A recipe from Thao & Philippe’s Kitchen!
Step 1: the base
Ingredients in the following proportions, in the total volume desired, are stirred until completely dissolved:
• 250 mL (approximately 1 cup) boiling water,
• 25 g of unflavored gelatin (about 3 packets of Knox brand gelatin), (flavored gelatin is ok too, it smells great but is more expensive, and we will reserve it for the 3rd layer below)
• 10 g (a heaping tablespoon) of sugar-free psyllium hydrophilic mucilloid fiber (brand name: sugar-free Metamucil). Talcum powder can also be used but it tends to settle more.
• Pour in a large container (more will follow so plan to not fill the container now. We used plastic salad bowls like this
Stir it every now and then (about every 20 min) until it starts to thicken. Otherwise the fibers will settle in the bottom. When it starts to thicken stop stirring, to not “break” the structure and introduce air bubbles (NO air bubbles at all of course!)
Let it congeal for a few hours in a refrigerator at 6°C. It should be really firm and jiggle when set.
This is the schematic of what we have with the first layer in orange.
Step 2: the inclusions
While the first layer is congealing, prepare the “inclusions”. We bought a bunch of small balloons like these:
Put some of the same mixture as above (without the Metamucil) so they appear echofree to make “cysts”. Prepare a smaller batch of mixture, and fill the balloon with a funnel. The balloon should be about 2-4 cm).
You can also fill a few balloons with a higher amount of Metamucil (triple it) to create more echogenic balloons)
To mimic “femurs” we simply took some drinking straws that we cut in exact numbers such as 20, 30, 40 and 50 mm. The exact numbers are so that when the students scan we know that values such as 37 mm are not well measured. These “femurs” will fill with gel in the next step when they get embedded.
To mimic “vessels”, use rubber tubing (not plastic as these are too hard to puncture). Old Foley catheters are good too if you can recover some.
To be really fancy, these can be filled with the same gelatine mixture (no Metamucil) with food colorant (so that we can test that the students aspirate color in their “puncture”. Again let the gel solidify in the whole tubing before cutting it in 10-15 cm segments.
This is the schematic of what we have with the first layer in orange, and the inclusion (in blue) on top of it
Step 3: anchoring the inclusions
Most likely all these inclusion will float, so they need to be anchored on the base. To do that the easiest is to place them on the base in Step 1 (when it has congealed) then fill a layer the thickness of the largest inclusion, and let that second layer congeal. When the balloons have congealed, cut the “knot” off so that the “lesion” looks cleaner delineated. Place the cut section “below” as much as possible. We tried anchoring with unfolded paper clips, but they created lots of artifacts when scanning and did not anchor very well.
When pouring the second layer, enter the cut straws (“femurs”) at an angle so that they fill with the liquid gel.
The inclusions will progressively “float” to the top of the second layer, so make it as thin as possible (certainly no thicker then the largest inclusion):
Let the phantom congeal, and very delicately stir the layer to prevent too much sedimentation of the fibers.
Step 4: Finishing layer
Finally one more layer, about 5-8 cm thick, is mixed and poured over the second (and already congealed layer) to form the “near field” when scanning. Here flavored gelatin is good as it will “hide” the inclusions better so that the students cannot see what they are trying to scan.
We did not add any antiseptic (such as cholorox) into the mixture and it will last 2 weeks in the refrigerator (we did not try longer and it may very well last longer).
We try to unmold the phantom but with uneven results (some cracked) so I think it is best to leave them in!
A piece of “plastic straw” to practice measuring and aiming. Note the different echogenicity of the various layers.
We probably need one phantom for every 4-5 students.
How to use the phantom
Insert a spinal needle (20 or 22 gauge) anywhere in the phantom at about 45degree, and get the student to image the whole length of the needle (minus the first 2-3 cm of course). This is an important step as they need to perfectly align the plane of the transducer with the needle. The tip of the needle has to appear as a bright echogenic dot at the end of the needle. Get the student to position and reposition the transducer several times until they can do this pretty fast. For right-handed students it may be easiest to hold the transducer with their right hand and hold the needle with the left hand. This tutorial is designed for conditions in which 2 operators are not available, and the physician has to guide herself. Of course there are many other ways to guide procedures but the fundamentals remain the same.
Students have a great tendency to try to look at the transducer and the needle to align each other. Although this is needed in the beginning, they should progress to be able to align the two without looking at the phantom, only by looking at the ultrasound screen. We ended up placing a sheet of paper to hide the transducer and needle.
One of my students in Mongolia, practicing the needle insertion and visualization
Example of needle visualization during an amniocentesis
The next step is for the student to move the needle “in” and “out” under ultrasound guidance and make sure that 1) they constantly image the tip of the needle and 2) get the reflex to STOP the needle if they loose the tip. Thus, just a simple "pull out-push in" motion with constant visualization of the tip of the needle.
Visualization of the needle tip, the needle and the target.
Now the student should be able to insert the needle under ultrasound guidance and track it as soon as it is 2-3 cm in the phantom.
The teaching points are:
1) Do a single insertion and when reposition is needed, do NOT totally remove the needle as this would require a reinsertion that would hurt the patient. Of course if the location has to be changed (baby moved!) then the needle has to be relocated.
2) Position the needle close to the transducer but at NO TIME can they touch (desterilize). If the needle touched the transducer, pull out, and resterilize the whole field.
Although not part of this tutorial it is important to teach how to properly sterilize the skin, give the time to the beta done to kill the germs. I teach to not cover the transducer with sterile cover and use no sterile gel. When properly done, the moisture of the betadyne is sufficient to act as a coupling agent, and if kept apart from the needle, the transducer does not need to be covered. This may need to be adapted to each institution’s guidelines. For procedure where fluid will be aspirated (amniocentesis, cyst aspiration…), the use of a plastic tubing (as below) between the needle and the syringe, allows a safer procedure as the needle is not held firmly in the patient.
3) Never do sideways movements inside the patient (the angle of the needle can only be changed in the very subcutaneous tissues). Doing a sideway movement with the needle runs the risk of the bevel of the needle cutting an umbilical artery. This is the only emergency C-section that this radiologist ever ordered while guiding a young resident !
The phantom makes it easy to recognize when the student pulls the needle sideways, as the bevel will cut in the gelatine and leave a clear mark! Same marking happens if the student pulls back the needle with the stylet not properly locked on the needle. Bubbles are drawn in the needle path and leave a clear mark.
4) Aim for a large (2-3 cm) target and get the student to insert the needle into the target. The student as to learn to 1) image the target, 2) rotate the transducer until the safest path to the target is imaged, and 3) insert the needle in the exact plane the transducer has defined. The needle should be visible after the first 2-3 cm in the phantom and remain constantly visible after. Note that since the phantom is fairly stiff, a lot of gel is needed to couple the edge of transducer to the phantom.
If the needle goes “below” the target, pull back (but not out) and oblique the needle more horizontally, it if goes “above” the target, pull back (but not out) and angle the needle more vertically. Be careful to NOT bend the needle!
5) The finale step is to practice aiming for smaller and smaller structures and practice until very familiar.
A few more tips:
When all these steps are mastered, teach the student that when inserting a needle they should neither go too fast (loose the tip of the needle) or too slow (as this could cause tenting of the membranes-red arrows in the picture-). When doing fetal procedure (such as IM injections in the fetus to paralyze it, or KCL) again a sharp push is needed or the fetus will move around.
Do not remove the stylet and reinsert the needle. This can cause the empty barrel of the needle to core pieces of membranes or particles that will block the needle. This will make aspiration difficult. One of the markers of a blocked needle, is that a vacuum is formed in the needle that causes a particular artifact:
While draining a cyst, check 1) there are no large vessels in front of the needle using color Doppler (especially if doing transvaginal as it is hard to tamponade a vaginal hematoma), and 2) as the cyst wall collapse reposition the needle to get as much of the fluid as possible.
During aspiration, the tip of the needle may become clogged as the walls of the cyst are aspirated. Reposition the needle gently, or rotate it to reorient the bevel.
Small residual fluid appears unavoidable after cyst aspiration
I hope this short tutorial s helpful to help students practice ultrasound guided procedures.