The agent used for perfusion imaging is Tc-99m
albumin (Tc-99m MAA).
Particle size is generally between 10 to 90 microns (90% of
and no particles
should be larger than 150 microns. Tc-99m MAA is injected slowly
percapillary arterioles, obstructing approximately 0.1% of their
number. The particles
clear by enzymatic hydrolysis and are phagocytized by RE cells
agent has a biologic
half-life in the lungs of between 6-8 hours). Normally, only 3 to
Tc-99m MAA will bypass the pulmonary vasculature. The critical
the lungs which
receive a dose of about 1 rad (1 cGy) from a typical 5 mCi dose.
kidneys and bladder
receive moderate exposure largely from the excretion of degraded
A 5 mCi dose usually contains on average 500,000 . The
temporarily occluded because the material degrades into smaller
are eventually phagocytized by cells of the reticuloendothelial
biological half-life of MAA in the lungs is about 6 to 8 hours
number of particles used for the exam should be reduced to about
100,000 in patients
with pulmonary arterial hypertension, and in those with known
left shunts. To
label a smaller number of particles while maintaining the dose of
Tc-99m, the vial should
be reconstituted with a larger amount of activity. A given number
mCi then represents a
smaller portion of the total vial, and thus contains fewer
minimum of 70,000
particles is necessary to obtain a diagnostic quality scan in an
It is estimated
that neonates have about 10% of the eventual number of adult
neonates, the number of particles should be reduced to between
to 50,000 to
maintain an adequate margin of safety. The number of capillaries
increases to half of the
adult value by age 3 years, and reaches an adult level by age 8 to
years. A dose should
be administered only if the preparation is less than 6 hours old.
is because Tc-99m
MAA tends to aggregate and Tc-99m slowly dissociates from Tc-99m
over time. Also,
decay of the radiopharmaceutical over time results in the need for
larger volume (and
hence greater number of particles) to obtain the same dose. For
imaging, a large field of
view camera with a parallel hole, high-resolution collimator
used. Images should
contain at least 750,000 counts.
Xenon-133 (Beta-minus decay):
Xe-133 is a fission product of U-235. It has a physical half-life
5.24 days and a
low energy gamma emission (81keV) that will be rapidly attenuated
therefore, scanning is performed posteriorly. The typical dose
the exam is 15-25
mCi, most commonly inhaled, but can be administered in saline IV.
critical organ is
the trachea and airways which receive about 100 mrad/mCi/liter/min
(total dose to the
trachea is approximately 28 mrad from inhalation and 11 mrad
IV injection [R]). Less than
of the dose is absorbed by the body.
The Xe-133 ventilation scan is generally performed prior to the
MAA perfusion exam,
as downscatter from the Tc-99m would severely degrade image
Some centers perform
the Xenon exam after the perfusion study by administering a very
dose (1-2 mCi) of
Tc-99m MAA and a much higher dose of Xenon-133 (20-30 mCi). A
window setting (10%)
on the Xenon-133 gamma energy is also applied to reduce the
contribution from scatter.
Images can then be obtained in the projection which best
Computer subtraction techniques have been developed to correct
Tc-99m scatter in
the Xenon-133 energy window, but the method is difficult to apply
not routinely used.
The gamma camera is placed behind the patient's back and a bolus
of Xe133 is injected into the mouthpiece of the spirometer system
the patient begins maximal inspiration . The xenon exam
- Single breath: The patient takes a single deep inspiration and
holds it for as long as possible. This view reflects regional
ventilation and can detect approximately 66% of ventilation
associated with obstructive airway disease.
- Equilibrium: During this phase, the patient performs normal
respirations for at least 3 minutes, and 5 minutes if
while rebreathing a mixture of Xenon-133 and oxygen. This view
demonstrates the overall lung volume as tracer equilibrates
aerated portions of the lungs. It is the least sensitive for
obstructive airway abnormalities. However, adequate duration of
rebreathing ensures diagnostic quality washout phase.
- Washout: Inhalation of Xenon-133 is discontinued and the
breaths room air or oxygen while exhaling the xenon into a
trap. Xenon-133 should clear from the lungs normally in 2-3
Retention of tracer activity beyond 3 minutes is observed in
air trapping (obstructive airway disease). The washout phase of
exam will detect about 90% of abnormalities associated with
airway disease (it is the phase of the exam that is most likely
ventilation abnormalities ). It is therefore more sensitive
single breath view in this regard. Posterior oblique views may
obtained to improve spatial localization of lung zones with slow
washout . Xenon is overall more sensitive than aerosolized
for detecting obstructive lung disease.
Xenon 127 (Electron capture):
Xe-127 is cyclotron produced and has a longer physical half-life
(36.4 days) than
Xe-133. Its gamma emissions: 172 keV (25%), *203 keV (68%), and
(18%) are of
higher energy than 140 keV emission of Tc99m. It can be used
post-perfusion to evaluate
specific defects. The higher photon energies require the use of a
medium or high energy
collimator. Additionally, the xenon trap system must be more
shielded and stored
for a considerably longer period of time prior to adequate decay
Tc-99m DTPA Aerosol:
Using an aerosol delivery system that generates submicronic
particles, 30 mCi of Tc-DTPA in 3ml of saline
(3 to 5 minutes of
rebreathing on the system with the oxygen at 8 to 10 liters/min.)
delivers about 500 to
750 uCi of tracer to the lungs . This dose yields 100K counts
2 minutes on a standard gamma camera with a low energy general
collimator . The typical radiation exposure to the lungs is
mrads. This is less than the several hundred millirad exposure
rebreathing ventilation exam. The dose to the lungs is also less
that from a Kr-81
exam . Images should be acquired for 100k counts or 5 minutes.
Exposure to personnel is usually less than that delivered during a
Xenon study. The count rate of the Tc-MAA activity (measured as
per minute) in the perfusion portion of the exam must be at least
times the count rate of the acitvity in the ventilation portion of
study . Verification of the count rates allows detection of an
inadequate number of injected Tc-MAA particles- which can occur
extravasation of the injected dose .
Nebulizers produce particles that are generally between 0.5 to 2
microns in diameter .
Because of their small size, these particles can be delivered to
alveoli like gases
during inhalation. Deposition of particles larger than 1 micron in
tree is influenced primarily by inertial impaction and
larger particles will deposit more proximal in the airways.
this procedure are
that collection devices are not necessary as they are for
persistence of activity in the lungs permits images in multiple
projections to be
obtained. Disadvantages are the inability to demonstrate areas of
excessive tracer is often seen in the proximal bronchial tree in
patients with obstructive
airway disease due to turbulent airflow. Tachypneic patients also
to have prominent
central deposition of the tracer.
Radioaerosol imaging studies can be performed in patients on
ventilator assistance, but
to avoid central tracer deposition from turbulent air flow, the
ventilator should be set
to the lowest peak flow rate possible and it is best to turn off
inhalation portion of the study. Nonetheless, the correlation
and aerosol deposition is poor in ventilated patients and better
definition of ventilated
segments can be obtained by using a gas .
The ventilation portion of the exam may be performed before or
study. If it is done after the Tc-99m MAA exam, the dose placed in
should be increased to 45 mCi (or more) and the inhalation of the
continue until the count rate obtained is 3
times the count rate of a posterior Tc-99m MAA image in order to
activity present in the lungs (perfusion defects will "fill-in"
areas of pulmonary embolism). Most authorities, however, believe
images should be obtained before the Tc-99m MAA exam.
Over time, aerosolized Tc-DTPA crosses the alveolar-capillary
membrane and enters the
bloodstream where it is then filtered by the kidneys and excreted.
critical organ is
the urinary bladder. In normal patients, the pulmonary clearance
Tc-DTPA has a
half-time of over 60 minutes (1 to 1.5 hours). The disappearance
from the lungs has
been found to be more rapid in patients with pulmonary embolism
(affected lung segments
primarily), lung injury (ARDS),
fibrosis, and in
smokers (as rapid as 20 minutes). Aerosolized Tc-PYP has a
significantly slower clearance
rate than Tc-DTPA in both smokers and non-smokers.
Tc04 is vaporized in a microfurnace to produce
labeled carbon particles (hexagonal platelets of metallic
each encapsulated within a thin layer of graphite carbon ).
material is produced by heating 5mCi of Tc-pertechnetate to very
degrees Celsius) in a crucible in the presence of 100% argon gas.
soot or ash material
is produced which is thought to be a Tc-carbon particle that is so
small it acts like a
gas . The median size of the technegas particles is between
and 0.15 microns (others report 30-60 nm [11,14]), and
there is good peripheral deposition even in patients with COPD.
the material is
produced in argon, inhalation may cause transient hypoxemia; this
be overcome by
giving oxygen via nasal canula. No severe adverse reactions have
reported to date.
There is longer pulmonary retention of Technegas with no effective
clearance half-time is 6 hours- the physical half-life of Tc-99m).
Since the material
produced is not filtered and contains up to 50% of the initial
radioactivity, a large
number of appropriately sized particles are inhaled with each
Thus, only a few
inspirations (typically 2 to 10) are needed to reach an adequate
Usually about 1 mCi
is deposited in the lung. Extrapulmonary activity in the
trachea, and stomach
can be seen in about 30% of patients. The exam may be technically
inadequate in up to 15%
of patients - particularly in severely ill patients that cannot
inhalation, or in patients with very shallow or rapid breathing
However, for SPECT perfusion imaging, Technegas has a more
tracer distribution compared to DTPA, partciularly in patients
obstructive airways disease .
If the Technegas
portion of the exam is performed following the perfusion study, a
counting rate of at
least two times the count rate of the perfusion exam is considered
adequate . Remember,
if the inhaled activity is insufficient, the perfusion
will continue to
dominate the final images .
If the reaction is produced in an atmosphere which contains 2 to
oxygen, a different
agent is produced called `Pertechnegas,' which is a vapor of
pertechnetate. This agent
also demonstrates excellent distribution in the lungs, but it is
rapidly (half time clearance from the lungs is 7 to 10 minutes
administration of the agent may be necessary during the exam.
Krypton-81m (Isomeric transition):
Produced from a Rubidium Krypton-81m generator. Krypton-81m
transition, with a gamma emission of 191 keV (65%), and a physical
half-life of only 13
Because it is not a nuclear byproduct material and it decays so
stable Krypton-81, Krypton-81m is not considered an environmental
storage/collecting devices are not necessary. As the energy is
than Tc-99m, and the
physical half-life is so short, ventilation images are usually
each perfusion view without moving the patient. This provides an
ventilation and perfusion. Regions of abnormal ventilation are
as areas of diminished or absent activity. In effect, the
gas decays before it
reaches the small airspaces of poorly ventilated regions. Because
physical decay occurs
before an equilibrium distribution can be attained, washout
obstructive lung disease cannot be obtained and some zones of mild
obstructive disease may
not be detected. The radiation dose to the lungs is significantly
than with other
agents (about 15 mrad per view). Because the parent isotope
81m has a physical
half-life of about 4.5 hours, the use of the generator is usually
working day which
makes it impractical for most clinical situations.
Modifications to the V/Q exam:
Decreased Particle Count:
A decreased number of particles (approximately 100,000) should be
used when performing
perfusion imaging in some patients:
1- Severe pulmonary hypertension:
These patients have a limited pulmonary vascular reserve, and
acute right heart failure may occur. Death has been reported in
patients immediately following injection of a standard dose of
2- Right to Left shunt:
Will see immediate renal activity. May produce symptoms due to
lodging of particles in the cerebral/coronary circulations.
3- Surgically absent lung (s/p pneumonectomy)
4- Patients with poor respiratory function:
Intubated ICU patient with cardiopulmonary instability.
5- Pediatric patients:
Etiologies of V/Q Mismatch:
* Pulmonary Embolism (thrombotic, septic, air, etc.):
Acute or Chronic. Note: Fat emboli typically produce a mottled
appearance to the scan due to the presence of many small fat
* Pleural effusion/Atelectasis: More typically, atelectasis
produces a ventilatory abnormality that demonstrates normal or
minimally reduced perfusion.
* Tumor/Hilar adenopathy: Bronchi are more resistant to
compression than are the pulmonary arteries because of their
* Vasculitis/Radiation Treatment: Can reduce regional lung
perfusion. Radiation treatment results in obliteration of the
microvasculature. Perfusion defects from radiation are usually
geometric and follow the treatment port. They are typically
non-segmental. Ventilation may also be reduced in the irradiated
but it is usually less affected than perfusion.
* Pulmonary artery atresia or hypoplasia. Segmental or branch
pulmonary artery stenosis.
* Fibrosing mediastinitis can lead to central vascular
* AVM: Short circuits delivery of the particulate tracer to the
regional pulmonary precapillary arterioles.
* CHF: Multiple non-segmental perfusion defects can be seen.
* Pulmonary artery sarcoma.
* Intravenous drug use: Can see bizarre perfusion patterns
resulting from embolization of materials such as talc. Multiple
defects are most commonly seen, but larger defects may be noted
occur in the absence of ventilatory abnormalities.
Etiologies of a
(Many small and medium sized defects scattered throughout both
* CHF: May be characterized by diffuse or scattered
perfusion defects. Typically there is redistribution with
the normal perfusion gradien; i.e.. upper lobes better perfused
lower lobes in an upright patient. A fissure sign refers to an
linear area of decreased perfusion due to pleural fluid tracking
* Lymphangitic carcinomatosis: Hematogenous microemboli which
from the capillaries into the lymphatics. Contour Mapping
lymphangitic carcinomatosis appears as linear defects outlining
margins of the bronchopulmonary segments.
* Non-thrombogenic emboli: Fat, Septic, Amniotic Fluid.
* Chronic Interstitial Lung Disease.
* Primary Pulmonary Hypertension : A characteristic
perfusion pattern consisting of multiple non-segmental perfusion
defects with a normal ventilation exam has been described in
with primary pulmonary hypertension (PPH) . The pattern is
to be secondary to vasoconstrictive occlusion of small pulmonary
arteries . The degree of heterogeneity to the perfusion
correlates with the clinical severity of PPH .
Unilateral decreased perfusion (or predominantly decrease
perfusion) can be
seen in 2% to 6% of V/Q scans [1,3].
* Pulmonary embolism: Thromboembolism as a cause of unilateral
decreased pulmonary perfusion was previously felt to be
Unilateral decreased perfusion can be secondary to PE in up to
cases . Generally perfusion defects are noted in the opposite
as well. However, chronic PE has been shown to be the etiology
unilateral hypoperfusion in up to 67% of cases .
* Pulmonary agenesis: There will also be absent ventilation and
the CXR will show a small, opaque hemithorax.
* Hypoplastic lung (Pulmonary artery atresia): There is usually
ventilation to a small lung which demonstrates no evidence of
perfusion. On CXR the involved lung is usually small,
contains few normal pulmonary markings [Miller, p.68].
* Swyer-James Syndrome: Characterized by bronchial destruction.
Ventilation images with Xenon-133 reveal decreased wash-in and
clearance of the tracer on the involved side. Images acquired
Krypton-81m generally demonstrate absent ventilation. Perfusion
typically be decreased and inhomogeneous, but may be severely
and nearly inapparent. In general, however, the disorder
more severe impairment of ventilation than of perfusion in the
* Massive pleural effusion
* Tumor/Mediastinal mass: A central mass can compress or
the pulmonary artery resulting in absent perfusion.
obstruction can produce hypoxic vasoconstriction.
* Pulmonary artery sarcoma
* Aortic dissection: Results in unilateral right lung absent
perfusion due to direct compression of the right pulmonary
the intramural hemorrhage within the adjacent ascending aorta.
* Fibrosing mediastinitis: Vessels (soft walls) will be
by the progressive fibrosis prior to occlusion of the bronchi
* Shunt procedures for congenital heart disease
* Lung transplantation with non-perfusion of the native
Artifacts on V/Q scintigraphy:
* "Hot Spots": Occur due to injection of blood clots which
inadvertently formed in the syringe.
* Effusions: If the patient is scanned supine, effusions may
collect posteriorly/superiorly and mimic a defect due to
* Liver uptake on ventilation images: Xenon is fat soluble (and
somewhat soluble in blood) and it may be deposited in the liver-
especially when there is fatty infiltration. Increased RUQ
should not be mistaken for RLL trapping.
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