body scanner (Signa Excite, GE Healthcare, Milwaukee, WI) and a 10-cm inner diameter birdcage MRI coil (Mayo Clinic Health Solutions, Rochester, MN, USA) were used in all of the following experiments. Unlike MRE, where shear wave propagation within a tissue is imaged for the calculation of its stiffness, the finger of interest was vibrated and the bulk motion of the corresponding muscle group was imaged with the proposed technique, named as Vibration imaging. A total of six healthy right-handed volunteers aged between 25 and 50 years (4 males and 2 females) were recruited for different parts of this study. Informed consent was obtained from each volunteer and the experiments were performed in accordance with the institutional review board.

Vibration Application

Selective vibration of individual fingers was performed with a pressure-activated driver system with four independent channels each devoted to a single finger (index, middle, ring and little). This system was derived from the single-channel driver system that is being used for liver MRE studies (6). The system included an active component, a passive component and a plastic tube that connected these two components and is shown schematically in Figure 1a. The active component was a modified acoustic speaker system (kept outside the scan room) designed as an enclosed system capable of conducting the cyclic pressure variations produced at the diaphragm into the scan room and the magnet bore via a primary connecting tube with a manifold near the subject containing an array of 4 auxiliary connecting tubes terminated in passive drivers. The passive driver components were made of 1.5-inch diameter polycarbonate cylinders 0.5 inch high, sealed on the bottom with rigid plastic and covered on the top with a thin, (0.005-inch) flexible polycarbonate membrane (Figure 1b). The four fingers were kept in contact with their respective passive driver components by double-stick tape. The typical positioning of the fingers is also indicated in Figure 1b, and with this configuration both the proximal and distal interphalangeal joints were vibrated. The auxiliary or supply lines for each driver could be opened and closed independently of the others to vibrate individual fingers or any combination of fingers desired. Figure 1a also shows the typical positioning of the volunteer: prone with either the right or left hand extended above the head, palm facing down unless otherwise indicated.

Figure 1. Vibration imaging experimental setup.

Figure 1

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(a) Schematic diagram of the pressure-activated driver system used in this study and the positioning of the volunteer (prone, palm facing down). (b) The four-channel passive driver array and the positioning of the individual fingers on these drivers. The auxiliary connecting tubes, which extend from a manifold in the primary connecting tube connected to the active driver, are also shown.

Vibration Imaging

An axial slice of the forearm, whose location is approximately shown in Figure 2a with respect to the deep-layer musculature of the forearm from Gray’s anatomy (14), was selected as the imaging plane since this location includes the functional compartments of all the fingers within the flexor muscles. Figure 2a shows the FDP muscle and the four tendons originating from this muscle and ending with the last four digits of the hand. With the application of the vibration, the functional compartments specific to the vibrated finger move in the direction indicated by the double-sided arrow in Figure 2a. Figure 2b shows a typical magnitude image of this imaging slice (in the right arm) without the externally applied vibrations. The lower portion of the image includes the flexor muscles flexor digitorum profundus and flexor digitorum superficialis (FDS). The upper part, separated from the flexor region by the two bones (radius and ulna, as indicated by the letters R and U), includes the extensor muscles. The flexor and extensor regions are indicated in the image as F and E, respectively.

Figure 2. Imaging slice position and the magnitude image.

Figure 2

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(a) The imaging slice location is shown with respect to the deep layer musculature of the forearm (from Gray’s anatomy). The direction of motion of the functional compartments is shown by the double sided arrow. (b) An anatomical MR image of the forearm at the slice location indicated in Figure 2a is shown. The regions of the extrinsic muscles, Flexors (F) and Extensors (E), and the bones, radius (R) and ulna (U) are indicated.

For most of the experiments, vibrations at a frequency of 90 Hz were applied and a gradient-recalled-echo MRE pulse sequence was used to encode the motion of the tissue into the phase of the MR images using a single 11.1-ms motion-encoding gradient (MEG) waveform (4). Since the finger-specific compartments move predominantly in and out of the imaging plane due to the orientation of the imaging slice, this through-plane component of the motion was sensitized to in these experiments by applying the MEG along that axis. Other relevant imaging parameters were field of view (FOV) = 12 cm, acquisition matrix = 256×64, frequency-encoding direction = L-R, TR/TE = 100/26 ms, total acquisition time = 51.2 s, number of excitations (Nex) = 1 and slice thickness = 5 mm.

The phase of an MR image obtained as described above provides an effective snapshot of the tissue motion. By adjusting the temporal relationship between the MEG and the actual motion between successive scans, four such phase images equally spaced over one period of the vibration were obtained. To extract information about the tissue motion occurring at the frequency of the vibration, a Fourier transformation was applied through the 4 phase images at each pixel and the complex-valued signal at the first harmonic (the frequency of vibration) was recorded. The amplitude of the first harmonic of the Fourier transform yields the maximum amplitude of the measured phase signal at each pixel and is directly proportional to amplitude of motion (4). This image is referred to as an amplitude map. The phase of the first harmonic yields information about the relative temporal phase of the motion throughout the image. These first-harmonic phase images were multiplied with normalized amplitude maps (i.e., amplitude-weighted) to highlight the phase information of high motion amplitude regions.

Vibration Imaging Experiments

A series of experiments were performed to test this technique in vivo and to demonstrate a few potential applications of this technique.

Study #1

To investigate the feasibility of imaging the subcompartments of the forearm flexor muscles, the four fingers of the right hand were vibrated individually and sequentially in 2 male and 2 female volunteers. The motion occurring within the finger-specific functional compartments was measured and the amplitude and temporal phase of the motion obtained for each finger was examined.

Study #2

To compare and contrast the definition of the functional compartments in dominant and subordinate arms, the technique was applied to the left and right hands of 2 right-handed volunteers and the amplitude maps were compared.

Study #3

To demonstrate that the technique is robust across different frequencies the applied vibrations, image acquisitions were performed individually on all the fingers of a single volunteer at three different frequencies (90 Hz, 120 Hz and 150 Hz), and the amplitude maps were examined to assess the difference in compartment location and localization between the different frequencies for each finger.

Study #4

To study the effects of the deep and superficial flexor muscles in isolation, data were obtained in a volunteer with vibrations focused to preferentially vibrate the distal interphalangeal joint by placing just the tip of the fingers in contact with the passive drivers, since the movement of the distal joint is predominantly controlled by the FDP (isolated vibration of the proximal joint is difficult with the current experimental setup). These results were compared to the data obtained from the normal vibration imaging experiments where both the distal and proximal interphalangeal joints were vibrated.

Study #5

To map the functional compartment locations of all the fingers in a single image and in a single acquisition, the fingers were vibrated simultaneously, but with different phases (i.e., time delays between the vibration and the motion-encoding gradients) for each finger to allow for isolation and identification of the compartments in 2 volunteers. This can be achieved by having independent channels of active and passive actuators devoted to each finger. However, in this work, this was accomplished by adjusting the lengths of the auxiliary connecting tubes extending from the manifold to each passive driver component. The amplitude maps of this combined motion were then examined and the finger-specific functional compartments were isolated manually (based on the visually observable amplitude difference and the boundaries between the subcompartments), color coded and overlaid onto the magnitude image of the forearm.

RESULTS

Study #1

Typical results obtained from the human volunteer vibration imaging experiments where individual fingers controlled by the FDS and FDP were vibrated independently are shown in Figure 3. Figure 3a shows the motion amplitude map obtained when the index finger was vibrated. Similarly, Figures 3b, 3c and 3d show the amplitude maps when digits 3–5 (middle, ring and little fingers, respectively) were individually vibrated. From these images it can be seen that there exist spatially distinct and localized regions within the flexor muscles with increased motion amplitude specific for different fingers (amplitudes typically ranging from 5 to 40 μm of through-plane motion) demonstrating the functional compartmentalization of these muscles. Two core regions are visible for each finger corresponding to the two flexor muscles, the FDP and FDS, indicated with the letters D and S for the deep and superficial muscles, respectively. No apparent gender-specific differences were observed between the data obtained from the male and female volunteers in this small study.

Figure 3. Functional compartments of the flexor muscles.

Figure 3

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Shown are the first-harmonic displacement amplitude maps of the forearm muscles for a volunteer when the index (a), middle (b), ring (c) and little (d) fingers were vibrated individually. Two core regions are visible for each finger corresponding to the two flexor muscles, the FDP and FDS, indicated with the letters D and S for the deep and superficial muscles, respectively.

Figures 4a and 4b show a representative amplitude map and amplitude-weighted first-harmonic phase map of the functional compartments specific for the ring finger, respectively. From the phase map, it can be seen that the functional subcompartments within both the flexor muscles (FDP and FDS) move in phase. From the motion amplitude map, in addition to the motion of the muscles in the flexor region of the forearm, motion can also be seen in the corresponding extensor muscles (green arrows). From the phase image, the flexor and extensor muscles (indicated by the white arrows) can be seen to be out of phase with each other since they form an antagonistic pair acting on the same joint. Similar behavior was observed with the vibration of the other fingers.

Figure 4. Phase opposition of the flexor and extensor muscles.

Figure 4

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(a) Motion amplitude map for a volunteer with the ring finger being vibrated at 90 Hz. (b) Corresponding first-harmonic phase map (with amplitude weighting). The flexor and extensor muscles are indicated by the arrows and the opposition in phase between these regions is evident.

Study #2

Figure 5 shows the comparison of the functional compartments of the index finger within the flexor muscles at comparable locations of the right and left arms of a right-handed volunteer. Examining the magnitude images, it can be seen that the cross sectional area of the dominant arm in Figure 5a is larger than that of the subordinate arm shown in Figure 5b. However, similar (mirror-image like) functional compartments within the flexor muscles of both arms can be visualized in the amplitude maps shown in Figures 5c and 5d (with slight differences due to slice position, arm position, and normal anatomic variations). Similar results were obtained in both volunteers and for each finger subcompartment.

Figure 5. Vibration imaging of dominant and subordinate arms.

Figure 5

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Shown are magnitude images and the corresponding functional compartment amplitude maps for the index finger of the right (a, c) and left (b, d) arms of a right-handed volunteer. The compartments were well visualized in both arms and the compartments are approximately mirror images of each other.

Study #3

Example data obtained from the multifrequency experiment are shown in Figure 6. The hand was held palm up for this experiment. The magnitude image of the imaging slice is shown in Figure 6a. Vibration imaging amplitude maps obtained when the middle finger was vibrated at 90, 120 and 150 Hz are shown in Figures 6b, 6c and 6d, respectively. In these maps, the location of the functional compartments of the finger can be visualized at each frequency of vibration. However, the amplitude of motion observed decreases as the operational frequency increases due to a combination of mechanical limitations of the driving system and the reduced motion of the compartments at the higher frequencies. Similar characteristics were observed with the other fingers.

Figure 6. Vibration imaging and frequency of operation.

Figure 6

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(a) Magnitude image of the forearm. Motion amplitude maps of the forearm muscles with the middle finger vibrated at 90 Hz (b), 120 Hz (c) and 150 Hz (d). The functional compartments are well visualized at all three frequencies and show similar spatial extent.

Study #4

Figure 7 shows an example of the data obtained from the vibration imaging experiment performed to isolate the deep and superficial flexor muscles in the forearm. Figure 7a shows the magnitude image for the imaging slice. Figure 7b shows the motion amplitude map obtained from the vibration imaging experiment when only the distal interphalangeal joint of the little finger was preferentially vibrated. In this case, the amplitude of motion in the functional compartment specific to the deep flexor muscle vibrates preferentially compared to the superficial flexor muscle. Figure 7c shows the corresponding, normal vibration imaging data when both the distal and proximal joints were vibrated. Both the deep and superficial muscles exhibit similar motion when both interphalangeal joints are involved. Similar patterns of behavior were observed for the other fingers as well.

Figure 7. Differentiation of the superficial and deep flexor muscles.

Figure 7

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(a) Magnitude image of the forearm. The motion amplitude maps with vibration of the little finger applied only to the distal interphalangeal joint (b) and to both the distal and proximal interphalangeal joints (c). These images show that high-amplitude motion occurs only in the flexor digitorum profundus with preferential vibration of the distal joint and increased motion of the flexor digitorum superficialis occurs with the involvement of the proximal joint.

Study #5

The results obtained from the experiment aimed at the simultaneous localization of the functional compartments of all the fingers are shown in Figure 8. Figures 8a and 8b show the magnitude image and the motion amplitude map, respectively, and Figure 8c shows the color-coded functional compartments of both the flexor muscles overlaid on the magnitude image from Figure 8a.

Figure 8. Functional compartments of the flexor muscles.

Figure 8

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(a) Magnitude image of the slice. (b) Motion amplitude map obtained with all 4 fingers being vibrated simultaneously. (c) Overlay image of the color-coded functional compartments of both flexor muscles for the four fingers on the anatomical image.

DISCUSSION

These preliminary results demonstrate that the flexor muscles possess functionally distinct compartments specific for each finger and that these compartments can be noninvasively visualized in vivo with the vibration imaging technique described in this work. With this technique, for each finger vibrated, two core regions of high-amplitude motion were observed within the flexor region representing the deep and superficial flexor muscle groups controlling the distal and proximal interphalangeal joints, respectively. It can also be seen that, in addition to these core regions for each finger, compartments that primarily move in response to adjacent fingers also had some low-amplitude motion when neighboring fingers were vibrated, suggesting an incomplete functional subdivision of these muscles. This is in agreement with the literature from electromyographic observations (15,16). The involvement of the extensor muscles is also noticeable from the individual finger motion amplitude maps.

The location of the functional compartments of the flexor muscles as attained from the vibration imaging technique were found to be similar to the results obtained from exercise-dependent MRI (2,3). The protocols for those studies included performing exercises with the finger of interest and localizing its functional compartment using a T2-weighted imaging technique capitalizing on the increase in the T2 relaxation time (10–30%) due to the active muscle water content (17–19). It has also been noted that the exercise-dependent muscle T2 increase is absent in patients with McArdle’s disease (20). Thus these techniques are subject-dependent and may result in fatigue in the exercised finger (3,21). Vibration imaging is performed with low amplitude, passive vibrations without active subject involvement, hence can be used in patients who are sensitive or are unable to respond appropriately to commands (e.g., pediatric patients or patients after hand trauma or surgery).

This technique could be a useful adjunct to conventional anatomic MR imaging and other functional tests such as electromyography. However, it should be noted that the experiments performed in this study are preliminary feasibility studies done on a small number of volunteers. Further investigations are needed to demonstrate the potential uses of this technique.

Potential research applications of vibration imaging may include exercise physiology studies that use electromyography, where improved localization of the functional compartments can help guide the positioning of the probe (15,22,23). The technique may find application in basic science studies of the human forearm where the function and contribution of intrinsic and extrinsic muscles is still under debate (24). While the vibration imaging technique demonstrated in this work is used qualitatively for the localization of the functional compartments, the technique is also a quantitative technique which directly measures the motion of the muscle. Therefore, the technique may potentially be used for quantifying tendon excursions or tendon gliding, which could be useful in the evaluation of tendon repair and tendon transfer studies (25,26). Similarly, the process of vibrating a finger could be used to induce shear waves in the carpal tunnel, facilitating the use of conventional MRE to assess the stiffness of the subsynovial connective tissue which also has the potential to be used as a marker for carpal tunnel syndrome (27,28).

Clinically, this technique may have application in identifying muscle function prior to surgical interventions such as for muscle or tendon transfers or nerve grafting to either the brachial plexus or more distal peripheral nerves innervating the hand in patients after trauma. The complexity and overlap of the innervation of the muscles of the hand is well known (29,30); using this technique prior to repair could better delineate functional and nonfunctional muscles, potentially allowing for more effective interventions. Vibration imaging could also be applied postoperatively to assess muscle function in a noninvasive fashion that would be complementary to electromyography and high-resolution anatomic imaging. In conclusion, these results support our hypothesis that the functional compartments of the multitendoned forearm muscles can be visualized noninvasively using the proposed vibration imaging technique by vibrating individual fingers and imaging the resulting motion within the muscles. This technique could be potentially useful for the localization of functional compartments applicable and/or complementary to studies such as electromyography for studying normal and abnormal hand biomechanics.

Acknowledgments

Grant Support: NIH EB001981

NIHMSID: NIHMS218299

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