What this page shows, in plain English

Tap any heading to expand it.

The big chart

Imagine slicing your nose into thin vertical slices, like a loaf of bread, from the tip backwards to your throat. For each slice we measured how much open space there is for air to pass. Left-to-right on the chart is how far back in your nose you are, in millimetres. Up-and-down is how much open space there is at that point, in square millimetres — a bigger number means more room.

Why there are two lines

Your nose is two separate passages, divided by a wall down the middle called the septum. One line is the left passage, one is the right. Where a line dips low, that is a tight spot.

The marked low points

The narrowest point found on each side. One of them is labelled “at boundary” — that means it sits right at the edge of where we chose to start measuring, so it is an artefact of where the line was drawn rather than a real narrowing. It should be ignored.

The pictures beside the chart

Cross-sections through your head at whichever point of the chart you are pointing at, so you can see the actual anatomy each number came from. The coloured areas are the air that was measured — a different colour for each side.

The summary numbers

Each one carries a range, not just a single figure. The result depends on exactly where you draw the line between “this pixel is air” and “this pixel is tissue”. We ran the whole thing three times with three different settings, and the range shows how much the answer moved. Numbers that move a lot are the ones to trust least.

The segmentation check picture

The boring but important one. It shows exactly which pixels the computer decided were airway, painted on top of the real scan. If that highlighting had leaked out into the room air around the head, or wandered into the sinuses, every number on this page would be wrong — so this picture is the proof that it did not.

Two things this page cannot tell you. First, your nose naturally swaps which side is more congested every few hours — this is normal and is called the nasal cycle. This scan is a single photograph of something that moves, so a difference between the sides here is not proof of a permanent blockage. Second, open space is not the same as how easily air actually flows through. This is a measurement of shape, not of breathing.

Nasal airway cross-sectional area — left vs right

GE Revolution Maxima · PNS-ORBIT-TMJ helical · Series 3 (bone kernel, 0.588 mm px, 0.625 mm slices) · measured on the axial volume (the shipped coronal/sagittal reformats have uneven spacing and were not used)

Minimum area — LEFT

77 mm²

at 0 mm behind the nostril-seal plane — this is the anterior cut plane itself: the curve only rises behind it, so the true minimum likely lies in the unmeasured vestibule (smallest value >3 mm inside: 87 mm² at 4 mm) · at −400/−600 HU: 85 / 65 mm²

Minimum area — RIGHT

47 mm²

at 42 mm · at −400/−600 HU: 58 / 36 mm²

Air volume — LEFT

13.0 ml

nostril-seal → choana · −400/−600 HU: 13.9 / 12.0 ml

Air volume — RIGHT

7.0 ml

nostril-seal → choana · −400/−600 HU: 7.9 / 6.4 ml

Left / Right volume ratio

1.87

−400/−600 HU: 1.76 / 1.88 · a single-scan snapshot — see nasal-cycle note

Rules behind every number: air = HU < −500; airway = 3D connected component seeded inside the nasal cavity; anterior boundary = first coronal plane behind which that component no longer reaches room air (the nostril openings; y = -84 mm — the vestibule tip anterior to it is not measured); posterior boundary = choana, the first plane where left and right air merge behind the septum (y = -13 mm); midline = bone mirror-symmetry plane at x = -2.6 mm (not the array centre); each in-plane air component goes to the side its centroid is on (so a bowed septum keeps each cavity whole), except components genuinely spanning both sides (a ~15 mm anterior stretch where air connects around the septum's free margin), which are split at that plane; maxillary/ethmoid/frontal/sphenoid sinuses and nasolacrimal ducts detached by watershed at their ostia.

Area vs position (front → back)

Left cavity Right cavity hover / drag to inspect · dots mark the minima

Coronal section at cursor

coronal CT section with left/right airway highlighted

blue = left cavity · orange = right cavity · violet = excluded air (sinuses, tear ducts) · R/L = patient right/left (radiological view)

↻ The nasal cycle makes single-scan L/R comparison unreliable. The turbinate lining congests and decongests in an alternating cycle over a few hours. A narrower side on this one scan may simply be the side that was congested at that moment — it is not evidence of a fixed anatomic blockage. Only the bony/cartilage anatomy is stable; the soft-tissue lining (most of what shapes these curves) is not.

Area is geometry, not airflow. Flow resistance depends on shape, wall state and flow regime, not on cross-sectional area alone. A small minimum area does not by itself mean a breathing problem, and a large one does not rule one out.

The threshold moves the numbers. Air was defined as HU < −500. Re-running the identical pipeline at −400 / −600 HU changes the left minimum to 85 / 65 mm² and the right minimum to 58 / 36 mm² — same anatomy, different rule. Treat absolute values as soft; the shape of the curves is more stable than any single number.

What is deliberately not measured. The nostril tip / vestibule in front of the seal plane, everything behind the choana (nasopharynx), the paranasal sinuses and the nasolacrimal ducts. The watershed cut at each sinus opening is approximate at this resolution (≈0.6 mm voxels), and partial-volume effects blur every air/tissue boundary.

Raw numbers (every 0.59 mm plane)

y (mm)	behind seal (mm)	left (mm²)	right (mm²)
-84.2	0.0	77	193
-83.6	0.6	81	190
-83.0	1.2	83	190
-82.4	1.8	83	185
-81.8	2.4	86	178
-81.2	2.9	89	170
-80.7	3.5	87	161
-80.1	4.1	92	152
-79.5	4.7	95	137
-78.9	5.3	103	125
-78.3	5.9	108	119
-77.7	6.5	114	110
-77.1	7.1	121	105
-76.5	7.6	127	101
-75.9	8.2	104	128
-75.4	8.8	107	122
-74.8	9.4	106	119
-74.2	10.0	107	112
-73.6	10.6	106	110
-73.0	11.2	104	106
-72.4	11.8	107	104
-71.8	12.3	106	104
-71.2	12.9	105	100
-70.7	13.5	107	101
-70.1	14.1	109	100
-69.5	14.7	111	104
-68.9	15.3	116	111
-68.3	15.9	120	114
-67.7	16.5	116	117
-67.1	17.0	117	113
-66.5	17.6	121	112
-66.0	18.2	121	113
-65.4	18.8	115	113
-64.8	19.4	111	108
-64.2	20.0	105	108
-63.6	20.6	108	105
-63.0	21.2	136	104
-62.4	21.8	154	103
-61.8	22.3	169	104
-61.3	22.9	191	84
-60.7	23.5	201	88
-60.1	24.1	208	88
-59.5	24.7	215	86
-58.9	25.3	202	94
-58.3	25.9	247	69
-57.7	26.5	250	69
-57.1	27.0	251	67
-56.5	27.6	255	71
-56.0	28.2	257	67
-55.4	28.8	252	67
-54.8	29.4	257	66
-54.2	30.0	238	71
-53.6	30.6	211	71
-53.0	31.2	186	71
-52.4	31.7	211	68
-51.8	32.3	231	64
-51.3	32.9	205	70
-50.7	33.5	202	67
-50.1	34.1	197	67
-49.5	34.7	209	54
-48.9	35.3	196	51
-48.3	35.9	200	50
-47.7	36.4	203	50
-47.1	37.0	205	49
-46.6	37.6	206	49
-46.0	38.2	201	49
-45.4	38.8	191	50
-44.8	39.4	197	51
-44.2	40.0	212	52
-43.6	40.6	224	56
-43.0	41.2	241	55
-42.4	41.7	247	48
-41.9	42.3	250	47
-41.3	42.9	247	48
-40.7	43.5	247	52
-40.1	44.1	251	50
-39.5	44.7	254	53
-38.9	45.3	259	60
-38.3	45.9	258	71
-37.7	46.4	264	83
-37.1	47.0	280	93
-36.6	47.6	288	110
-36.0	48.2	285	129
-35.4	48.8	281	147
-34.8	49.4	287	164
-34.2	50.0	288	175
-33.6	50.6	292	188
-33.0	51.1	290	199
-32.4	51.7	288	195
-31.9	52.3	283	187
-31.3	52.9	272	176
-30.7	53.5	252	151
-30.1	54.1	217	83
-29.5	54.7	168	67
-28.9	55.3	155	68
-28.3	55.8	159	69
-27.7	56.4	158	72
-27.2	57.0	158	66
-26.6	57.6	152	68
-26.0	58.2	152	68
-25.4	58.8	151	67
-24.8	59.4	151	63
-24.2	60.0	151	59
-23.6	60.6	152	57
-23.0	61.1	156	57
-22.5	61.7	157	56
-21.9	62.3	164	53
-21.3	62.9	169	59
-20.7	63.5	176	60
-20.1	64.1	187	59
-19.5	64.7	201	63
-18.9	65.3	215	66
-18.3	65.8	222	72
-17.7	66.4	227	82
-17.2	67.0	228	92
-16.6	67.6	232	110
-16.0	68.2	228	130
-15.4	68.8	225	154
-14.8	69.4	217	184
-14.2	70.0	207	198
-13.6	70.5	194	207