A Brief History and Summary of Stereoscopy in Medicine

written by Lindsay Cole, Canada

Introduction

In life, most humans see the world from two dis­tinct view­points: one from each eye.  The left eye receives an image slight­ly dif­fer­ent from the right eye because it is focus­ing on the object of inter­est from a slight­ly dif­fer­ent angle.  The brain uses these dif­fer­ences to per­ceive depth.

The term ‘Stere­oscopy’ refers to what is now more wide­ly known as 3D imag­ing, i.e., record­ing and pre­sent­ing visu­al infor­ma­tion in three dimen­sions.  When a stereo­scop­ic image is viewed prop­er­ly, the brain per­ceives depth as it would in the phys­i­cal world.  Thus, a flat paper or screen can car­ry spa­tial infor­ma­tion. This sim­ple con­cept can be adapt­ed into draw­ing, pho­tog­ra­phy and video for true 3D visualization. 

Charles Wheat­stone dis­cov­ered the con­cept of stere­oscopy and built the first view­ing devices in 1832¹.  By 1852 the Brew­ster-type stere­o­scope was pub­licly avail­able¹.  This hand­held device allowed the com­mon indi­vid­ual to view side-by-side (SBS) stereopho­tog­ra­phy for­mats and helped incite the great pop­u­lar­i­ty of stere­oscopy in the mid 19th cen­tu­ry¹.  Though this pop­u­lar­i­ty increased and decreased through­out the decades, the con­cept was con­tin­u­ous­ly applied to a vari­ety of prac­tices includ­ing art, film, pho­tog­ra­phy, his­to­ry, and medicine. 

In the present day a vari­ety of view­ing devices exist.  Polar­ized light pro­jec­tion and fil­tered glass­es allow both left and right eye images from one screen.  It is cur­rent­ly wide­ly used, and known to the gen­er­al pub­lic via 3D movies.  SBS stereo images can also be “free-viewed” by those indi­vid­u­als who have trained their eyes to diverge on the page: each eye focused on its cor­re­spond­ing image.  This easy access allows the visu­al­iza­tion of true 3D depth, as it is nat­u­ral­ly per­ceived by the brain, for a vari­ety of pur­pos­es with­out the need for advanced or expen­sive technologies.

Stereoscopy in Medical Education

The use of stere­oscopy for med­ical edu­ca­tion began in the ear­ly 20th cen­tu­ry.  Some sources indi­cate that Dr. Daniel John Cun­ning­ham was the first to use SBS stere­o­graphs to teach human anato­my^2,3.  In 1909 his work was com­piled into sev­er­al vol­umes of Stereo­scop­ic Stud­ies of Anato­my, even­tu­al­ly com­pris­ing the Edin­burgh Stereo­scop­ic Atlas of Anato­my^3,4.  By the mid-cen­tu­ry mark, a resur­gence in the pop­u­lar­i­ty of stereo­scop­ic view­ing sparked col­lab­o­ra­tion between anatomist Dr. David L. Bas­sett and William B. Gru­ber, inven­tor of the View­mas­ter sys­tem.  They cre­at­ed a col­lec­tion of stereo­scop­ic disks fea­tur­ing dis­sec­tions by Bas­sett and pho­tographs by Gru­ber: A Stereo­scop­ic Atlas of Human Anato­my⁴.

Stereo­scop­ic anatom­i­cal teach­ing con­tin­ues to be of use today.  The reten­tion of knowl­edge in med­ical stu­dent groups shown 3D stereo­scop­ic videos is greater than that of groups shown 2D videos^2,6.  This result is sig­nif­i­cant in anatom­i­cal rela­tion­ships, while func­tion­al knowl­edge appears unaf­fect­ed².  The strength of this learn­ing impact is still unknown.  Some stud­ies deter­mined no sig­nif­i­cant dif­fer­ence in writ­ten test scores between 3D and 2D video learn­ing⁷.  These incon­clu­sive results are poten­tial­ly due to the type (ie. func­tion­al vers­es spa­tial) of ques­tions test­ed.  The ben­e­fit of 3D learn­ing varies from per­son to per­son.  A large pre­dic­tor of a student’s suc­cess in anatom­ic learn­ing is visu­ospa­tial abil­i­ty; stereo­scop­ic learn­ing aids may be more ben­e­fi­cial for those with low­er nat­ur­al visu­ospa­tial scores^6,7,8.  It effec­tive­ly evens the play­ing field.  Unfor­tu­nate­ly, though many med­ical schools cur­rent­ly give stu­dents access to 3D com­put­er mod­els, they are dis­played or pro­ject­ed onto flat screens, and thus lose the actu­al stereo­scop­ic ben­e­fit^6,8.  Mod­ern teach­ing pro­grams with true stereo­scop­ic pro­jec­tion of CT vas­cu­lar mod­els have been intro­duced through polar­ized pro­jec­tion^7,9.  When intro­duced to stereo­scop­ic tech­niques of learn­ing, stu­dents find it use­ful and are inter­est­ed in using it in the future^6,10.

Figure 1: Stereoscopic teaching tools throughout the decades (1905–1960).

In the 21st cen­tu­ry, stereo­scop­ic teach­ing has been thor­ough­ly mod­ern­ized and adapt­ed, sig­nif­i­cant­ly in the field of neu­ro­surgery, where many prac­tice-hours are need­ed despite lim­it­ed avail­abil­i­ty of patients and OR time¹⁰.  Vir­tu­al real­i­ty (VR) sim­u­la­tions in 3D space allow res­i­dents to gain expe­ri­ence in the field with­out risk to patients¹⁰.  These can be both hap­tic (touch sim­u­la­tion) and non-hap­tic, and stud­ies have shown them to be sig­nif­i­cant­ly ben­e­fi­cial com­pared to con­trol groups with­out VR, though some uncer­tain­ty exists^11,12,13.  The hepat­ic por­tion of the sim­u­la­tion train­ing may not con­tribute sig­nif­i­cant­ly to the ben­e­fit, sug­gest­ing that the improved learn­ing out­comes is due to the stereo­scop­ic VR¹³.  Stereo­scop­ic mod­els are also used for learn­ing pur­pos­es in oth­er spe­cial­i­ties¹⁰.

Stereoscopy in Surgery 

Stereo­scop­ic tech­niques in surgery are not sim­ply lim­it­ed to train­ing and sim­u­la­tion.  As ear­ly as 1922, sur­geons had adapt­ed the knowl­edge of stere­op­sis for use in the OR¹⁵.  Gun­nar Holm­gren retained depth when mag­ni­fi­ca­tion was required by first using binoc­u­lar micro­scopes at the Uni­ver­si­ty Clin­ic of Stock­holm¹⁵.  This tech­nique was quick­ly employed world­wide and had espe­cial ben­e­fit in the small surg­eries of the head and neck¹⁵.

Stere­oscopy is also a use­ful phe­nom­e­non in laparo­scop­ic surg­eries.  Tra­di­tion­al­ly, the laparo­scope video is dis­played on a 2D mon­i­tor and there­fore lacks poten­tial­ly cru­cial depth infor­ma­tion.  Stereo­scop­ic laparo­scopes allow true 3D rela­tion­ships to be deter­mined with­out open­ing the body cav­i­ty to human eyes¹⁶.  This is done sim­ply by includ­ing two visu­al view­points and then view­ing them via any stereo­scop­ic method.  As min­i­mal­ly inva­sive surg­eries, and there­fore laparoscopy, increased in com­plex­i­ty, the dif­fi­cul­ties of per­form­ing pro­ce­dures with­out depth became appar­ent¹⁷.  In the ear­ly 1990s stereo­scop­ic laparoscopy was used for the first time on human patients: a laparo­scop­ic chole­cys­tec­to­my per­formed at Nuclear Research Cen­ter Karl­sruhe¹⁷.  The incor­po­ra­tion of stere­oscopy in laparo­scop­ic pro­ce­dures increas­es the ease of tasks that require 3D visu­al­iza­tion such as organ mobi­liza­tion and sutur­ing¹⁷.  A 2017 lit­er­a­ture review by Schwab et al. col­lect­ed objec­tive and sub­jec­tive infor­ma­tion from patients and sur­geons using 2D vs 3D laparo­scop­ic sys­tems dur­ing chole­cys­tec­to­my oper­a­tions¹⁸.  They found that there were no increased neg­a­tive out­comes in either method, and that many sur­geons per­ceived bet­ter depth per­cep­tion and con­trol with the 3D sys­tems¹⁸.  Stereo­scop­ic laparoscopy can also be com­bined with laparo­scop­ic ultra­sound for aug­ment­ed real­i­ty views of inter­nal struc­tures¹⁶. 

Image-guid­ed surgery (IGS) is wide­ly used in a vari­ety of fields¹⁴.  IGS allows a 3D sur­gi­cal plan to be devel­oped on a 3D mod­el of the patient which is cre­at­ed from pre­op­er­a­tive imag­ing¹⁴.  3D mod­els are also used for pre-plan­ning, head frame place­ment and to deter­mine acces­si­bil­i­ty before radio­surgery¹⁹.  This short­ens pro­ce­dure times, opti­mizes patient expe­ri­ence and decreas­es repeat pro­ce­dures¹⁹.  Unfor­tu­nate­ly, most of these mod­els are viewed on a 2D mon­i­tor, elim­i­nat­ing the ben­e­fit of true stereo­scop­ic effect¹⁴.  It would be ben­e­fi­cial and fea­si­ble to include stereo­scop­ic view­ing through a polar­ized dis­play¹⁴.

3D imag­ing can be per­formed intra­op­er­a­tive­ly with­in the OR through stereo­scop­ic record­ing and polar­iza­tion¹⁴.  These images may be com­bined with the pre­op­er­a­tive mod­els for intra­op­er­a­tive visu­al­iza­tion¹⁴.  The ease of incor­po­rat­ing stere­oscopy into surgery increas­es as tech­nol­o­gy devel­ops.  Some­thing as com­mon­ly avail­able as a smart­phone can be adapt­ed into a view­ing device²⁰.  Suc­cess­ful robot-assist­ed oph­thalmic surgery sim­u­la­tions have been per­formed by attach­ing a stereo­scop­ic cam­era to a binoc­u­lar sur­gi­cal micro­scope and dis­play­ing the resul­tant image on a smart­phone VR head­set²⁰.

High-def­i­n­i­tion stereo­scop­ic 3D imag­ing in real-time is cru­cial in the emerg­ing field of telesurgery.  With ultra-fast 5G inter­net con­nec­tion it is pos­si­ble to per­form robot-assist­ed laparo­scop­ic surg­eries thou­sands of kilo­me­tres from the patient site^21,22.  These meth­ods, though still new, could poten­tial­ly com­bat sur­geon short­ages, remote access prob­lems, and dis­ease spread²¹.

Figure 2: An example of telesurgery technologies.

Stereoscopy in Ophthalmology

Stere­oscopy is an impor­tant part of vision.  Screen­ing and test­ing for stereo­scop­ic acu­ity can help diag­nose a vari­ety of ocu­lar con­di­tions includ­ing stra­bis­mus and ambly­opia²³.  There are sev­er­al stereo­scop­ic tests rang­ing in speci­fici­ty and ease of use for the appro­pri­ate patient case and physi­cian concern. 

In 1960 Bela Julesz used ran­dom-dot stere­o­cards to test for stereo­scop­ic acu­ity²³.  These ran­dom pat­terns of dots appear flat and unin­ter­est­ing when viewed monoc­u­lar­ly, but when the test sub­ject uses intact binoc­u­lar vision through a stereo­scop­ic view­ing device, shapes emerge in the depth²³.  The TNO stereotest uti­lizes the same con­cept and tests most­ly for ambly­opia²³.  It is viewed in red/blue anaglyph and con­sists of recog­nis­able images instead of depth clues²³.  TNO tests are designed so that when view­ing monoc­u­lar­ly, an incor­rect image is still vis­i­ble²³.  A quan­ti­ta­tive aspect is also added²³.  The Lang test is sim­i­lar again, but uses a pano­graph­ic view­ing tech­nique where no spe­cial glass­es are required²³.  The most com­mon stereotest is the Tit­mus test, in which raised shapes and objects are detect­ed when viewed through polar­ized glass­es²³.

Lang’s two pen­cil test, first described in 1983, removes the need for any spe­cial­ized stere­ode­vice²⁴.  It requires that the patient cor­rect­ly line up a pen­cil with one held by the exam­in­er; binoc­u­lar per­for­mance is com­pared against monoc­u­lar in each eye^23,24,25.  This is a qual­i­ta­tive test and is not used for diag­no­sis, but has been shown to dis­tin­guish gross stra­bis­mus at a high sen­si­tiv­i­ty and speci­fici­ty com­pared to ran­dom-dot and TNO with a neg­a­tive pre­dic­tive val­ue of 100%²⁵.

Figure 3: Stereoscopic acuity tests.

Stere­oscopy is not only used to test for ocu­lar con­di­tions; it’s also used to treat them.  Non-sur­gi­cal treat­ments of eye move­ment and ocu­lar mus­cle dis­or­ders such as stra­bis­mus are known as orthop­tics: these include stereo­scop­ic exer­cis­es.  Dr. Louis Javal intro­duced orthop­tic treat­ment for stra­bis­mus in the late 19^th cen­tu­ry, using ear­ly Wheat­stone stere­o­scopes to induce prop­er binoc­u­lar focus²⁶.  The binoc­u­lar train­ing pro­vid­ed by these treat­ments is used in replace­ment of and to sup­port surgery²⁶.  The over-reliance and mis­use of ear­ly orthop­tics dam­aged their ther­a­peu­tic rep­u­ta­tion around the turn of the cen­tu­ry, but treat­ments were mod­ern­ized in 1919 by doc­tors E. E. and M. C. Mad­dox and even­tu­al­ly re-pop­u­lar­ized their use²⁶.  M. C. Mad­dox cre­at­ed stere­o­cards for the amblyoscope (lat­er the syn­op­tophore), a reflect­ing stere­o­scope-like device that allows for indi­vid­ual eye stim­u­la­tion with lights, diver­gence angle mea­sure­ment, and deter­mi­na­tion of the area of sup­pres­sion²⁷.

In 1927 Dr. Carl Sat­tler pub­lished the first set of orthop­tic stere­o­cards for stra­bis­mus diag­no­sis and treat­ment that were avail­able for use at home²⁸.  They were inex­pen­sive and avail­able for par­ents to pur­chase for their chil­dren²⁸.  Oth­er sim­i­lar sets were soon avail­able and wide­ly used until the 1950s²⁸.

Figure 4: Orthoptic stereocards for strabismus diagnosis and treatment.

The ben­e­fit of orthop­tic exer­cis­es con­tin­ues to be of con­tro­ver­sy today.  Oph­thal­mol­o­gists dis­pute whether lengthy exer­cis­es sup­port marked ther­a­peu­tic improve­ment²⁶.  Stud­ies have shown that stereo­scop­ic orthop­tic train­ing for only two weeks (15 mins twice a day) mild­ly increas­es the angle of fusion in stra­bis­mus patients and decreas­es the required pris­mat­ic cor­rec­tion²⁹.  Longer train­ing can lead to greater improve­ment and the remis­sion of tropia²⁹.  These ben­e­fits depend on the individual’s moti­va­tion, time com­mit­ment, and the nature of their pre­sent­ing stra­bis­mus^29,30.  Exode­vi­a­tion seems to ben­e­fit more than esode­vi­a­tion, while ver­ti­cal devi­a­tion shows no effect²⁹.  The out­come of orthop­tic train­ing depends large­ly on the sever­i­ty of stra­bis­mus pri­or to start­ing ther­a­py: bet­ter results cor­re­late with milder cas­es³⁰.  Non-sur­gi­cal treat­ments are there­fore reserved for mild or small angle stra­bis­mus^26,29,30.  They can­not over­come large dis­par­i­ties that would ben­e­fit from surgery²⁶.  These inher­ent lim­i­ta­tions of orthop­tics are often mis­un­der­stood, lead­ing to mis­ap­pli­ca­tion of orthop­tic ther­a­pies, poor out­comes and decreased clin­i­cal sup­port²⁶.

Stere­oscopy can also be used in screen­ing for glau­co­ma³¹.  Ocu­lar fun­dus images are often ana­lyzed for diag­nos­tic char­ac­ter­is­tics of the optic nerve head; this is espe­cial­ly true for nor­moten­sive glau­co­ma that can­not be detect­ed with a tonome­ter³¹.  Impor­tant fea­tures such as the cup depth, disc shape and cup to disc ratio are used in screen­ing^31,32.  Cor­rect diag­no­sis of glau­co­ma based on these fea­tures is aid­ed by stereo­scop­ic analy­sis^31,32,33,34.  Oph­thal­mol­o­gists can view these stereo­scop­ic images either dig­i­tal­ly or on film³³.  When com­par­ing glau­co­ma detec­tion using stereo­scop­ic ver­sus mono­scop­ic images, the stereo­scop­ic screen­ing detects cas­es at increased sen­si­tiv­i­ty and repro­d­u­ca­bil­i­ty³².  Stereo­scop­ic images more accu­rate­ly indi­cate glau­co­ma patients that are near­er the cup to disc ratio thresh­old³².  Fur­ther­more, com­put­er-aid­ed machine-learn­ing detec­tion meth­ods that focus on ana­lyz­ing stereo­scop­ic images of the ocu­lar fun­dus out­per­form those that rely only on a sin­gle image with no depth infor­ma­tion^31,34.

Conclusion

This is by no means a com­pre­hen­sive col­lec­tion of the uses of stere­oscopy in med­i­cine.  The increased infor­ma­tion of 3D con­tributes ben­e­fits to many areas of imag­ing not men­tioned above, such as screen­ing mam­mog­ra­phy, echocar­dio­g­ra­phy, bron­choscopy and scintig­ra­phy^35,36,37,38.  Humans exist in and process the phys­i­cal world in three dimen­sions; it is no sur­prise that the best med­ical obser­va­tions also include depth information. 

Stere­oscopy remains a long-stud­ied and under-used tech­nol­o­gy in med­i­cine.  It is used wide­ly in some spe­cial­i­ties, includ­ing anato­my, neu­ro­surgery, laparo­scop­ic surgery, imag­ing and oph­thal­mol­o­gy, but remains a rel­a­tive­ly unknown con­cept in the gen­er­al med­ical and non-med­ical pop­u­la­tion.  Com­mon crit­i­cisms of stere­oscopy in all fields include the need for addi­tion­al view­ing devices, glass­es or tools^10,23,32.  While this is poten­tial­ly true, nov­el and inex­pen­sive stereo­scop­ic devices are avail­able in the major­i­ty of cas­es^20,28.

The pop­u­lar­i­ty of stere­oscopy has ebbed and flowed over the many decades since Charles Wheat­stone first described its prin­ci­ple¹.  The extent of its use in med­i­cine has fol­lowed a sim­i­lar process.  The cur­rent progress in com­put­er tech­nol­o­gy allows for increased con­sump­tion of mod­ern 3D movies, and increased inno­va­tion in the med­ical field.  We exist in a cur­rent stereo­scop­ic boom.  There are more oppor­tu­ni­ties for stere­oscopy in vir­tu­al care and remote pro­ce­dures^21,22.  More 3D tools are avail­able to med­ical stu­dents^6,8.  For record­ed depth infor­ma­tion to be of ben­e­fit, 3D data need to be viewed not from flat screens, but from stereo­scop­ic view­ers.  Only in this way is the patient tru­ly represented.

References

1. Pel­lerin D. Stere­oscopy: The Dawn of 3D. May BH, edi­tor. Lon­don: Lon­don Stereo­scop­ic Com­pa­ny; 2021. 
2. Bernard F, Richard P, Kahn A, Fournier HD. Does 3D stere­oscopy sup­port anatom­i­cal edu­ca­tion? Surg Radi­ol Anat. 2020 Jul;42(7):843–852. doi: 10.1007/s00276-020–02465‑z. Epub 2020 Apr 4. PMID: 32248256.
3. Cun­ning­ham DJ. Stereo­scop­ic stud­ies of anato­my, vol 3. New York: Impe­r­i­al Pub­lish­ing Com­pa­ny; 1911.
4. Cun­ning­ham DJ. Stereo­scop­ic stud­ies of anato­my, vol 2. New York: Impe­r­i­al Pub­lish­ing Com­pa­ny; 1909.
5. Bas­sett DL. A Stereo­scop­ic Atlas of Human Anato­my. Port­land, Ore­gon: Sawyer’s Inc.; 1962.
6. Cui D, Wil­son TD, Rock­hold RW, Lehman MN, Lynch JC. Eval­u­a­tion of the effec­tive­ness of 3D vas­cu­lar stereo­scop­ic mod­els in anato­my instruc­tion for first year med­ical stu­dents. Anatom­i­cal Sci­ences Edu. 2016 Jun;10(1):34–45. doi: 10.1002/ase.1626
7. Goodarzi A, Mon­ti S, Lee D, Gir­gis F. Effect of Stereo­scop­ic Anaglyph­ic 3‑Dimensional Video Didac­tics on Learn­ing Neu­roanato­my. World Neu­ro­surg. 2017 Nov;107:35–39. doi: 10.1016/j.wneu.2017.07.119. Epub 2017 Jul 29. PMID: 28765017.
8. Luurse­ma JM, Ver­wey WB, Kom­mers PAM, Annema JH. The role of stere­op­sis in vir­tu­al anatom­i­cal learn­ing. Inter­act­ing with Com­put­ers. 2008 Sep;20(4–5):455–460. doi: 10.1016/j.intcom.2008.04.003
9. Cui D, Lynch JC, Smith AD, Wil­son TD, Lehman MN. Stereo­scop­ic vas­cu­lar mod­els of the head and neck: A com­put­ed tomog­ra­phy angiog­ra­phy visu­al­iza­tion. Anat Sci Educ. 2016 Mar-Apr;9(2):179–85. doi: 10.1002/ase.1537. Epub 2015 Apr 30. PMID: 25929248.
10. Jacques­son T, Simon E, Dauleac C, Mar­gueron L, Robin­son P, Mertens P. Stereo­scop­ic three-dimen­sion­al visu­al­iza­tion: inter­est for neu­roanato­my teach­ing in med­ical school. Surg Radi­ol Anat. 2020 Jun;42(6):719–727. doi: 10.1007/s00276-020–02442‑6. Epub 2020 Feb 29. PMID: 32114650.
11. Rol­land JP, Wright DL, Kancher­la AR. Towards a nov­el aug­ment­ed-real­i­ty tool to visu­al­ize dynam­ic 3‑D anato­my. Stud Health Tech­nol Inform. 1997;39:337–48. PMID: 10168929.
12. Unger B, Tor­don B, Pisa J, Hochman JB. Impor­tance of Stere­oscopy in Hap­tic Train­ing of Novice Tem­po­ral Bone Surgery. Stud Health Tech­nol Inform. 2016;220:439–45. PMID: 27046619.
13. Erolin C, Lamb C, Soames R, Wilkin­son C. Does Vir­tu­al Hap­tic Dis­sec­tion Improve Stu­dent Learn­ing? A Mul­ti-Year Com­par­a­tive Study. Stud Health Tech­nol Inform. 2016;220:110–7. PMID: 27046562.
14. Christo­pher LA, William AMS, Cohen-Gadol AA. Future Direc­tions in 3‑Dimensional Imag­ing and Neu­ro­surgery Stere­oscopy and Autostere­oscopy. Neu­ro­surg. 2013;72:A131-A138. doi: 10.1227/NEU.0b013e318270d9c0
15. Uluç K, Kujoth GC, Başkaya MK. Oper­at­ing micro­scopes: past, present, and future. Neu­ro­surg Focus. 2009;27:E4. doi: 10.3171/2009.6.FOCUS09120
16. Kang X, Aziz­ian M, Wil­son E, Wu K, Mar­tin AD, Kane TD, Peters CA, Cleary K, Shekhar R. Stereo­scop­ic aug­ment­ed real­i­ty for laparo­scop­ic surgery. Surg Endosc. 2014 Jul;28(7):2227–35. doi: 10.1007/s00464-014‑3433‑x. Epub 2014 Feb 1. PMID: 24488352.
17. Beck­er H, Melz­er A, Schurr MO, Buess G. 3‑D video tech­niques in endo­scop­ic surgery. Endosc Surg Allied Tech­nol. 1993 Feb;1(1):40–6. PMID: 8050009.
18. Schwab K, Smith R, Brown V, Whyte M, Jour­dan I. Evo­lu­tion of stereo­scop­ic imag­ing in surgery and recent advances. World J Gas­troin­test Endosc. 2017 Aug 16;9(8):368–377. doi: 10.4253/wjge.v9.i8.368. PMID: 28874957; PMCID: PMC5565502.
19. Ford E, Purg­er D, Trygges­tad E, McNutt T, Christodouleas J, Rig­a­mon­ti D, Shokek O, Won S, Zhou J, Lim M, Wong J, Klein­berg L. A vir­tu­al frame sys­tem for stereo­tac­tic radio­surgery plan­ning. Int J Radi­at Oncol Biol Phys. 2008 Nov 15;72(4):1244–9. doi: 10.1016/j.ijrobp.2008.06.1934. PMID: 18954719.
20. Ho DK. Using smart­phone-deliv­ered stereo­scop­ic vision in micro­surgery: a fea­si­bil­i­ty study. Eye (Lond). 2019 Jun;33(6):953–956. doi: 10.1038/s41433-019‑0356‑8. Epub 2019 Feb 12. PMID: 30755728; PMCID: PMC6707160.
21. Mohan A, Wara UU, Arshad Shaikh MT, Rah­man RM, Zai­di ZA. Telesurgery and Robot­ics: An Improved and Effi­cient Era. Cureus. 2021 Mar 26;13(3):e14124. doi: 10.7759/cureus.14124. PMID: 33927932; PMCID: PMC8075759.
22. Zheng J, Wang Y, Zhang J, Guo W, Yang X, Luo L, Jiao W, Hu X, Yu Z, Wang C, Zhu L, Yang Z, Zhang M, Xie F, Jia Y, Li B, Li Z, Dong Q, Niu H. 5G ultra-remote robot-assist­ed laparo­scop­ic surgery in Chi­na. Surg Endosc. 2020 Nov;34(11):5172–5180. doi: 10.1007/s00464-020–07823‑x. Epub 2020 Jul 22. PMID: 32700149.
23. Pat­eras E, Plak­it­si A, Chatzi­pan­telis A. Stereo­scop­ic Vision & Test­ing Tech­niques – Overview. Bio­med. J. Sci­en­tif­ic and Tech­ni­cal Res. 2020 Mar;26(5):20252–20260. doi: 10.26717/BJSTR.2020.26.004405
24. Lang J. Der Tre­f­fver­such zur Prü­fung des Stere­ose­hens [The two-pen­cil test for test­ing stere­op­sis]. Klin Mon­bl Augen­heilkd. 1983 Jun;182(6):576–81. Ger­man. doi: 10.1055/s‑2008–1054858. PMID: 6876657.
25. Nong­puir ME, Shar­ma P. Hor­i­zon­tal Lang two-pen­cil test as a screen­ing test for stere­op­sis and binoc­u­lar­i­ty. Indi­an Jour­nal of Oph­thal­mol­o­gy 58(4):287–90. doi: 10.4103/0301–4738.64125
26. Parks MM. Stra­bis­mus care: Past, present and future. Doc Opthal­mol. 1973 Feb;34:301–315. doi: 10.1007/BF00151817
27. Syn­op­tophore (Major Amblyoscope) Infor­ma­tion Guide. HS Clement Clarke Opthalmic. Har­low, Essex: Haag-Stre­it UK; Nov 2014.
28. Lan­deck­er H. What do you see?. Hid­den Trea­sure: The Nation­al Library of Med­i­cine. New York: Blast Books; 2011
29. Borowiec-Woj­tanows­ka A, Bara­nows­ka-George T. Lecze­nie chorych z małym katem zeza ćwiczeni­a­mi na telestere­oskopie Starkiewicza i syn­opto­forze [Treat­ment of patients with small angle squint by exer­cis­ing with Starkiewicz’s tele-stere­oscopy and syn­op­tophore]. Klin Ocz­na. 1994 Jun-Jul;96(6–7):197–9. Pol­ish. PMID: 7897972.
30. Goldrich SG. Opto­met­ric ther­a­py of diver­gence excess stra­bis­mus. Am J Optom Phys­i­ol Opt. 1980 Jan;57(1):7–14. doi: 10.1097/00006324–198001000-00002. PMID: 7377279.
31. Liu Y, Yip LWL, Zheng Y, Wang L. Glau­co­ma screen­ing using an atten­tion-guid­ed stereo ensem­ble net­work. Meth­ods. 2021 Jun 19:S1046-2023(21)00162–6. doi: 10.1016/j.ymeth.2021.06.010. Epub ahead of print. PMID: 34153436.
32. Shrestha R, Budenz DL, Mwan­za JC, Tulenko SE, Fleis­chman D, Gow­er EW. Com­par­i­son of ver­ti­cal cup-to-disc ratio esti­mates using stereo­scop­ic and mono­scop­ic cam­eras. Eye (Lond). 2021 Dec;35(12):3318–3324. doi: 10.1038/s41433-021–01395‑3. Epub 2021 Jan 29. PMID: 33514892; PMCID: PMC8602644.
33. Hasan­reisoglu M, Priel E, Naveh L, Lusky M, Wein­berg­er D, Ben­jami­ni Y, Gaton DD. Dig­i­tal ver­sus film stereo-pho­tog­ra­phy for assess­ment of the optic nerve head in glau­co­ma and glau­co­ma sus­pect patients. J Glau­co­ma. 2013 Mar;22(3):238–42. doi: 10.1097/IJG.0b013e31823298da. PMID: 21946551.
34. Norouz­i­fard M, Daw­da A, Abdul-Rah­man A, Gho­lamHos­sei­ni H, Klette R. Super­pix­el seg­men­taion meth­ods on stereo fun­dus images and dis­par­i­ty map for glau­co­ma detec­tion. 2018 Inter­na­tion­al Con­fer­ence on Image and Vision Com­put­ing New Zealand (IVCNZ). 2018 Nov:1–6. doi: 10.1109/IVCNZ.2018.8634732.
35. Ferre R, Goumot PA, Mesurolle B. Stereo­scop­ic dig­i­tal mam­mo­gram: Use­ful­ness in dai­ly prac­tice. J Gynecol Obstet Hum Reprod. 2018 Jun;47(6):231–236. doi: 10.1016/j.jogoh.2018.03.009. Epub 2018 Apr 3. PMID: 29621618.
36. Harake D, Gnanap­pa GK, Alvarez SGV, Whit­tle A, Punithaku­mar K, Boech­ler P, Noga M, Khoo NS. Stereo­scop­ic Dis­play Is Supe­ri­or to Con­ven­tion­al Dis­play for Three-Dimen­sion­al Echocar­dio­g­ra­phy of Con­gen­i­tal Heart Anato­my. J Am Soc Echocar­dio­gr. 2020 Nov;33(11):1297–1305. doi: 10.1016/j.echo.2020.06.016. Epub 2020 Sep 9. PMID: 32919855.
37. Nobuya­ma S, Sato T, Han­da H, Nishine H, Inoue T, Mineshi­ta M, Miyaza­wa T. Com­par­i­son of Air­way Mea­sure­ments for Tra­cheo­bronchial Steno­sis Between Stereo­scop­ic Bron­cho­scope and MD-CT. J Bron­chol­o­gy Interv Pul­monol. 2017 Oct;24(4):296–302. doi: 10.1097/LBR.0000000000000409. PMID: 28957890.
38. Tana­ka C, Fujii H, Ike­da T, Jin­no H, Naka­hara T, Suzu­ki T, Kita­gawa Y, Kita­ji­ma M, Ando Y, Kubo A. Stereo­scop­ic scinti­graph­ic imag­ing of breast can­cer sen­tinel lymph nodes. Breast Can­cer. 2007;14(1):92–9. doi: 10.2325/jbcs.14.92. PMID: 17245002.