High Resolution in Vivo Magnetic Resonance Imaging of the Skin
and comparison to High Frequency Ultrasound
A. Liffers1, M.Vogt1, H. Ermert1, C. Müller2, A. Falk2, L. Heuser2, S. El Gammal3
1 Department of Electrical Engineering, 2 Institute for Radiology and Nuclear Medicine, University Hospital
and 3 Dermatologic University Hospital, Ruhr-University, Bochum, Germany
INTRODUCTION
RESULTS
For noninvasive imaging and characterization of the skin,
high resolution imaging techniques have to be applied. While
high frequency ultrasound (HFUS) achieves a resolution in the
range of 10 µm, magnetic resonance imaging (MRI) on a
1.5 Tesla, special equipped clinical whole body imager obtains a
resolution of several 10x10 µm2 in plane and several 100 µm
slice thickness [1]. Additionally, MRI offers the possibility for
representing tissue characteristics. In [2] different relaxation
times T1 and T2 were determined for different skin tissues (tumor, epidermis, corium, subcutis) by means of high-resolution in
vitro MR microscopy in a 9.4 Tesla main field. With the implementation of special low noise surface coils and a modified three
dimensional Fast Imaging with Steady State Precession (3DFISP) sequence, high resolution (80x40 µm2 in plane, 800 µm
slice thickness) in vivo measurements were performed in a Siemens Magnetom Vision (1.5 Tesla). Since HFUS is an established method for skin imaging, 100 MHz ultrasonic images of
the same areas have been taken for comparison.
With the used 3D-FISP sequence, T1 weighted images are
expected. But due to the short T2 time of the skin, the resulting
MR images show a T2-T1-contrast mixture. In Figure 1 a section
of a MR image of the skin at the shin bone is shown. With the
achieved resolution and SNR, one can recognize the sweat
glands at a hair follicle (1), the boundary between hypodermis
and subcutaneous fat (2), and a blood vessel (3). The epidermis
(4) is barely visible because of its lack of water i. e. H1 protons.
1
A simulation tool was developed to determine homogeneity,
sensitivity and SNR per measured voxel for coils with various
geometry. Rf coils were optimized in SNR vs. field of view by
varying coil radius, wire radius, wire material and number of
turns.
In vivo measurements were performed with a stacked three
turn 7.5 mm radius circular loop made of 0.5 mm radius, silver
plated copper wire. The coil was tuned to the resonance frequency of 63.6 MHz and matched to 50 Ω. A special frame was
designed which fixed the coil localized within the gradient field
and also fixed the examined skin under the coil.
The tomograph is equipped with a gradient set with a maximum gradient of 23 mT/m. The high resolution in plane of
80 µm (interpolated to 40 µm) times 40 µm is obtained with a
modified 3D-FISP sequence with an asymmetric echo in readout
direction. With gradients operated at full capacity, a relative long
readout time is necessary. Thus a long but acceptable echo time
of 10.5 ms has to be chosen. A higher echo time would be critical because the relaxation time T2 of skin is about 20-40 ms [2].
The images were taken with a field of view of 40 mm, 512x1024
(phase x readout) matrix. The slice thickness was 800 µm, but
can easily be reduced down to 100 µm. The repetition time was
67 ms. The acquisition time was 15 min.
For ultrasonic imaging, a mechanical scanner with a single
element transducer working in the 100 MHz range was used. A
special imaging technique was applied with the focused transducer to optimize the resolution and the sensitivity. The lateral
sampling is performed by mechanical movement of the transducer. The axial resolution of this system is 9 µm, and the lateral
resolution is 27 µm [3].
4
2
1.5
3
2
2.5
mm
1
METHODS
Very low noise radio frequency (rf) coils are required to improve MR images of the human skin. Since the reduction of the
imaging volume covered by the rf coils improves signal to noise
ratio (SNR), we addressed the design and implementation of
small rf surface coils.
1
0.5
2
3
4
5
6
7
8
9 mm
Figure 1: MRI
Figure 2 shows the HFUS image of the same area. The
sweat glands (1) and the boundary (2) can also be identified
clearly. The subcutaneous fat appears dark as it is a hypoechoic
area, while fat gives a high MR-signal because of its long T1
time.
1
0.5
1
1.5
40
dB
30
20
2
2.5
mm
10
2
1
2
3
4
5
6
7
8
9 mm
0
Figure 2: HFUS image
CONCLUSIONS AND FUTURE WORK
MRI as well as HFUS show a high potential for noninvasive
skin imaging. Clinical HFUS images of different tumors have
been presented in [3]. With the achieved resolution and SNR,
clinical MR images of tumors will be acquired and evaluated.
The future aim is the discrimination of different tissue types like
in [2]. For that purpose, an implementation of T1 and T2weighted sequences is necessary. A hybridism of MRI and
HFUS promises additional information.
REFERENCES
[1]
[2]
[3]
Song H.K., Wehrli F.W., Ma J.: In vivo MR microscopy
of the human skin. Magn Res Med 37, 185-191, 1997
el Gammal, S., Hartwig, R., Aygen, S., Bauermann, T., el
Gammal, C., Altmeyer, P, J. Invest. Dermatol., 12871292, 1996
Ermert, H., Vogt, M., Passmann, C., EL Gammal, S.,
Kaspar, K., Hoffmann, K., Altmeyer, P., Skin Cancer and
UV Radiation, Altmeyer, P., Hoffmann, K., Stücker, M.,
(Eds.), Springer-Verlag, 1023-1051, 1997
ISMRM Proc. International Society for Magnetic Resonance in Medicine (8th Scientific Meeting) (2000), p. 1400.
Proc. ISMRM, 8th Scientific Meeting and Exhibition, Denver, April 2000, p. 1400