Con way
Drs
and
Hayes
We thank Dr Hall of the newer data
for raising are currently
only recently become cy of three-compartment share
Dr
costs
that
Hall’s
some
an interesting in unpublished
aware
that wrist
concern
about
reason of the
that
we describe
Since most form, we have
about the still exists.
added
wrist
unpublished
point.
controversy arthrography
the
three-compartment
for this
gested
respond:
financial
and
arthrography
(with
the
efficaWe time
entail.
authors’
It is
permission)
results.
The group at the Mallinckrodt Institute of Radiology (Gilula LA, Wilson AJ, Mann FA, oral communication, December 1, 1989) examined 250 patients who had undergone threecompartment wrist arthrography and found 47 examples of “one-way communications” between the joints of the wrist. Forty-four of these would not have been detected with an isolated injection into the radiocarpal joint. The investigators followed a protocol similar to that suggested by Dr Hall, namely, maximal distention of the joints examined. Our experience (Conway WF, Hayes CW, unpublished material, December 1, 1989), although involving fewer patients (n 50), is qualitatively similar to the Mallinckrodt experience. In addition, with injection into the distal radioulnar joint,
we
on the
were
able
to demonstrate
undersurface
(TFCC)
of the
that were
two
triangular
not visualized
partial-thickness
injection
carpal joint. The group at the University of Utah (Manaster BJ, oral communication, December 1, 1989), however, had a different experience. Examining 50 three-compartment wrist arthrograms, they found a minimal number of false-negative results with respect to ligamentous and capsular damage when injection
into
the
radiocarpal
joint
was
examined
separately.
The
one exception to this finding was the detection of a significant number of partial-thickness undersurface tears of the TFCC during injection into the distal radioulnar joint. The Utah group
consider
that
since
their
surgeons
do
not
operate
on
such tears, there is limited value in their detection. Surgeons at our institution, however, have expressed interest in operating on well-documented partial-thickness tears of the TFCC. At present, we believe that the preponderance of information
(1,2)
(Gilula
et al,
Hayes, unpublished three-compartment that some questions Dr
Hall
in
vantages
personal
communication)
(Conway
and
material) argues for the added efficacy of wrist arthrography, although we admit still remain. For this reason, we join with
recommending
further
documentation
of three-compartment
wrist
of the
ad-
arthrography.
References 1. Levinsohn EM, Palmer AK, Coren AB, Zinberg E. Wrist arthrography: the value of the three compartment injection technique. Skeletal Radio! 1987; 16:539-544. 2. Quinn SF, Pittman CC, Belsole R, Greene T, Rayhack J. Digital subtraction wrist arthrography: evaluation of the multiple-cornpartment
technique.
AJR
1988;
problem
could
be eliminated
by disrupting
to Faraday’s
law
of electromagnetic
induction,
whereby a current can be induced in a closed-loop conductor as it moves through a magnetic field (2). In a soon-to-be-published article (3), we describe a commercially available truly MR-compatible halo device that is constructed
entirely
of nonferromagnetic
and
nonconductive
ma-
terials (Ambulatory Halo System; All Orthopedic Appliances, Division of Kirschner Medical, Greenwood, SC). The advantages of using this particular halo device in patients undergoing MR imaging are as follows: (a) there are no associated deflection forces, (b) heating is insignificant, (c) there are no imaging artifacts, (d) and there is no likelihood of induced current. References 1.
complex
into the radio-
this
als, according
tears
fibrocartilage
with
that
the electrical continuity of the supporting frame of the device. Besides the problem of image quality, there is also a potential safety hazard that should be considered when using MR imaging in the presence of a halo vest. Specifically, there is a theoretical risk of inducing electrical current in the ring portion of any halo device that is made from conductive materi-
2.
3.
Malko JA, Hoffman JC Jr. Jarrett PJ. Eddy-current-induced artifacts caused by an “MR-compatible” halo device. Radiology 1989; 173:563-564. Ballock RT, Hajek PC, Byrne TP, Garfin SR. The quality of magnetic resonance imaging, as affected by the composition of the halo orthosis. J Bone Joint Surg [Am] 1989; 71:431-434. Shellock FG, Slimp C. MRI-cornpatib!e halo vest for cervical spine fixation. AJR (in press).
Dr Malko
responds:
The origin of all the induced eddy currents is, of course, ultimately described by Faraday’s law, as discussed by us in our reference 1 . However, this law concerns the generation of an electromotive force (EMF) (potential difference), while the actual current generated by this EMF depends on the circuit impedance, as described by Ohm’s law. Disrupting the circuit is one way to increase the impedance and effectively eliminate the current, thereby eliminating the artifact and any safety problem associated with the current (eg, heating); the device is then MR-compatible. However, from a practical standpoint it seems to us that, all other things being equal, it would be better to simply use a nonconducting device from the start and not have to worry about any possible failure of the impedance-increasing
mechanism;
lock and Slimp
appears
John
PhD
A. Malko,
Department 1364 Clifton
the
to accomplish
device
described
by
Shel-
this.
of Radiology, Emory University Road, NE, Atlanta, GA 30322
Hospital
151:1173-1174.
Erratum William
F. Conway,
Department Box
615,
MD,
PhD,
of Radiology, MCV
Station,
Medical
Eddy-Current-induced “MR-compatible” Frank
Section
of Magnetic
Medical 8700
VA
PhD,
and
Resonance
W.
Hayes,
MD
of Virginia
“Use
Gina
Imaging,
Caused Slimp,
of Barium
in Evaluation
Gastrointestinal
23298-0615
Artifacts Device
Halo
G. Shellock,
Curtis
College
Richmond,
U
From:
and
by an
of Disorders
Current
Status.”
of the Radiology
Upper
1989;
173:601-608 Page 605, the first sentence after Value of the Barium Examination
the section
head
“Diagnostic
Compared
with
Endoscopy”
should studies
three prospective, blinded value of a radiographic
read “In comparing
examination
RT
Tract:
published
the past 5 years, the diagnostic
and endoscopy
of the upper
GI tract have
been
(39,52,55).”
Cedars-Sinai
Center
Beverly
Boulevard,
Los
Angeles,
CA
90048
Editor:
The article by Malko et al (1), which appeared in the November 1989 issue of Radiology, described moderate-to-severe artifacts caused by eddy currents induced in the conducting frame of a halo device by the radio-frequency excitation field during
586
magnetic
Radiology
#{149}
resonance
(MR)
imaging.
The
authors
sug-
May
1990
Drs
and
Hayes
We thank Dr Hall of the newer data
for raising are currently
only recently become cy of three-compartment share
Dr
costs
that
Hall’s
some
an interesting in unpublished
aware
that wrist
concern
about
reason of the
that
we describe
Since most form, we have
about the still exists.
added
wrist
unpublished
point.
controversy arthrography
the
three-compartment
for this
gested
respond:
financial
and
arthrography
(with
the
efficaWe time
entail.
authors’
It is
permission)
results.
The group at the Mallinckrodt Institute of Radiology (Gilula LA, Wilson AJ, Mann FA, oral communication, December 1, 1989) examined 250 patients who had undergone threecompartment wrist arthrography and found 47 examples of “one-way communications” between the joints of the wrist. Forty-four of these would not have been detected with an isolated injection into the radiocarpal joint. The investigators followed a protocol similar to that suggested by Dr Hall, namely, maximal distention of the joints examined. Our experience (Conway WF, Hayes CW, unpublished material, December 1, 1989), although involving fewer patients (n 50), is qualitatively similar to the Mallinckrodt experience. In addition, with injection into the distal radioulnar joint,
we
on the
were
able
to demonstrate
undersurface
(TFCC)
of the
that were
two
triangular
not visualized
partial-thickness
injection
carpal joint. The group at the University of Utah (Manaster BJ, oral communication, December 1, 1989), however, had a different experience. Examining 50 three-compartment wrist arthrograms, they found a minimal number of false-negative results with respect to ligamentous and capsular damage when injection
into
the
radiocarpal
joint
was
examined
separately.
The
one exception to this finding was the detection of a significant number of partial-thickness undersurface tears of the TFCC during injection into the distal radioulnar joint. The Utah group
consider
that
since
their
surgeons
do
not
operate
on
such tears, there is limited value in their detection. Surgeons at our institution, however, have expressed interest in operating on well-documented partial-thickness tears of the TFCC. At present, we believe that the preponderance of information
(1,2)
(Gilula
et al,
Hayes, unpublished three-compartment that some questions Dr
Hall
in
vantages
personal
communication)
(Conway
and
material) argues for the added efficacy of wrist arthrography, although we admit still remain. For this reason, we join with
recommending
further
documentation
of three-compartment
wrist
of the
ad-
arthrography.
References 1. Levinsohn EM, Palmer AK, Coren AB, Zinberg E. Wrist arthrography: the value of the three compartment injection technique. Skeletal Radio! 1987; 16:539-544. 2. Quinn SF, Pittman CC, Belsole R, Greene T, Rayhack J. Digital subtraction wrist arthrography: evaluation of the multiple-cornpartment
technique.
AJR
1988;
problem
could
be eliminated
by disrupting
to Faraday’s
law
of electromagnetic
induction,
whereby a current can be induced in a closed-loop conductor as it moves through a magnetic field (2). In a soon-to-be-published article (3), we describe a commercially available truly MR-compatible halo device that is constructed
entirely
of nonferromagnetic
and
nonconductive
ma-
terials (Ambulatory Halo System; All Orthopedic Appliances, Division of Kirschner Medical, Greenwood, SC). The advantages of using this particular halo device in patients undergoing MR imaging are as follows: (a) there are no associated deflection forces, (b) heating is insignificant, (c) there are no imaging artifacts, (d) and there is no likelihood of induced current. References 1.
complex
into the radio-
this
als, according
tears
fibrocartilage
with
that
the electrical continuity of the supporting frame of the device. Besides the problem of image quality, there is also a potential safety hazard that should be considered when using MR imaging in the presence of a halo vest. Specifically, there is a theoretical risk of inducing electrical current in the ring portion of any halo device that is made from conductive materi-
2.
3.
Malko JA, Hoffman JC Jr. Jarrett PJ. Eddy-current-induced artifacts caused by an “MR-compatible” halo device. Radiology 1989; 173:563-564. Ballock RT, Hajek PC, Byrne TP, Garfin SR. The quality of magnetic resonance imaging, as affected by the composition of the halo orthosis. J Bone Joint Surg [Am] 1989; 71:431-434. Shellock FG, Slimp C. MRI-cornpatib!e halo vest for cervical spine fixation. AJR (in press).
Dr Malko
responds:
The origin of all the induced eddy currents is, of course, ultimately described by Faraday’s law, as discussed by us in our reference 1 . However, this law concerns the generation of an electromotive force (EMF) (potential difference), while the actual current generated by this EMF depends on the circuit impedance, as described by Ohm’s law. Disrupting the circuit is one way to increase the impedance and effectively eliminate the current, thereby eliminating the artifact and any safety problem associated with the current (eg, heating); the device is then MR-compatible. However, from a practical standpoint it seems to us that, all other things being equal, it would be better to simply use a nonconducting device from the start and not have to worry about any possible failure of the impedance-increasing
mechanism;
lock and Slimp
appears
John
PhD
A. Malko,
Department 1364 Clifton
the
to accomplish
device
described
by
Shel-
this.
of Radiology, Emory University Road, NE, Atlanta, GA 30322
Hospital
151:1173-1174.
Erratum William
F. Conway,
Department Box
615,
MD,
PhD,
of Radiology, MCV
Station,
Medical
Eddy-Current-induced “MR-compatible” Frank
Section
of Magnetic
Medical 8700
VA
PhD,
and
Resonance
W.
Hayes,
MD
of Virginia
“Use
Gina
Imaging,
Caused Slimp,
of Barium
in Evaluation
Gastrointestinal
23298-0615
Artifacts Device
Halo
G. Shellock,
Curtis
College
Richmond,
U
From:
and
by an
of Disorders
Current
Status.”
of the Radiology
Upper
1989;
173:601-608 Page 605, the first sentence after Value of the Barium Examination
the section
head
“Diagnostic
Compared
with
Endoscopy”
should studies
three prospective, blinded value of a radiographic
read “In comparing
examination
RT
Tract:
published
the past 5 years, the diagnostic
and endoscopy
of the upper
GI tract have
been
(39,52,55).”
Cedars-Sinai
Center
Beverly
Boulevard,
Los
Angeles,
CA
90048
Editor:
The article by Malko et al (1), which appeared in the November 1989 issue of Radiology, described moderate-to-severe artifacts caused by eddy currents induced in the conducting frame of a halo device by the radio-frequency excitation field during
586
magnetic
Radiology
#{149}
resonance
(MR)
imaging.
The
authors
sug-
May
1990
