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294 P A R T III / Assessment of Heart Disease
clinical question at hand. If a provider is attempting to establish
the diagnosis of coronary artery disease, then it is recommended MAGNETIC RESONANCE
that cardiac medications be held. In a patient with known disease IMAGING
undergoing risk stratification, continuing medications is a reason-
able option. Patients should be warned that they may activate ra- MRI is a diagnostic noninvasive imaging modality that does not
diation detectors (e.g., at the U.S. border crossings and airports) use ionizing radiation like CT or nuclear imaging. It is composed
in the days following injection of radiotracers. A letter is com- of two parts: (1) an external superconducting magnet, and (2) var-
monly given to a patient planning a trip in the days following ious coils. The superconducting magnet is described in terms of
SPECT or PET imaging.
the density of its field strength. Tesla (T) is a unit of measure for
magnetic field density: the higher the tesla, the greater the mag-
netic field density. The most common field density of clinical
Radionuclide Ventriculogram
magnets usedfor cardiovascular imaging is 1.5 T. However, this
A radionuclide ventriculogram (RNVG), also known as an density may change as the field progresses. The other major com-
MUGA (multiple gated acquisition) study, is a procedure in ponent of MRI is an array of severaldifferent types of coils. Vari-
which a small amount of the patient’s blood is withdrawn, la- ous coils are responsible for transmitting radiofrequency (RF)
beled with a technetium 99m-labeled radionuclide, and then pulses, transmitting magnetic gradient pulses, and receiving RF
reinjected. Using the electrocardiogram (ECG) signal for tim- pulses (echoes). These echoes are then processed into an image.
ing (gating), images are acquired throughout the cardiac cycle. Protons, the positively charged nucleus of atoms, possess a
Radioactive scintillation counts from corresponding time seg- concept known as “spin.” “Spin” is the movement of the positive
ments are summed to augment image clarity. In this way, the charge around the nucleus and possesses magnetic fields. These
manner in which the radioactivity (and hence the blood pool) protons are oriented randomly in nature. When protons of the
changes over the cardiac cycle is demonstrated. 27 This summed object to be imaged are exposed to the environment of a super-
cardiac cycle can be played back as a cine loop video display of conducting magnet, randomly oriented magnetic fields become
a normally recurring cardiac cycle. LV ejection fraction (EF), oriented in one of two ways: a higher energy state or a lower en-
right ventricular (RV) EF, and segmental wall motion can be ergy state. To recap, the object that is placed into the bore of the
precisely calculated. The video display resembles a contrast LV magnet now contains protons that are no longer oriented ran-
or RV angiogram. Variation in radioactive counts over time is domly but are oriented in one of two energy states.
analyzed to provide information about systolic function. Coils transmit RF pulses and magnetic gradient pulses to the ob-
Rhythm disturbances, such as atrial fibrillation or frequent pre- ject within the bore of the magnet. This energy is used to change
mature contractions altering beat-to-beat filling and cycle the direction of the magnetic fields of the protons to assume differ-
z
length, may decrease quantitative accuracy. RNVG is com- ent energy levels in different imaging axes (x, y, z). After the trans-
monly used to evaluate cardiac function in patients with heart mitted pulses are turned off, the protons reassume their energy state
failure and valvular heart disease or to monitor the potential prior to the transmission of the RF pulse (the higher or lower en-
cardiotoxic effects of the commonly used chemotherapeutic ergy states). This change in energy state requires releasing the previ-
agents. ously absorbed energy and generating an echo. Coils receive these
echoes and, based on the timing and location, generate an image.
Patient Preparation for RNVG In order to yield diagnostic-quality cardiovascular images, both
There is no special patient preparation for an RNVG study. The respiratory and cardiovascular motion must be taken into ac-
patient may continue to take all medications. The entire study count. Respiratory motion is compensated for by either breath
usually takes approximately 30 minutes. Patients should be holding or acquiring images based upon the movement of di-
warned that they may activate radiation detectors (e.g., at U.S. aphragm during respiration. Acquiring data over multiple cardiac
border crossings and airports) in the days following injection of cycles and thus gating acquisition to the patient’s ECG, compen-
radiotracers. A letter is commonly given to a patient planning a sates for cardiovascular motion.
trip in the days following an RNVG. Gadolinium chelates are the most commonly used IV MRI
contrast agents. Gadolinium, an extracellular compound, alters
the magnetic properties of adjacent protons within the patient.
Risks of Radionuclides
The need for contrast depends upon the diagnostic indication for
In contrast to radioactive substances used for therapeutic (tissue
the study. Indications for gadolinium in cardiovascular imaging
ablation) purposes, radiopharmaceuticals used for imaging have
include myocardial viability, stress perfusion, vessel angiography,
short half-lives (minutes to several hours), contributing to their
or mass perfusion. Oral contrast agents are not used currently for
relatively rapid decay in the body. They are used in very small
MRI. Gadolinium is excreted from the body by the kidneys and
amounts. Thus, there is no need to isolate patients who have had
can be removed with dialysis.
these studies, and no particular precautions are needed for dis-
posal of body substances (including blood, urine, or stool). The
risk to a fetus is small, but it is recommended that patients who Diagnostic Indications
are pregnant or breast-feeding not undergo injection of ra- for Cariovascular MRI
dionuclides. Personnel should remember that radioactivity de-
creases dramatically with distance from the source. Personnel Cardiovascular MRI provides diagnostic information on anatomy,
who work in nuclear medicine departments wear detection physiology, and tissue characterization (Table 14-1). MRI pro-
badges like those worn by radiology personnel to monitor their vides anatomicaldescriptions of the size, location, and connection
exposure. of all four cardiac chambers and valves (Fig. 14-3), which is