J. Neurol. Neurosurg. Psychiat., 1967, 30, 159

Magnetic probe for the stereotactic thrombosis ofintracranial aneurysms 1"2

JOHN F. ALKSNE, AARON G. FINGERHUT, AND ROBERT W. RAND

From Harbor General Hospital, Torrance, California, and UCLA School of Medicine,Departments of Surgery (Divisions of Neurosurgery) and

Departments of Radiology

The development of a method for the stereotacticthrombosis of intracranial aneurysms should signific-antly reduce the morbidity and mortality associatedwith this disease. By removing the necessity forvisual exposure of the aneurysm, stereotactic surgeryovercomes the problems of brain retraction andmanipulation of blood vessels which constitute themajor limiting factors in current surgical therapy(McKissock, Richardson, and Walsh, 1962). More-over, because a stereotactic probe is inserted througha small burr hole, the operation can be performedunder local anaesthesia. Consequently, it may bepossible to institute stereotactic surgery immediatelyafter the patient is admitted to the hospital andthereby remove the risk of recurrent haemorrhagewhich accompanies the prevailing practice of delayedtreatment. The techniques of stereotaxis have beenadequately developed to allow accurate placing of aprobe against an intracranial aneurysm, but aneffective means of thrombosing aneurysms has notbeen available.We have previously reported a technique for

creating a focal intra-arterial thrombus by theattraction of intravascular iron powder to an extra-vascular magnet (Alksne and Fingerhut, 1965;Alksne, Fingerhut, and Rand, 1966a and b;Fingerhut and Alksne, 1966). On the basis of ourlaboratory experiments, we have developed a mag-netic stereotactic probe for clinical use which isinexpensive, easy to use, and effective. The con-:struction and use of this probe is the subject of thisreport.

SPECIFICATIONS

'Structurally, the most important consideration indesigning the magnetic probe was to reduce the probe'ssize to the minimum while maintaining its strength at'Supported in part by funds from general research grant U.S.P.H.S.no. 1-SO1-FR-0551-03-5 and U.S.P.H.S. research grant NB no.66441-01.'Constructed by Trenton H. Wells, Mechanical Developments-Company, Southgate, California.

the maximum. Functionally, the probe needed to providefor the injection of an iron suspension directly into theaneurysm. In addition, the necessity for three to five daysof chronic implantation required the magnetic componentto be detachable.

SIZE We have arbitrarily selected X inch as the maximumallowable diameter for the probe although it is possiblethat a larger probe could be tolerated in certain areas ofthe brain. Because the strength of a magnet variesdirectly with its total mass, we have used magneticmaterial for the entire intracranial portion of the probe.The length of the magnet, therefore, depends on thedistance from aneurysm to scalp.

DIRECT INJECTION We demonstrated in animal experi-ments that direct injection of iron suspension wasessential if aneurysms of 1 cm. or more in diameter wereto be totally thrombosed. Furthermore, in two patientson whom we operated before the development of thedirect injection technique, transient neurological deficitsdeveloped when an iron suspension was introduced intothe carotid artery. We have therefore concluded that allof the iron in all cases should be injected directly into theaneurysm using a needle which passes through thecentre of the probe.

DETACHABILITY Laboratory experiments indicate thatthe magnet should be in contact with the metallicthrombus for three to five days to ensure that the clotwill not be dislodged. During this time early organizationoccurs. The magnet, therefore, must be detachable sothat it can be disconnected from the extracranial stereo-tactic device and fixed rigidly to the skull. The varyingdistance from aneurysm to skull dictates that the detach-able magnets be available in lengths from 11 to 3 in.

MATERIALS

After pilot experiments, we selected Alnico 5 permanentmagnets as the material of choice. The magnets are bestdescribed by stating their dimensions rather than theirstrength because the common unit of physical measure,the gauss, does not adequately indicate the factor whichwe consider most important. The gauss reading of amagnetic material, which represents the number of lines

159

Protected by copyright.

on February 23, 2020 by guest.

http://jnnp.bmj.com

/J N

eurol Neurosurg P

sychiatry: first published as 10.1136/jnnp.30.2.159 on 1 April 1967. D

ownloaded from

John F. Alksne, Aaron G. Fingerhut, and Robert W. Rand

FIG. 1. The magnetic stereotactic probe is shown dis-assembled. Above is the magnet. Note that it is bluntedon one end for ease ofinsertion and threaded on the otherfor attachment to the non-magnetic extension which isalso shown.

The special needle can be seen to be a composite of ashort 27gauge needle soldered into an 8 in. 22 gauge needle.When the probe is assembled, the needle can be inserted sothat the 27 gauge portion protrudes beyond the end of themagnet.

of force per square centimetre, is greatest at the pole ofthe magnet. For creating metallic thrombi, however, thefield strength 1 cm. beyond the magnet is more important.The distant field can be enhanced by increasing thelength of a bar magnet although the gauss reading at thepole remains the same. This apparent inconsistency isexplained by the fact that the lines of force are projectedfarther in space by the longer magnet even though theyare not increased in number (Parker and Studders, 1962).

Cylindrical Alnico 5 permanent bar magnets Xin. indiameter and 1 to 3 in. in length form the basic unit ofthe probe. A conduit inserted in a slot down the core ofthe magnet allows a 22 gauge needle to be passed thefull length of the probe. One end of the magnet is bluntedfor ease of insertion. The other end is threaded forattachment to a non-magnetic extension which adaptsto the stereotactic device (Fig. 1). The magnetsshould be remagnetized before each use to ensure maxi-mum strength.The injection needles are constructed by inserting a

- in. segment of 27 gauge needle stock into a blunt, 8 in.22 gauge needle (Fig. 1). Needles fabricated from ferro-magnetic material facilitate the injection of iron suspen-sion through the magnetic field by shielding the lumen.Otherwise, the system tends to become a magnetic filterand retain the iron within the needle.

PROCEDURE

The location of the aneurysm is determined by angio-graphy. The patient is placed in the stereotactic device'with the dome of the aneurysm as the target point. The

'We have used the Rand-Wells stereotactic instrument.

FIG. 2. Lateral carotid angiogram showing the magneticprobe abutting an anterior communicating artery aneurysm.The probe has been inserted stereotactically through afrontal burr hole.

site for the burr hole is selected to allow the probe topass through the least important brain area.The magnetic probe is inserted under x-ray control

until the aneurysm is contacted (Fig. 2). The needle isthen inserted through the magnetic probe until it projects2 to 4 mm. into the aneurysm. Immediate injection of ironsuspension is begun.To facilitate slow, continuous injection, we have used

an automatic injector2 set at a flow rate of 0 5 ml. perminute. Our iron for injection consists of 1 g. of type SFCarbonyl iron powder3 in 10 ml. of 25% human serumalbumin.4 The suspension is mixed in the syringe andtubing every two minutes to prevent settling. This isaccomplished by placing a three-way stopcock at thejunction between tubing and needle which allows theentire contents to be agitated by barbotage.

Because the iron microspheres are radio-opaque, thedevelopment of the metallic thrombus can be followedon plain radiographs (Fig. 3). The total clot is about 250larger than the iron bolus due to natural surrounding-thrombus. Between 5 and 10 ml. of iron suspension mustbe injected for complete obliteration of an aneurysmI cm. in diameter. Repeat arteriograms must be performedto determine the actual size of the thrombus but it isimportant that the total amount of contrast agent usedis kept to a minimum.

After the aneurysm is obliterated, the magnet isdetached from the stereotactic device and fixed to theskull so that it remains abutting the aneurysm (Fig. 3).Two screws are placed at the margin of the burr hole.The screw heads and the threaded end of the magnet arethen encased in cranioplasty acrylic. As a result, themagnet is fixed rigidly to the skull but can be easilywithdrawn at any time by removal of the screws.

2Supplied by Trenton H. Wells, Mechanical Developments Co.,Southgate, Calif.3Supplied by General Analine and Folm Co., 435 Hudson St.. NewYork, N.Y.4Supplied by Cutter Laboratories, Berkeley, Calif.

R .!! m MI 'm

160

Protected by copyright.

on February 23, 2020 by guest.

http://jnnp.bmj.com

/J N

eurol Neurosurg P

sychiatry: first published as 10.1136/jnnp.30.2.159 on 1 April 1967. D

ownloaded from

Magnetic probe for the stereotactic thrombosis of intracranial aneurysms

FIG. 5. Lateral radiograph of the head, same patient as inFigs. 2 and 3, taken six weeks after aneurysm thrombosis.The magnets were removed on the sixth postoperative day.Note that the metallic thrombus is essentially unchanged.

FIG. 3. Lateral radiograph of the skull taken one daypost-operatively on the same patient whose angiogram isshown in Figure 2. Two probes were necessary to obtaincomplete thrombosis of the aneurysm. Note that themagnets now end flush with the skull. The screws can beseen which, together with the threaded ends of the magnets,are encased in acrylic to create a rigid support. The entireprobe is a permanent magnet.

The radio-opaque metallic thrombus can be seen in theaneurysm. Note that the iron does not completely fill theaneurysm. Nevertheless, repeat angiography failed to fillthe aneurysm indicating that the remainder was filled withradiolucent natural thrombus.

A. Irtracranr at<S\ arteriatt

D.0.

FIG. 4.

a \RMS !,,A&-, ,,99a2DJJ2 e-BM,I

FIG. 4. Drawing to show the results of our experimentswith laboratory models to determine the optimal magnetorientation. Comparing A and B one can see that ananeurysm is more readily filled with iron when both northpoles are approximated if the magnets approach each otherat an angle about 90°. Comparing C and D it is apparentthat the aneurysm is much more easily bridged with ironwhen a north and south pole are approximated if themagnets approach at 180°.

B.

Is LLLud/11111-111.1.1 .

161

Protected by copyright.

on February 23, 2020 by guest.

http://jnnp.bmj.com

/J N

eurol Neurosurg P

sychiatry: first published as 10.1136/jnnp.30.2.159 on 1 April 1967. D

ownloaded from

John F. Alksne, Aaron G. Fingerhut, and Robert W. Rand

DISCUSSION

Our clinical experience with the magnetic stereotacticprobe is not yet sufficient to make any statisticallyreliable statements about efficacy, morbidity, or

mortality. Nevertheless, the probe has been success-ful for creating a thrombus in every instanceattempted, including both laboratory and clinicalexperience. In our initial clinical cases we experienceddifficulty in completely thrombosing large aneurysms.Subsequently, we have found that the simultaneousinsertion of two probes through separate burr holesappears to overcome this limitation. We haveconcluded that any aneurysms greater than 1 cm.in any dimension will require more than one mag-

netic probe.When two probes are required, our experiments

with laboratory models indicate that the polarorientation of the magnets should be dependent on

their angle of intersection (Fig. 4). The magneticfield within the aneurysm will be strongest when likepoles are approximated if the magnets approach eachother at an angle of less than 1800. If the magnetsapproach each other at 1800, however, the magneticfield will be strongest if unlike poles are approxi-mated.Our laboratory experience indicates that a metallic

thrombus, once produced, yields permanent occlu-sion. The two clinical cases available for follow-upsupport this finding (Fig. 5). It will, of course, beimportant to have long-term studies on numerouscases before a final conclusion can be drawn.

In our seven cases we have experienced no difficultyin approaching the aneurysm stereotactically withthe magnetic probe or penetrating the aneurysmalfundus with the special needle. We have alwaysbegun the injection of iron suspension simultaneouslywith penetration of the aneurysm so that anytendency for leakage will be immediately arrested bythe metallic thrombus.

In an attempt to increase the magnetic field acrossthe aneurysm we are continuing experiments with

electromagnets and with permanent magnets using alarger mass external to the head. The major disad-vantage of both of these systems is that they arecumbersome for prolonged magnet contact. In anattempt to obtain better distribution of the magneticfield, we are also experimenting with an auxiliarytransnasal magnet placed in the sphenoid sinus.At the present time, the detachable permanent

magnet, adapted for needle insertion into theaneurysm, is the most effective means of creatingstereotactic metallic thrombosis. Utilizing a standardtechnique, we shall attempt to obtain a series ofcases to determine whether aneurysm thrombosiswith this probe provides a superior method oftherapy.

SUMMARY

The technique of magnetically controlled metallicthrombosis is briefly reviewed. Using Alnico 5permanent magnets, magnetic probes have beenconstructed for use in the stereotactic thrombosis ofintracranial aneurysms. The probes are describedand their application to the clinical situation isdiscussed.

We wish to express our appreciation to Mr. Jack Catchof Magnet Sales Co., Los Angeles, for his cooperationand advice in magnet selection and production.

REFERENCES

Alksne, J. F., and Fingerhut, A. G. (1965). Magnetically controlledmetallic thrombosis of intracranial aneurysms. Bull. LosAngeles neurol. Soc., 30, 153-155.

____ _9 and Rand, R. W. (1966a). Magnetically controlledmetallic thrombosis of intracranial aneurysm. Surgery, 60,212-218.

(1966b). Magnetically controlled focal intravascularthrombosis in dogs. J. Neurosurg., 25, 516-525.

Fingerhut, A. G., and Alksne, J. F. (1966). Thrombosis of intracranialaneurysms: An experimental approach utilizir'g magneticallycontrolled iron particles. Radiology, 86, 342-343.

McKissock, W., Richardson, A., and Walsh, L. (1962). Middle-cerebral aneurysms. Lancet, 2, 417-421.

Parker, R. F., and Studders, R. J. (1962). Permanent Magnets andTheir Application. John Wiley and Sons, New York.

162

Protected by copyright.

on February 23, 2020 by guest.

http://jnnp.bmj.com

/J N

eurol Neurosurg P

sychiatry: first published as 10.1136/jnnp.30.2.159 on 1 April 1967. D

ownloaded from