Lung Ventilation - Part I in V/Q Lung Scan

1. Types of Ventilation
1. ¹³³Xe
2. ¹²⁷Xe
3. ^81mKr
4. ^99mDTPA
5. Technegas


2. Comparing of the different ventilation agents

	P or V first	Gamma Energy	Washout	Room Pressure
133Xe	Ventilation	80 keV	Yes	Negative
127Xe	Perfusion	203 keV	Yes	Negative
81mKr	Perfusion	190 keV	No	N/A
Technegas	Perfusion	140 keV	No	N/A
99mDTPA	Either	140 keV	No	N/A

	T1/2	Views	Cost
133Xe	5 days	1 (POST)	Average
127Xe	36 days	1 (POST or best view)	More than 133Xe
81mKr	13 seconds	Multiple	Very Expensive
Technegas	6 hours	Multiple	Unknown
99mDTPA	6 hours	Multiple	Less than X133

3. When does the room need to have negative pressure?
1. When the radiopharmaceutical is a gas and has a significantly long T1/2
1. ¹³³Xe and ¹²⁷Xe require a negative pressure room
2. Because of ^81mKr T_1/2 is so short, a negative pressure room is not required
2. Technegas and ^99mTcDTPA are not gases, therefore a negatively pressured room is not necessary
3. Criteria regarding a negatively pressured room
1. Xenon deliver systems contain traps made of charcoal and are placed at the exhaust port
2. Exhaled xenon gas that leaves the trap must be below certain concentration levels in air
1. Restricted area maximum concentration is 1 x 10-5Cμ/ml
2. Non-restricted area maximum concentration is 3 x 10-7μCi/ml
3. If the sum of all exhaust rates is greater than the sum of the air supply coming into the room, then the room has negative pressure
4. Flow is measured in cubic feet per minute (CFM)
5. CFM must be measured quarterly
4. Charcoal trap must be measured monthly to assure that you are below the maximum concentration level
1. Collect activity from a Xenon study by placing a plastic bag around the exhaust port
2. Measure the activity with your gamma camera
5. Negative pressured room must also be tested for the time it takes for a xenon spill to be ventilated from the room
1. If a xenon spill occurs, the door must be shut in order to contain the gas
2. Once the air has been completely re-circulated, the gas removed, the door may then be opened


4. Why should a ¹³³Xe be completed before the perfusion procedure? (Refer to graph below)

1. Concern relates to a concept known as cross talk
1. ¹³³Xe has the lower gamma energy
2. If ^99mTcMAA is completed first, the gamma energy from ^99mTcMAA spills into (cross talk) the ¹³³Xe window adding unwanted counts in the image. This would also be defined as Compton scatter from the ^99mTc atoms
3. If ¹³³Xe is completed first, this does not become an issue
4. CAVEAT: Literature cites some "evidence" to indicate that cross talk is not a problem, and that computer subtraction technique will eliminate/solve the cross talk issue. I disagree.


5. Ideally perfusion should be done before ventilation
1. Rationale: if the perfusion images are normal, then there is no reason to continue with ventilation procedure (if there is a cold defect in perfusion that raises the question of PE)
2. Cold Defects - Matched and mismatched
1. Matched defects indicate COPD (or related). This ventilation defect may also affect the vascular supply to the same area, resulting in a cold perfusion and cold ventilation defect. This would occur if the vascular and bronchial are both necrotic or badly damaged
2. Mismatch would indicate PE (cold perfusion/hot ventilation)
3. In order to complete the ventilation images, consideration of the gamma energy on the ventilation agents must be considered (refer to table)
1. ¹²⁷Xe and ^81mKr have higher energy gamma than Tc99m, hence ventilation would follow perfusion
2. Why are these two agents not routinely used? (cost and availability)


6. Ventilation procedure using ¹³³Xe or ¹²⁷Xe
1. Requires a negative pressure room
2. Requires a special apparatus for administration, Xenon Delivery System (see the above diagram)

3. Patient is setup so that the posterior portion of the lungs are imaged - NOTE: can only taken in one projection
4. ¹³³Xe is usually done before the perfusion study; however, if ¹²⁷Xe is used, then ventilation may follow perfusion (note energy)
5. If ¹²⁷Xe is used, only one modification must be made with the delivery device, and that is it must have increased shielding for the higher energy gamma
6. There are three phases to the study
1. Initial Breath (closed system) - via an inhalation of a radioactive bolus which identifies total lung capacity
1. Patient inhales a bolus of ¹³³Xe through a bio-filter
2. Patient holds his/her breath for 20 seconds
3. Delivery system is defined as closed (refer to diagram)
1. When patient exhales the gas, it enters the delivery system
2. Study enters its second phase
2. Equilibration - In a closed system, the gas is continually recycled to the patient, filtering out CO₂ on exhalation and enriching the inhalation with O₂, until the concentration of xenon in the lungs = the concentration in the ventilation system (indicates lung volume)
1. Note the pathway of the exhalation: CO₂ absorber, anhydrous crystals, xenon chamber, bio-filter, patient
2. Defined as a closed system - follow the red arrows
3. While the patient is breathing, air and xenon follow the red arrows
4. Additional O₂ is added to the closed system
5. One minute images are taken for two to three minutes
3. Washout - The ventilation system is then opened and the exhaled gas leaves the patient and exits the ventilation system. The faster the xenon clears from the lung, the better ventilation is in the lungs. Poorly ventilated areas, that contain COPD, will have residual xenon remaining in the lungs for an extended period of time (greater than several minutes)
1. As patient exhales, gas follows the red arrows to blue arrows
2. System is defined as an open-system
3. Charcoal filter traps the Xenon as it leaves the system
4. One minute images are taken for three to five minutes
5. Patient is kept on the system until all the gas has washed out of the lungs
6. Delayed washout or residue activity indicates COPD

4. Here is an example of what a Xenon Delivery System looks like


7. ^81mKr procedure
1. Because of the cost and availability of the Rb/Kr generator, many imaging centers do not use this procedure
1. Consider the purchase of 1 generator per day times 365 days per year
2. Also consider the limited shelf life of the Rb/Kr generator
3. ⁸¹Rb T1/2 = 30.5 minutes and has a useful shelf life of approximately 6 hours
4. ^81mKr T1/2 = 13 seconds
5. Gas is continually inhaled while the patient is being imaged
6. The inhaled gas reaches an equilibrium count rate that is proportional to the ventilation
7. Because the energy is higher than Tc99m, perfusion maybe completed first
8. Images maybe taken at any angle in which the perfusion defect is noted
9. Negative pressured room is not required because of the short T1/2 of ^81mKr
10. Because of the limited counts in the ⁸¹Rb generator after 3PM (note 30.5 minute T1/2) an alternative form of ventilation must be considered

11. Here is an example of a ^81mKr Delivery system. Image rollover may not work in Bb


8. Comments on ^99mDTPA aerosol system
1. ^99mTcDTPA is injected into the nebulizer (refer to diagram)

2. O₂ enters the nebulizer converting liquid (^99m TcDTPA) into small liquid droplets via an ultrasound or positive-pressure nebulizer
3. Droplet size smaller than 2.0 μm reaches the alveoli
4. Patient inhales and droplets coat the inside of the lungs


http://www.uky.edu/Pharmacy/research/lunab25b.jpg

5. Concern related on aerosol droplets having extensive uptake in the bronchial tree depends on: particle size, air flow rates, and turbulence
6. The delivery tubing filters out larger droplets (10 to 15um), while droplets that are <0.1 μm escape as expired air
7. Note the one way value in the diagram allows the expired air (droplets) to be trapped by the filter
8. Note the extra tubing for patients that are on ventilators (tubing is called the "tail")
9. Dose
1. 3 to 5 ml placed into the nebulizer
2. Approximately 35 mCi (if the study is done before perfusion)
3. No more 10 to 15% of the dose reaches the lungs, however, based on the amount of type the drug is administered the dose may actually be less
10. Administration imaging the aerosol (ventilation) procedure
1. Usually the patient breaths on this system five minutes
2. After ventilation, images are taken for approximately 150k counts or five minutes
3. In patients with severe COPD or other ventilation problem, mucous plugs prevent the droplets from reaching the alveoli. In addition, the mucous plugs appears hot (droplets adhere to the plugs). This may reduce diagnostic accuracy. (What happens if a mucous plug prevents droplets from entering a segment in the lung that appears cold in perfusion)
4. If a patient suffers from COPD, numerous mucous plugs will appear on the scan. I refer to this condition as "hot grapes."
11. If perfusion is done first, then the ventilation dose must be significantly higher (60mCi or greater) and the perfusion dose must be significantly less (1 mCi). However, most clinics perform ventilation prior to perfusion.
12. Problem with the ^99m Tc energy is noted when both perfusion and ventilation use the same radionuclide
1. The amount of counts coming from whichever procedure is done second must be at least 3 times greater than the counts coming from the first procedure
2. As an example, if ventilation is done first and too many counts come from the ventilation images, then activity from the ventilation images may shine through into the perfusion image. If a segment in the lung contains a PE, it would normally look cold in a perfusion defect. However, if a ventilation image contains too many counts, activity from the ventilation image may shine through, causing the perfusion image to contain activity in the area of the PE, this could lead to a false negative diagnosis
13. Improving the quality of the aerosol image
1. The patient should be dosed in the upright position, which allows for better distribution of the droplets

 


2. Patients with ventilators require a special hookup
   1. The ventilation system is considered a closed system. The pressure generated by the ventilator must be able to inflate the lungs
   2. Therefore, the aerosol system must be attached in such a way that the system remains closed. This is what is being demonstrated in the image above
3. Patients on ventilators present additional difficulty
1. Increased mucous build-up in the main-stem bronchi may require the patient to be suctioned prior to dosing (reducing clumping of activity in the main stem bronchi)
2. A 10% concentration of ethanol to ^99m TcDTPA improves distribution of the particle by reducing the surface tension on the particle and increasing its density (Journal of Nuclear Medicine: 26(6):643-6, 1985 Jun)
3. Positive end-expiratory pressure (PEEP) is the pressure applied by a ventilation device. Excessive pressure (causing turbulence) of greater than 20 may cause abnormal clumping of the droplets in the main stem bronchi. Reducing PEEP while the patient is being dosed with ^99m TcDTPA may reduce this effect and improve the quality of the scan
4. Diagram shows how PEEP effects the alveolus

5. Note: When there is excessive activity in the main stem bronchi, image quality is reduced, thus reducing being collected by the area of interest - the lungs.


14. Technegas
1. Is an ultra-fine microaerosol, graphite coated with ^99mTc atoms wrapped around the radionuclide. They care called "Buckyballs"
1. Diameter range is 5 to 35 nm
2. Is a hexagonal crystal of native ^99m Tc metal that is "shrink-wrapped" in a graphite
3. 7 to 10 mCi dose

http://lungspect.com/generator/howtousethegenerator.html?showall=1
2. How are Buckyballs made?
1. (a) Turn Argon flow on and place crucible into Technegas generator
2. (b) ^99mTcO₄^- is placed into alcohol moistened graphite crucible
3. (c) 6 L shielded chamber is then closed and simmer stage is initiated
4. The crucible is heated to evaporate all the liquid and air within the camber. This replaces the Argon
5. (d) Select start button and a 6 minute process to produce Buckyballs is initiated
   1. Unit starts "simmering" process @ 1500^oC
   2. Then for 15 seconds, the "burn" stage, generates a temperature of 2500^o C
   3. Creating the Buckyballs
6. (e) The patient then inhales via an attached breathing apparatus 2 to 3 times and approximately 1.0 mCi dose is administered
7. Because the particles are hydrophobic and chemically inert they remain lodged in the alveoli
8. This ventilation procedure is performed prior to the perfusion study
9. Multiple images can be taken (just like ^99mDTPA)

http://www.veccsa.com/en/Technegas.htm

3. Comparison of Technegas to DTPA

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Ventilation Lung Procedure - Aerosol
Ventilation Lung Procedure - Xenon

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