July Didactics recap

7/16: Physics Basics: 

Take home points: 

1. Ultrasound images are made from ultrasonic waves being sent through tissue, bouncing off, and returning to the probe. 
   
2. Artifacts are made from these soundwaves hitting tissues and creating different distortions in the returning images. These may be used to aid diagnosis or obstruct/distort the images structures. 
   
3. Ultrasonic waves can be unsafe when applied at too high of frequency and/or for too long. Sonographers need to consider the thermal index (heat generated from soundwaves) and Mechanical index (shaking of the tissues from the soundwaves) when imaging delicate structures (i.e. fetus, orbits). Goal is to use the ultrasound as low as reasonably achievable. 
   

In depth Notes: 

• Not discussing doppler
  

-        Ultrasound
 Sound above 20k Hz. 
We use 1-15 MHz.
Image is made by sound bouncing off of tissues.
-        Waves: come in cycles.
o   Frequence (in MHz) is how many cycles per second
o   Length is distance of cycle (m)
o   Due to time, shorter lengths make high frequency and vice versa
o   Propagation velocity (phi) is c/f
-         Material properties: 
(K) bulk modulus (how compressible it is) as the BM goes up, then so does bulk modulus.
-          Ro is density. Mass per volume. This is inversely proportional to propagation velocity
o   Dense tissues will not let waves travel well through them.

The probe 
-          How do you make the waves? 
You send electricity through the housing, then through the piezoelectric crystals which vibrate and make waves. The acoustic lens and match layer focus them.
-              Piezoelectric crystals when electricity is applied to them, they deform (change shape) and this displaces some of the electrodes making a charge w/ positive on one side and negative on other. The charge can make energy.
o   This powers lighters, electronic drums, ink jet printers. 
o   In ultrasound the charge we make gets turned into a sound wave.
o   Our probes can receive and emit the waves
-              Attenuation
o   As waves move through a tissue, the amplitude (intensity) of the wave lessens making the image less refined.
o   Attributers

• Absorption: where the sound waves get converted to another energy (heat) that the tissue takes up.
  
• Tissue viscosity. The increased viscosity decreases frequency and flow.  This is due to greater bond strength between the molecules.
  
• Frequency can address this. Higher frequency (shorter wavelengths) gets absorbed faster in the superficial tissues.
  
• Higher frequency is absorbed faster.
  
• Tissues have their own attenuation. Air and bone have high attenuation coefficients. Fat, blood, and liver are lower.
  
• Absorption is increased w/ frequency and viscosity.
  

- Reflection
o   What makes us able to use ultrasound. This is when a fraction of emitted waves returns to the probe to make the images.
o   Some go directly back to the probe, but not all. Some
-              Refraction When the waves go through the tissue and bends and continues traveling through the tissue.
o   The change in the speed of the wave changes as it goes through changes due to density of the tissue increasing making the wave change velocity.
-              These are dependent on acoustic impedance. This is how a sound moves through a medium (propagates). It is relative to the density of the fluid and speed of sound in the medium (Z = ro c)
o   This is notable when you have 2 structures w/ 2 different properties abutt (bone and soft tissue) this is when you have acoustic impedance mismatch.
o   Impedance is increased w/ density, bulk modulus.
-              Reflection is noted when you have tissues w/ very different acoustic impedances. This is what makes them appear bright (when the impedance is seen on surface of the bone)
-              Refraction is when there’s a difference of impedance, but not the massive dramatic change. This is when you have tissues w/ a curve.
-              Final tissue loss is scattering:
o   When there is a particle that is smaller than the wavelength of the tissue you’re talking about. This has the signal dissipate in the tissue.
-              These drive the creation of Artifacts
o   Posterior shadowing: the waves hit high attenuation tissues so you can’t see through them (bones, stones)
o   Posterior enhancement: waves pass through low attenuation tissue. This gets more waves through the initial fluid, so the second tissue gets more energy in it making it appear brighter (see in tissue behind fluid filled sacs)
o   Reverberation: when the echoes reverberate. This is a highly reflective structure (pleural line) that keeps sending signals back to the probe w/ different depths. You see this in needles too.
o   Mirror artifact: when you have high reflection tissue. Waves hit the highly reflective tissue (diaphragm)
o   Edge: highly refractive tissue makes lines at the edges. See this in vessels when you hit the structure w/ curved edge, and no echoes come back to them bouncing off.
Beam properties:
o   The probe: electricity goes into the probe, hits the backing, vibrates the crystals to make waves, then matches them in a similar direction, then the Lens focuses the beam.
o   Linear array: sequential firing down the beam. This is done in a linear pattern. Each goes out and comes back in a perpendicular pattern.

•  Only one component at a time. And receives only their area they sent out.
  

o   Phased array. In this, all crystals fire at once. We can change the frequency these are fired and where it is fired. This gives us a small footprint and wide field of view.
o   properties of the waves emitted from the probe
§  When probes fire, it will be wide, then narrow to a focus point, then broaden out again.
§  Focus: point where beams converge.
§  Near field (Fresnel zone): close to field. Waves converging and thus your resolution and clarity is better
§  Far field (Fraunhofer zone): opposite. Spacing of waves is reduced and see poor image resolution.
§  Focal zone: best resolution
§  Probe and focal zones: small footprint means the beam narrows and short focus. A larger footprint allows broader space and more time before the wave disperse.  Frequency of probe also contributes.
·      High frequency probes have a closer near field. Where curves have broader nearfields.
o   Note, most of this is about the main beam. However, there are also other beams.
§  Side lobes: Side beams. On the edges and they make artifacts. When the crystals get charged, they get longer and thus it has more area that sends off these side lobes.
§  Grating lobe: even more off the sides.
-              Artifact:
o   Side lobes: the probe takes the side lobes waves back and transpose onto the main image. Can fan through side lobes.
§  This can make stuff look present that isn’t there
§  Tends to have more linear appearances. Don’t respect anatomical boundaries
o   Beam width this is based on that the beam sends out waves that are diverging after the focal zone. Looks like a hazy abnormality. Can adjust the focal zone to address.

• Image adjustments to counter physics and artifacts
  

o   Get the right depth 
o   Get the right focal zone. Have the object in this area.
o   Minimize far field
o   Optimize gain: this is a pre-processing tactic. Does not change the tissue, but instead changes the display.
§  Time gain compensation: this is where you can change gain at different gains at different points along the tissue. Less stuff is going to the deeper tissues, so you try increasing the deep image areas.
o   Tissue harmonic imaging: when you get waves back that you sent out AND from the tissues. Can be helpful in deep structures.

• Safety: (shake and bake)
  

o   These are measures of the safety of the waves.
o   TI: Thermal index. This is related to intensity, duration, and absorption of the wave. Need to be aware of this.
§  Continuous wave doppler (because you give it constantly you have a high risk of it).
o   MI: mechanical “shaking” of tissues. Risk of making a cavity from popped bubbles. We want MI low to keep people safe. When you make this high, you make holes or pop hollow tissues.
-    ALARA: want the POCUS waves to be used at levels as low as reasonably achievable.

Didactics 7/23
Take home points: 

1. Ultrasound guided vascular has been shown to be cause less complications than blind/landmark guided access 
   
2. On difficulty patients denying USIV placement is causing harm
   
3. Ultrasound guided subclavians were found to have less complications than blind. There are multiple methods you can use to perform this procedure safely using POCUS. 
   

Vascular Access Revolution: The impact of Ultrasound. 
-              Discuss the literature on vascular access via POCUS
-              Evidence dating from the late 90’s that if you don’t use ultrasound for vascular access of difficult patients, you are harming the patient.
-              Complications are often from consultants putting in lines w/o POCUS, often after a miss. These are where you see AV fistula or other complications.
POCUS guided IJ: 

• General tips: 
  
  • Machine is opposite to side doing practice on the contralateral side of the study
    
  • Lay in Trendelenburg and resuscitate them. Give oxygen for comfort.
    
  •    When doing it, you want to track the true needle tip.
    
    • To find, jiggle w/o advancing to give you a signal.
      
    • Then do microtilts to follow it down to the vessel.
      

-              Was disruptive innovation initially!
-              Seems simple. Can see it press the IJ or
o   Evidence.
o   3 RTC studies. Milling ’05, Leung ’06, Karakitos. Looked at complications and found much less likely for carotid puncture, unsuccessful cannulation, hematoma, pneumothorax. Leung was Australian study and did w/o ultrasound trained people (unlike Milling) showing 15.4 less unsuccess, 7.7 less hematoma, 5 less carotid, 1.5 for pneumo (all ARR)
o   So, 10-30% decrease in harm w/ US guided IJ

Subclavians: 
looked at blind subclavian vs US guided and massive harm reduction w/ US. Study broke this by expertise and even high experience providers had greater complications w/ blind insertion vs POCUS.
o   Even if you’re great at blind SC, you likely haven’t done enough to fail.
o   US guided subclavian:
§  Can do!
§  Put on the clavicle, then slide laterally, in a long axis, then you need to disassociate from the clavicle. And rotate the axis to look at the vessel in a long axis.
§  In live studies, you may have an external jugular joining a larger subclavian and may see a valve where it joins.
§  Veins also can look like have bulbs
§  Can do it in a short axis too.
·      Use doppler to confirm which is the artery and vein and memorize which is the correct one by nearby structures. Can also do color but pulse wave doppler is better.
§  Vezzani looks at infraclavicular cannulation of the subclavian vein in cardiac surgery patients. Long vs short axis. Found the short axis did better.
§  Subramony looked at ultrasound guided vs landmark method for subclavian vein in the ED and when educated, found in 2022 that US guided was better. Found 60% landmark and 70% w/ US in subclavian.
§  Supraclavicular looks that if you can jam the probe in there near sternum, then you can see the subclavian joining the brachiocephalic
§  Can use the endocavitary in this area and in suprasternal notch.
·      Also found an article that when you insert the guidewire, the direction of the J tip on the wire will send the line where it needs to go, so point it towards the feet.
-              PIV:
o   AEUS chairmen found in 2014 that routine PIV US guided not strongly supported
o   But then found US guided PIV found on patient survey found that they were discharged home vs getting a central line due to no access. And had higher satisfaction.
o   We noticed that central venous placement drops off as more PIVs are done. Noted in 2012 at Thomas Jefferson by Arthur Au
-              Shokoohi found ultrasound guided PIV made CVC use drop off logarithmically.
-              Do Echo-enhanced needles make a difference in access? Turns out not a huge difference except that does prevent double wall (back walling)
-              Peds: ultrasound had higher success than landmark.
-              Key of all these: stay on target. Do not bring your probe to the needle but bring the needle to the probe.
o   Learners struggle when they have to manipulate the needle and the probe.

7/30 – DVT

• Ultrasound takes ~3 minutes and allows for timely clinical diagnosis.
  
• Diagnosis is made by compressing the vein and assessing compressibility.
  
  • If not compressible → blood clot present.
    
• Most common clot locations: groin and behind the knee (areas where veins bend, leading to venous stasis).
  

Anatomy & Key Veins to Assess

• From proximal to distal: Common femoral vein (CFV) → Deep femoral vein → Femoral vein (FV).
  
• The common femoral vein is sometimes called the superficial femoral vein—but it is a deep vein and requires anticoagulation.
  
• Great saphenous vein joins the CFV.
  
• After the femoral artery bifurcates, a branch comes off the femoral vein.
  
• Progress distally to assess the popliteal vein, then scan down to see the trifurcation.
  

Technique

• Differentiate arteries vs veins: use color Doppler or compression.
  
• Sensitivity for calf DVT is low → if clinical suspicion remains high and study is negative, have patient come back for a repeat exam.
  
• Positioning: Reverse Trendelenburg to pool blood in veins.
  
• Equipment: Linear probe (superficial imaging).
  
• Compression is the key step in your exam.
  
• Consider scanning in prone position.
  
• Elevate head to increase venous distension.
  

Pitfalls

• Vessels may be deep, while superficial structures (e.g., lymph nodes) can be distractors—train your eye to know expected depth and location.
  
• Anatomy reminders:
  
  1. Veins and arteries often run in pairs.
     
  2. Always check depth and expected location.
     
• If hard to locate vessels → use color flow and augmentation (squeeze calf to see increased flow).
  

Acute vs Chronic DVT

• May be difficult to differentiate—patients can have acute-on-chronic thrombus.
  
• In chronic DVT → incomplete collapse due to central reabsorption.
  
• A true negative requires complete collapse of the vein.
  

Other Findings on DVT Ultrasound

• Baker’s cyst (stellate shape, connects deep to joint)
  
• Cellulitis
  
• Gastrocnemius tear
  
• Groin hematoma
  
• Pseudoaneurysm
  

Controversies

• Is 2-point scanning (CFV, popliteal) enough?
  
• Diagnostic accuracy in novice scanners.
  
• Scan types:
  
  • 2-point: CFV, Popliteal
    
  • 3-point: CFV, FV, Popliteal
    
  • Whole leg: CFV, FV, Popliteal, Anterior tibial (ATV), Posterior tibial (PTV), Peroneal veins
    
• Comprehensive duplex US?
  
  • 1993: US vs venography → no isolated segment thrombosis found.
    
  • 1998: RCT → Whole leg vs abbreviated scan had same complication rate.
    
• Majority of clots occur in proximal CFV or popliteal vein.
  
• Deep femoral vein thrombosis is <1% of cases.
  
• Evidence that we’re missing VTE with limited scans is outdated.
  
• Serial 2-point US + D-dimer vs whole leg US:
  
  • RCT: If D-dimer negative → 2-point scan sufficient.
    
  • If positive D-dimer + high clinical suspicion → repeat US.
    
• Even with whole leg US → some patients may still develop PE within a week (tech limitations, disease progression).
  
• Whole leg US is not superior to 2- or 3-point US.
  

• The more scans you do, the better your accuracy.
  
• Venography was stopped due to increased VTE incidence from cannulation.
  

 

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