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The wrist, short axis view, focusing on the

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carpal tunnel space and the median nerve on MRI.

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In the middle, T1 fat weighted.

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In the right-hand corner, wearing the

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blue trunks, heavily water weighted image.

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In the left-hand corner, wearing the

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red trunks, heavily T2 weighted image,

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somewhat heavily water weighted.

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I point this out because the median nerve,

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which is here, is going to be bright on the

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heavily water weighted sequences like PD spar,

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special, spare, and many gradient echoes.

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But on the T2, it should be gray.

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Should not be brighter than muscle.

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Let's look at the anatomy.

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If we go to the proximal carpal area, the proximal

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aspect of the carpal tunnel, we've got the pisiform.

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And the tubercle of the scaphoid

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connected to it is the flexor retinaculum.

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The flexor retinaculum also has a little fascicle

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that goes over this way that helps encase

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Guion's canal called the ligamentum palmare only.

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The flexor retinaculum forms the

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anterior boundary of the carpal space.

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The deep transverse carpal

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ligament forms the deep boundary.

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The median nerve has a variable position.

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In this case, it sits between the flexor superficialis

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of digits 2 and 3, between 2 and 3 and 4.

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It has a juicy, but slightly triangular shape.

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If it sits between these tendons right here, the flexor

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digitorum superficialis of 2 and 3, and the profundus,

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and the flexor pollicis, as it is prone to do 20

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percent of the time, then it'll be more slit-like.

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But that's not the variation that we see here.

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Sometimes the median nerve will bifurcate

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prematurely at the level of the pisiform.

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When that happens, the accompanying vessels

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with it, like the median artery seen here,

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or veins, are large, bifid, or trifid.

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In other words, there are anomalies.

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In patients who are examined in the proximal carpal

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row, you should see a little bit of interspersed

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fat best appreciated on the T1 weighted image.

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Complete absence of fat on the T1 weighted

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image at any level through the carpal tunnel

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space is an indicator that there is mass effect.

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The patient may have clinical carpal tunnel syndrome.

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So we go back more proximally, look at where our median

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nerve goes, it goes more towards the radial side.

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Now we go distally, we're at the level of the pisiform.

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Now we leave the pisiform, but

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we don't pick up the hamate yet.

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Now we're at the mid portion

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of the carpal tunnel space.

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There's our flexor retinaculum.

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There is our median nerve.

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There is our flexor digitorum profundus fourth digit.

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And superficialis fourth digit, as well

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as the other deep and superficial flexor

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tendons of the second and third digits.

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And there is our median nerve with

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the proper signal as described before.

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Let's keep following it more distally

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to the level of the hook of the hamate.

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Where the carpal tunnel normally

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gets a little more shallow.

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And things look a little more squished.

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There's a little less fat.

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But we still see some fat deep within

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the carpal tunnel bony canal or space.

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Loss of that little fat stripe can be a

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very subtle indirect sign that there's

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too much mass effect in the carpal tunnel.

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Now within the carpal tunnel, we've got

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profundus tendons and superficialis tendons.

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And the flexor pollicis longus,

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but not the flexor carpi radialis.

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And now the flexor retinaculum runs from the

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hamulus of the hamate over to the greater

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multangular, or trapezium, forming the

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anterior boundary of the carpal tunnel space.

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Don't forget, when you're looking at the carpal

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tunnel space, to examine the thickness, fullness,

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and lack of fat in a normal thenar eminence.

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For in carpal tunnel syndrome, this

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space will get smaller and fat-laden.