Abstract
purpose. To investigate both feasibility and clinical potential of magnetic
resonance imaging–dynamic color mapping (MRI-DCM) in measuring the
motion of soft tissues in the orbit and in the diagnosis of orbital
disorders by detecting changes in motion.
methods. Sequences of MRI scans were acquired (acquisition time, 5 seconds) in a
shoot–stop manner, while the patient fixated at a sequence of 13 gaze
positions (8° intervals). Motion was quantified off-line (in
millimeters per degree of gaze change) using an optical flow algorithm.
The motion was displayed in a color-coded image in which color
saturation of a pixel shows the displacement and the hue the
displacement’s orientation. Six healthy volunteers and four patients
(two with an orbital mass and two with acrylic ball implant after
enucleation) were studied.
results. The technique was found to be clinically feasible. For a gaze change of
1°, orbital tissues moved between 0.0 and 0.25 mm/deg, depending on
the type of tissue and location in the orbit. In the patients with an
orbital mass, motion of the mass was similar to that of the medial
rectus muscle, suggesting disease of muscular origin. In the enucleated
orbits, soft tissue motion was decreased. One eye showed attachment of
the optic nerve to the implant, which could be verified by biopsy.
conclusions. MRI-DCM allows noninvasive and quantitative measurement of soft tissue
motion and the changes in motion due to pathologic conditions. In cases
in which the diagnosis of a tumor in the apex is in doubt, it may
reduce the need for biopsy. In contrast to static computed tomographic
(CT) scans and MRIs, it can differentiate between juxtaposition and
continuity and may be a new and promising tool in the differential
diagnosis of intraorbital lesions.
The introduction and refinement of noninvasive imaging techniques
such as computed tomographic (CT) scans and magnetic resonance imaging
(MRI) have been revolutionary in the differentiation of orbital
diseases. However, these techniques are static and in a number of cases
leave unanswered questions about the origin of lesions and the
relationship between tissues during motion. This relationship, the
kinetics of the eye and orbital tissue due to gaze, is highly complex
and incompletely understood.
1 Cinematic MRI, and also
dynamic CT and dynamic ultrasound, were developed to surmount these
limitations and to evaluate the motion of tissues in the orbit in
relation to gaze changes.
2 3 However, cinematic MRI scans
and the other two modalities are evaluated by inspection of videos and
consequently allow only qualitative judgments that are subject to a
large intra- and interobserver variability. By measuring motion
quantitatively, this may be avoided. In addition, data reduction can be
achieved. Clinicians can gain an understanding of orbital motion by
inspecting a single image, instead of a video of often several
minutes’ duration.
We have developed a new technique, MRI-dynamic color mapping (MRI-DCM)
to quantitatively measure the motion of orbital tissues, using
cinematic MRI with short acquisition times (5 seconds/image), combined
with powerful image-processing techniques.
2 3 4 The
purpose is to express the motion of orbital soft tissues in millimeters
per degree of change in gaze and display these in a color-coded image
in which the hue of a pixel is determined by the orientation and its
saturation by the length of the underlying motion
vector.
5 6 This technique allows the study of motion in
relation to gaze changes, but not yet of saccades and pursuit
movements, because the temporal resolution of orbital cinematic MRI
currently does not allow it.
Disorders of orbital tissues can all influence soft tissue motion: for
example, space-occupying lesions, enucleation with prosthesis
implantation, Graves’ orbitopathy, or trauma. Measuring such changes
may aid in localizing and differentiating orbital tumors, exploring
prosthesis motility, and observing tissue attachments in the case of
enucleation and trauma. In addition, it is known that in Graves’
orbitopathy, motility disturbances are related to muscle tightness and
swelling, increased intraorbital pressure, and inflammatory changes in
muscles and intraorbital fat tissue. After decompression surgery,
motility disturbances may either increase or decrease, and surgical
management of these disturbances is not as straightforward as in other
cases of acquired strabismus. By measuring the motion of orbital
tissues, we may be able to more fully understand the causes of motility
disorders in Graves’ orbitopathy, especially after decompression
surgery.
The purpose of the present study was to investigate the feasibility and
usefulness of MRI-DCM and to establish the additional value of MRI-DCM
in the differential diagnosis of orbital lesions.
This study introduces a new technique for measuring the motion of
soft tissues in the orbit, MRI-DCM. The technique allows the
noninvasive determination of the kinetics of orbital tissues such as
muscle, optic nerve, orbital fat, and tendon in millimeters per degree
change in gaze position. The technique is feasible using clinical MRI
equipment, is tolerated by subjects and patients and, by its
quantitative nature, avoids intra- and interobserver variation. It has
approximately the same cost as conventional MRI, and, if a static MRI
is requested, can be performed simultaneously at that appointment.
There are a few disadvantages to MRI-DCM. It can be difficult in
patients who cannot concentrate for an extended period, especially
elderly patients who may find the large number of targets confusing. By
improving the optical flow algorithm, we have (after this study was
completed) been able to reduce this number. The technique is
contraindicated in patients with pacemakers and arterial clips and in
some patients with metal implants. Optical flow and DCM are
image-processing methods that are most sensitive to tissues that show
many MRI features (and have relatively inhomogeneous signal
intensities). Motion is more difficult to measure reliably in tissues
that are relatively smooth, such as vitreous and fat. Up to now, only
two-dimensional motion can be measured.
MRI-DCM allows measurement of soft tissue motion in normal subjects.
Motion tends to range between 0 and 0.25 mm/deg, depending on the type
of tissue and the position of the tissue in the orbit relative to the
eyeball. The measured range of motion for the lens is close to the one
expected from the calculations
(Table 1) . No objective measurement is
currently available for orbital kinetics. The only possible validation
would be by an invasive technique, probably influencing the very
kinetics it is meant to measure.
MRI-DCM allows additional information beyond CT and MRI in orbital
lesions and, in contrast to these techniques, allows a differentiation
between juxtaposition and continuity of tissues. In patient A, MRI-DCM
showed that the mass was continuous with the medial rectus muscle (the
most likely cause being an origin in the muscle) thus facilitating the
diagnosis of myositis. It may be of clinical value in differentiating
the origin of a retrobulbar lesion and may replace the need for a risky
biopsy in the apex of the orbit.
MRI-DCM allows measurement of soft tissue motion after enucleation. In
patients C and D, soft tissue motion in the entire enucleated orbit
(0.0–0.14 mm/deg) was less than that in the healthy contralateral
orbit (0.0–0.24 mm/deg). This is in agreement with earlier
nonquantified
observations.
9
Little is known about the anatomy in the orbit after enucleation and
implant. MRI-DCM allows differentiation between juxtaposition of the
optic nerve to and continuity with the scleral cover of the implant
after enucleation. Much to our surprise we discovered that after
enucleation, the optic nerve stump showed regrowth to the sclera cover.
This finding has been confirmed by biopsy.
10 Such
attachments have previously not been recognized, probably because on
static CT and MRI scans, it is impossible to differentiate between
juxtaposition and continuity of structures. Further studies should be
undertaken to reveal the clinical significance of this phenomenon.
We want to stress that the common basis for this last conclusion is
that the more similar the motion of two adjacent structures (i.e., the
more similar their colors in a MRI-DCM image), the more likely it is
that they are continuous. In this study we examined only this aspect of
orbital kinetics. We hope to extend our studies to disorders of ocular
motility in the future.
Presented in part at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, May, 1997.
MDA is supported by a grant from the Fischer Fund.
Submitted for publication October 1, 1999; revised February 14, 2000; accepted March 31, 2000.
Commercial relationships policy: N.
Corresponding author: Michael D. Abràmoff, University Medical Center Utrecht, Room E03.136, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands.
m.d.abramoff@oogh.azu.nl
Table 1. Range of Orbital Soft Tissue Motion Measured with MRI-DCM in 6
Normal Subjects
Table 1. Range of Orbital Soft Tissue Motion Measured with MRI-DCM in 6
Normal Subjects
Structure | Orientation | Motion (mm/deg) |
Lens | Left–right | 0.19–0.25 |
Optic nerve (anterior part) | Left–right | 0.13–0.19 |
Optic nerve (posterior part) | Left–right | 0.0–0.05 |
Medial rectus muscle | Anterior–posterior | 0.7–0.12 |
Lateral rectus muscle | Anterior–posterior | 0.8–0.12 |
Orbital fat | Left-anteriorly–right anteriorly | 0.0–0.06 |
Gentry LR. Anatomy of the orbit. Neuroimag Clin. 1998;8:171–194.
Bailey CC, Kabala J, Laitt R, et al. Cine magnetic resonance imaging of eye movements. Eye
. 1993;7:691–693.
[CrossRef] [PubMed]Demer JL, Miller JM. Magnetic resonance imaging of the functional anatomy of the superior oblique muscle. Invest Ophthalmol Vis Sci
. 1995;36:906–913.
[PubMed]Amartur SC, Vesselle HJ. A new approach to study cardiac motion: the optical flow of cine MR images. Magn Res Med
. 1993;29:59–67.
[CrossRef] Stuijfzand EP, Abràmoff MD, Zuijderveld KJ, et al. Fast kinematic magnetic resonance imaging of the eye and orbit. RSNA Electr J. 1997.1.available at http://ej.rsna.org/EJ_0_96/0032–97.fin/ej0032–97.html
Abràmoff MD, Niessen WJ, Viergever MA. Objective quantification of the motion of soft tissues. IEEE Trans Med Imag. In press.
Barron JL, Fleet DJ, Beauchemin SS. Performance of optical flow techniques. Int J Comp Vis
. 1994;12:43–77.
[CrossRef] Collin JRO. A Manual of Systematic Eyelid Surgery. 1989; 2nd ed. 121–126. Churchill Livingstone Edinburgh.
Ghabrial RPM, Harrad RA, Hunter J, Kabala J, Macey D. Assessment of the anophthalmic socket with dynamic cine-MRI. Orbit
. 1997;16:207–216.
[CrossRef] Abràmoff MD, Stuijfzand EP, Mali WPTM, Mourits MPh. Rapid dynamic video shows reattachment of optic nerve after enucleation [ARVO Abstract]. Invest Ophthalmol Vis Sci. 1997;38(4)S390.Abstract nr 582