# N-localizer

Figure 1. Depiction of the N-localizer and its intersection with the computed tomography (CT) image plane. (A) Side view of the N-localizer. The CT image plane intersects two vertical rods and one diagonal rod. (B) CT image. The intersection of the CT image plane with the N-localizer creates two fiducial circles and one fiducial ellipse. The relative spacing between the ellipse and the two circles varies according to the height at which the CT image plane intersects the diagonal rod. Measuring this spacing permits calculation of the point where the CT image plane intersects the diagonal rod.
Figure 2. Three N-localizers are attached to a stereotactic instrument. The three N-localizers are placed around the hemi circumference of the stereotactic instrument. The instrument shown is the prototype that was built by Russell Brown in order to test the concept of the N-localizer.[1] The small spheres inside the instrument were used to simulate intracranial tumors. The vertical rod at the right rear of the instrument is larger in diameter than the other rods. This large rod facilitates unambiguous interpretation of the fiducial landmarks in a tomographic image.[2][3]
Figure 3. Depiction of three N-localizers and their intersection with the computed tomography (CT) image plane. The quadrilateral represents the CT image plane. The oval and the arch represent the stereotactic instrument. The vertical and diagonal lines that are attached to the oval represent the three N-localizers. The three points where the CT image plane intersects the diagonal rods are depicted by the dots. These points of intersection determine the spatial orientation of the CT image plane relative to the stereotactic instrument, and permit the transfer of patient information from the two-dimensional coordinate system of the planar image into the three-dimensional coordinate system of the stereotactic instrument.

The N-localizer is a device that enables guidance of stereotactic surgery or radiosurgery using tomographic images that are obtained via medical imaging technologies such as X-ray computed tomography (CT), magnetic resonance imaging (MRI) or positron emission tomography (PET).[4][5][6][7][8][9][10][11] The N-localizer comprises a diagonal rod that extends from the top of one vertical rod to the bottom of a second vertical rod, and is shaped like a capital "N" (Figure 1A). This device was invented in 1978 by the American physician and computer scientist Russell Brown.[12][13][14][15][16][17]

This invention stimulated renewed interest in, and further development of, image-guided surgery, specifically, stereotactic surgery and radiosurgery. It has achieved widespread clinical use in the Brown-Roberts-Wells (BRW),[18] Leksell[19] and Cosman-Roberts-Wells (CRW)[20] stereotactic systems and other stereotactic and radiosurgical instruments.

## How it works

Medical images that are obtained using technologies such as computed tomography (CT), magnetic resonance imaging (MRI) and positron emission tomography (PET) are two-dimensional, planar, tomographic images of patient anatomy. Treatment modalities such as stereotactic surgery and radiosurgery operate in the three-dimensional space of the patient. Hence, the central problem with using CT, MRI or PET to guide stereotactic surgery or radiosurgery is the transfer of patient information from the two-dimensional coordinate system of the planar CT, MRI or PET image into the three-dimensional coordinate system of the stereotactic or radiosurgical instrument. The N-localizer provides an elegant solution to this problem by creating fiducial marks or landmarks in each tomographic image. These fiducial marks specify the spatial orientation of the image plane with respect to the N-localizer (Figure 1). The N-localizer comprises a diagonal rod that extends from the top of one vertical rod to the bottom of a second vertical rod. This combination of rods creates two circles and one ellipse in a tomographic image. The ellipse moves away from one circle and towards the other circle as the image plane moves upward with respect to the N-localizer. Measuring the relative distances between the ellipse and the two circles permits calculation of the point where the image plane intersects the diagonal rod. The attachment of three N-localizers to a stereotactic instrument (Figure 2) permits calculation of three points where the image plane intersects the three diagonal rods (Figure 3). Because three points determine a plane in three-dimensional space, these three points of intersection determine the spatial orientation of the image plane relative to the stereotactic instrument. Therefore, the spatial orientation of any patient anatomy that is seen in the planar image is also determined relative to the stereotactic instrument. Because the spatial orientation of the patient anatomy is determined relative to the stereotactic instrument, patient information may be transferred from the two-dimensional coordinate system of the planar image into the three-dimensional coordinate system of the stereotactic instrument.[21]

## References

1. ^ Brown, Russell A. (June 1979). "A computerized tomography-computer graphics approach to stereotaxic localization". Journal of Neurosurgery 50 (6): 715–20. doi:10.3171/jns.1979.50.6.0715. PMID 374688.
2. ^ Brown RA, Roberts TS, Osborn AG (March–April 1981). "Simplified CT-guided stereotaxic biopsy". American Journal of Neuroradiology 2 (2): 181–184. PMID 6784559.
3. ^ Brown, Russell A. (October 2013). "The mathematics of the N-localizer for stereotactic neurosurgery". Cureus 5 (10): e142. doi:10.7759/cureus.142.
4. ^ Heilbrun MP, Roberts TS, Apuzzo ML, Wells TH Jr, Sabshin JK (August 1983). "Preliminary experience with Brown-Roberts-Wells (BRW) computerized tomography stereotaxic guidance system". Journal of Neurosurgery 59 (2): 217–222. doi:10.3171/jns.1983.59.2.0217. PMID 6345727.
5. ^ Thomas DG, Anderson RE, du Boulay GH (January 1984). "CT-guided stereotactic neurosurgery: experience in 24 cases with a new stereotactic system". Journal of Neurology, Neurosurgery & Psychiatry 47 (1): 9–16. doi:10.1136/jnnp.47.1.9. PMC 1027634. PMID 6363629.
6. ^ Leksell L, Leksell D, Schwebel J (January 1985). "Stereotaxis and nuclear magnetic resonance". Journal of Neurology, Neurosurgery & Psychiatry 48 (1): 14–18. doi:10.1136/jnnp.48.1.14. PMC 1028176. PMID 3882889.
7. ^ Thomas DG, Davis CH, Ingram S, Olney JS, Bydder GM, Young IR (January 1986). "Stereotaxic biopsy of the brain under MR imaging control". AJNR American Journal of Neuroradiology 7 (1): 161–163. PMID 3082131.
8. ^ Heilbrun MP, Sunderland PM, McDonald PR, Wells TH Jr, Cosman E, Ganz E (1987). "Brown-Roberts-Wells stereotactic frame modifications to accomplish magnetic resonance imaging guidance in three planes". Applied Neurophysiology 50 (1-6): 143–152. doi:10.1159/000100700. PMID 3329837.
9. ^ Maciunas RJ, Kessler RM, Maurer C, Mandava V, Watt G, Smith G (1992). "Positron emission tomography imaging-directed stereotactic neurosurgery". Stereotactic and Functional Neurosurgery 58 (1-4): 134–140. doi:10.1159/000098986. PMID 1439330.
10. ^ Levivier M, Massager N, Wikler D, Lorenzoni J, Ruiz S, Devriendt D, David P, Desmedt F, Simon S, Van Houtte P, Brotchi J, Goldman S (July 2004). "Use of stereotactic PET images in dosimetry planning of radiosurgery for brain tumors: clinical experience and proposed classification". Journal of Nuclear Medicine 45 (7): 1146–1154. PMID 15235060.
11. ^ Perry, Janice (October 2, 1983). "New system may improve brain surgery". Times Daily. Retrieved 19 May 2011.
12. ^ Brown RA, Nelson JA (2012). "Invention of the N-localizer for stereotactic neurosurgery and its use in the Brown-Roberts-Wells stereotactic frame". Neurosurgery 70 (2 Supplement Operative): 173–176. doi:10.1227/NEU.0b013e318246a4f7. PMID 22186842.
13. ^ Brown RA (December 2014). "The N-localizer and volume imaging". Cureus 6 (12): e232. doi:10.7759/cureus.232.
14. ^ Brown RA (October 2014). "The mathematics of three or more N-localizers for stereotactic neurosurgery". Cureus 6 (10): e218. doi:10.7759/cureus.218.
15. ^ Brown RA, Nelson JA (January 2014). "The history and mathematics of the N-localizer for stereotactic neurosurgery". Cureus 6 (1): e156. doi:10.7759/cureus.156.
16. ^ Brown RA, Nelson JA (September 2013). "The origin of the N-localizer for stereotactic neurosurgery". Cureus 5 (9): e140. doi:10.7759/cureus.140.
17. ^ US patent 4608977, Brown, RA, "System using computed tomography as for selective body treatment", issued September 2, 1986
18. ^ Brown RA, Roberts TS, Osborn AG (1980). "Stereotaxic frame and computer software for CT-directed neurosurgical localization". Investigative Radiology 15 (4): 308–12. doi:10.1097/00004424-198007000-00006. PMID 7009485.
19. ^ Leksell L, Jernberg B (1980). "Stereotaxis and tomography. A technical note.". Acta Neurochirugica 52 (1-2): 1–7. doi:10.1007/BF01400939. PMID 6990697.
20. ^ Couldwell WT, Apuzzo ML (1990). "Initial experience related to the Cosman-Roberts-Wells stereotactic instrument. Technical note". Journal of Neurosurgery 72 (1): 145–8. doi:10.3171/jns.1990.72.1.0145. PMID 2403588.
21. ^ Brown, Russell A. (July–August 1979). "A stereotactic head frame for use with CT body scanners". Investigative Radiology 14 (4): 300–4. doi:10.1097/00004424-197907000-00006. PMID 385549.