Cardiac Procedures & Stress Testing – Boards Practice

What does the acronym MUGA stand for?

Multiple Gated Acquisition

What two main things does a MUGA (gated blood pool) study measure?

Ejection fraction and wall motion

What radiopharmaceutical is used for a MUGA scan?

Tc-99m-labeled red blood cells

Why is a MUGA more reproducible scan-to-scan than echocardiography for measuring EF?

It counts activity directly from the blood pool averaged over hundreds of heartbeats, so it doesn't depend on operator angle

What is the most common real-world reason a MUGA is ordered today?

Serial LVEF monitoring during cardiotoxic chemotherapy

How does sinus arrhythmia affect a gated blood pool study?

It degrades the data because varying R-R intervals mismatch the frame segments; filtering bad beats lengthens the exam

In RBC labeling, what does the stannous ion (Sn2+) do?

Reduces Tc-99m pertechnetate from +7 to +4 so it binds hemoglobin inside the RBC

Inside the red cell, reduced Tc-99m binds to which part of hemoglobin?

The beta chain

Why does reduced Tc-99m stay inside the red blood cell?

Pertechnetate crosses the RBC membrane freely, but once reduced it can't efflux back out, so it's trapped

Which RBC labeling method has the LOWEST and most variable labeling efficiency?

In vivo

What is the approximate labeling efficiency of the in vivo RBC method?

About 60-90% (lowest, most variable)

What is the approximate labeling efficiency of the modified in vivo/in vitro ("in vivtro") RBC method?

About 90-95%

What is the approximate labeling efficiency of the in vitro (UltraTag) RBC method?

Greater than 95-98%, the most consistent

In the in vivo RBC labeling method, when is the Tc-99m pertechnetate injected?

15-30 minutes after the cold stannous pyrophosphate injection

Describe the modified in vivo/in vitro RBC labeling method.

Stannous pyrophosphate IV, then blood is withdrawn and incubated with Tc-99m pertechnetate in a shielded syringe, then reinjected

In the in vitro (UltraTag) kit, what is the job of sodium hypochlorite?

Oxidizes extracellular stannous ion so pertechnetate can't be reduced outside the cell

Why is free (unbound) pertechnetate bad for a MUGA?

It raises background (thyroid, stomach, salivary, urinary) and degrades the background subtraction used to calculate EF

Which RBC labeling method is preferred for serial studies where small EF changes must be trusted?

In vitro (UltraTag)

What does "in vitro" literally mean?

In glass - the labeling happens outside the body in a vial or syringe

Which RBC labeling method starts in vivo but finishes in vitro?

Modified in vivo ("in vivtro")

What is the minimum number of frames per cardiac cycle for a MUGA assessing systolic function?

16 frames (24-32 preferred)

What is the preferred frame range for a MUGA assessing systolic function?

24-32 frames per cycle

How many frames per cardiac cycle are used to assess DIASTOLIC function on a gated blood pool study?

32-64 frames

How many frames per cycle is sufficient for a stress gated blood pool acquisition?

16 frames

How long is each acquisition per exercise stage during stress gated blood pool imaging?

About 2-3 minutes per stage

What energy window is used for a Tc-99m MUGA?

140 keV with a 10% window (+/-10%)

How many total counts are typically collected for a planar MUGA?

3-7 million total counts

How many counts per frame are recommended for a planar MUGA?

200,000-400,000 counts per frame

What are the standard MUGA views?

Anterior, best-septal LAO (~45 degrees, caudal tilt), and left lateral (~70 degrees LAO)

What is the angle of the "best-septal" LAO view used to separate the LV from the RV?

About 45 degrees LAO, often with a caudal tilt

How does the beat (arrhythmia) rejection window for planar systolic-function MUGA compare with gated blood-pool SPECT?

Planar is tighter (~+/-10-15%) than gated blood-pool SPECT (~+/-15-35%)

What acquisition mode is recommended whenever arrhythmia is present on a gated study?

List mode, so beats can be re-binned retrospectively

What happens to EF if too FEW frames per cycle are acquired?

The true peak of systole can be missed, which underestimates EF

What is the trade-off of adding more frames per cardiac cycle?

Better timing resolution but fewer counts (poorer statistics) per frame

What does gated blood-pool SPECT add over planar MUGA?

Regional/segmental wall-motion assessment

Can a gated blood-pool SPECT EF be used interchangeably with a planar MUGA EF?

No - values are not numerically interchangeable without a correction factor

Define stroke volume.

The volume of blood ejected by a ventricle during one systolic contraction

What is the formula for cardiac output?

CO = stroke volume x heart rate

A patient has a stroke volume of 70 mL and a heart rate of 80 bpm. What is the cardiac output?

5,600 mL/min (5.6 L/min)

Define end-diastolic volume (EDV).

The ventricle's largest volume, right before contraction

Define end-systolic volume (ESV).

The ventricle's smallest (residual) volume, right after contraction

What is the count-based ejection fraction formula for a gated blood pool study?

EF = (EDC - ESC) / (EDC - Background)

Calculate LVEF: EDC 42,000, ESC 18,000, background 4,000.

63%

Calculate LVEF: EDC 30,000, ESC 15,000, background 5,000.

60%

Calculate LVEF: EDC 50,000, ESC 35,000, background 10,000.

37.5%

What is the normal LVEF cutoff?

50% or greater

In the MUGA EF formula, what does the numerator (EDC - ESC) represent?

The counts that left the LV during systole

Where should the MUGA background ROI be placed?

Adjacent/lateral to the LV, not over the spleen or aorta

What happens to EF if the background ROI is placed too close to the LV and picks up spleen or aortic counts?

EF is falsely raised

What happens to EF if the background ROI is placed outside the body?

EF is falsely lowered

How does arrhythmia ("diastolic frame dropout") affect a MUGA EF?

Underestimates end-diastolic counts, so LVEF is underestimated

On LAO MUGA views, breast attenuation in women can mimic what?

A photopenic defect or pericardial effusion

What does phase analysis show?

WHEN each part of the LV starts contracting (timing)

What does amplitude analysis show?

HOW MUCH each part of the LV contracts (regional stroke volume/strength)

What mathematical method does phase analysis use to fit each pixel's time-activity curve?

Fourier-based fit

Regions of the LV contracting at very different times is called what?

Left ventricular dyssynchrony

Phase analysis is increasingly used to predict response to which therapy?

Cardiac resynchronization therapy (CRT)

Which conduction problems can phase analysis help detect?

Left and right bundle branch block (LBBB/RBBB)

Which chemotherapy drugs are known to be cardiotoxic and prompt serial MUGA monitoring?

Anthracyclines (doxorubicin, daunorubicin) and trastuzumab (Herceptin)

During cardiotoxic chemo, what is the typical action if LVEF is greater than 50%?

Periodic monitoring - continue therapy

During cardiotoxic chemo, what is the typical action if LVEF is 30-50%?

Frequent monitoring - closer surveillance

During cardiotoxic chemo, what LVEF is a contraindication to continuing the therapy?

Less than 30%

According to the 2018 MUGA vs. cardiac MRI comparison, what percentage of oncology patients were misclassified at the 50-55% LVEF threshold?

20-35%

What is the valvular regurgitation index formula on a gated blood-pool study?

LV stroke counts / RV stroke counts

A regurgitation index (LV stroke counts / RV stroke counts) that is clearly elevated above 1 suggests what?

Mitral or aortic regurgitation

What patient prep is needed for a resting MUGA?

None; place 3 ECG leads with a strong upright R wave

What is the typical adult dose for a MUGA (gated blood pool)?

15-30 mCi Tc-99m RBCs

When is MUGA imaging typically started after the labeled RBC injection?

About 15-20 minutes post-injection

What part of the ECG triggers gated acquisition?

The R wave

What one question does a stress MUGA answer?

Does the LVEF go up with exercise the way a healthy heart should?

Why is a supine/semi-upright bicycle used for stress MUGA instead of a treadmill?

The chest must stay still under the camera; on a treadmill the heart would bounce out of the field of view

What Tc-99m RBC dose is typically used for a stress MUGA, and why the higher end?

About 20-30 mCi; the higher end gives enough counts for short stage acquisitions

In which view is the stress MUGA acquired throughout the study?

Best-septal LAO, kept in the same position throughout

How is workload typically increased during a bicycle stress MUGA?

About 25-watt steps every 2-3 minutes

When during each exercise stage is the gated stress MUGA image acquired?

During the last part of each stage, once the heart rate has leveled off

What is a NORMAL LVEF response to exercise on a stress MUGA?

LVEF rises by at least 5 percentage points

Resting LVEF is 60%. At peak exercise it is 62%. How is this response classified?

Abnormal - it rose by less than 5 percentage points

What normally happens to end-systolic volume with exercise on a stress MUGA?

It gets smaller (the ventricle squeezes harder)

What is an abnormal end-diastolic volume response on stress MUGA?

Clearly dilated

A new regional wall-motion abnormality in a coronary territory on stress MUGA suggests what?

Coronary artery disease (ischemia)

In aortic or mitral regurgitation, what does an EF that drops with exercise indicate?

Early warning that the LV is starting to fail, even with a normal resting EF

The EF response to exercise on stress MUGA reflects what?

Contractile reserve

If a patient can't exercise, which pharmacologic agent is used for a stress blood-pool study?

Dobutamine

Why don't vasodilators (adenosine, regadenoson, dipyridamole) work for a stress blood-pool study?

They increase blood flow, not how hard the heart squeezes

As heart rate climbs during stress MUGA and the R-R interval shortens, why does gating still work?

Each beat is still divided into the same number of frames

Which medication class blunts the heart-rate response during a stress MUGA and is held only per physician order?

Beta blockers

What does myocardial perfusion imaging compare to detect flow-limiting coronary disease?

Perfusion at rest vs. stress

Tl-201 behaves like an analog of which ion?

Potassium

How does Tl-201 enter myocardial cells?

Active transport via the Na+/K+-ATPase pump, proportional to blood flow

How do Tc-99m sestamibi and tetrofosmin localize in the myocardium?

Passive diffusion, then trapping in mitochondria driven by the negative mitochondrial membrane potential

Which myocardial perfusion agent shows significant redistribution?

Tl-201

Why does Tl-201 redistribution allow "fill-in" of ischemic areas on delayed images?

It washes out of normal myocardium faster than ischemic-but-viable myocardium

Why do Tc-99m sestamibi/tetrofosmin studies need separate rest AND stress injections?

They show minimal to no redistribution - each injection is a fixed snapshot

Over what time does Tl-201 redistribution imaging occur?

3-4 hours after stress (or a 24-hour delay option)

In a 1-day Tc-99m rest-stress protocol, which dose is given first?

The lower rest dose, then the larger stress dose

Why must the rest dose come before the stress dose in a 1-day Tc-99m protocol?

Stress-then-rest would leave residual activity that contaminates and obscures reversible stress defects

What is the typical rest dose for a 1-day Tc-99m sestamibi/tetrofosmin protocol?

About 8-12 mCi

In a 1-day Tc-99m protocol, how much larger is the stress dose than the rest dose?

About 3-3.5 times larger (~24-36 mCi)

What dose is given each day in a 2-day Tc-99m MPI protocol?

About 25-30 mCi each day

What is the advantage of a 2-day Tc-99m protocol?

Better counts/target-to-background in larger patients (full dose each day)