Page 144 - Feline Cardiology

Chapter 11: Hypertrophic Cardiomyopathy 143

In human cardiology, cardiac MRI (cMRI) is frequently the cat with 2 leads placed cranially at the level of the
used to evaluate cardiac function, calculate LV mass, and heart just to the left and right of the sternum, and 2
assess regional wall motion abnormalities in various car- caudal leads placed in the left and right ventral caudal
diomyopathies (Devlin et al. 1999; Soler et al. 2003; Rathi abdomen, with maximal space between the cranial and
et al. 2004). Delayed gadolinium contrast enhancement caudal leads. The lead with the highest amplitude R
is seen in regions of myocardial fibrosis in patients with wave is chosen for gating. If S waves have greater ampli-
diseases such as HCM (Moon et al. 2003, 2004; tude than R waves, that lead may be selected and inverted.
Choudhury et al. 2002). Advantages of cMRI over echo- The pulsed gradient fields may distort the ECG once the
cardiography include independence from an acoustic patient has been placed inside the MRI machine, and
window, limitless imaging planes, no translational arti- respiration often induces significant ECG artifact. ECG
facts, more accurate 3-dimensional data acquired for gating compensates for cardiac motion and synchro-
determination of intracardiac volumes and myocardial nizes image acquisition to the cardiac cycle. Prospective
mass, ability to detect myocardial fibrosis or infarction ECG gating is routinely used with fast gradient echo Cardiomyopathies
via contrast enhanced imaging, information on myocar- sequences, but it may not acquire the first 10% and the
dial perfusion, and more comprehensive visualization of last 20% of the cardiac cycle. For diastolic function
cardiac and vascular structures (Devlin et al. 1999; studies, a retrospective ECG gating technique using cine
Grothues et al. 2002; Bogaert et al. 2003). Major limita- pulse sequence should be utilized that acquires data
tions of cMRI in veterinary medicine are the require- regardless of the ECG and then retrospectively calculates
ment for general anesthesia for image acquisition, the the appropriate cardiac phases based on the stored ECG
high cost, a lack of experience in obtaining and analyzing and imaging data. If ECG gating is impossible, periph-
cMRI, and lack of efficient semiautomated image analy- eral pulse gating from the lingual artery may be used,
sis. Currently, cMRI in animals is limited to cardiovascu- where gating should occur on the sharp upstroke of the
lar research settings. arterial flow tracing.
Spin echo and gradient echo techniques are the two
Technique of cardiac MRI in cats main pulse sequences used in cMRI. The spin echo tech-
In order to perform cMRI with high spatial and tempo- nique is often used for examining cardiac anatomy,
ral resolution, a state-of-the-art MRI machine is neces- where the blood pool is black and the myocardium is
sary. Cardiac MRI requires a strong magnetic field of at white (hyperintense). Spin echo is often utilized to eval-
least 1–1.5 Tesla, which allows for high temporal resolu- uate for arrhythmogenic right ventricular cardiomyopa-
tion at fast heart rates. Strong and fast gradient coils thy, where fatty infiltration of the myocardium is better
(15 mT/m) are necessary. Surface phased array receiver identified (Tandri et al. 2003). Gradient echo and cine
coils may be useful to define a smaller field of view MRI are often used for cardiac function analysis, where
(FOV), improve signal to noise ratio (SNR), and improve the blood pool is white and the myocardium is dark gray.
spatial resolution in small animals. The three major sources of artifact with cMRI are respi-
Electrocardiographic (ECG) gating is mandatory for ratory motion, blood flow artifact from rapid or slow
adequate image acquisition. With ECG gating, image flowing blood, and cardiac motion. Turbulent blood
acquisition is synchronized to occur during the same flow creates a signal void (black) when using gradient
instant of the cardiac cycle for several hundreds of con- echo sequencing. ECG gating and a fast acquisition rate
secutive heartbeats. This “snapshot” approach delivers reduce cardiac motion artifact. Breath-hold imaging (at
hundreds of static images throughout the cardiac cycle end-expiration) negates respiratory artifact.
of the beating heart. Without ECG gating, the cardiac During image acquisition, three-plane spatial local-
motion would render an MRI image uninterpretable, in izing images are first obtained, depicting the heart in
contrast to an MRI image of a static structure (e.g., sagittal, transverse, and frontal views. With the patient
brain) wherein gating is not needed. Obtaining a quality in dorsal recumbency, the long-axis of the heart is ori-
ECG tracing for proper ECG gating on the R wave is a ented obliquely to the localizing imaging planes. The
common initial obstacle in cats because of their small sagittal view often resembles the lateral view of a tho-
QRS complexes. In people, 2–5% of cases have an unreli- racic radiograph, where the LV apex and left atrium are
able ECG signal, and triggering must be based on visualized. Initially a long-axis view is acquired primarily
peripheral pulse gating. The ventral surface of veterinary from the sagittal view where a line is placed connecting
patients must be shaved before MRI compatible special- the LV apex and the center of the mitral valve, and slices
ized ECG electrode pads (Quatrode®, In Vivo Research, are adjusted to bisect the midleft ventricular level at the
Inc.) are placed. Electrode position may vary between 2–3 o’clock position of the heart on the frontal or cross-
patients, but generally the optimal placement in cats is sectional view. Appropriate 4-chamber long-axis views
in a rectangular arrangement on the ventral surface of depict a symmetrical LV with adequate visualization of