An important application of cardiac MRI
is to characterize tissues using special pulse sequences. Tissue characterization can be useful for subjects with cardiac masses and pericardial disease, as well as intrinsic myocardial diseases such as ischemic heart disease and cardiomyopathies. The most widely used tools are T1- and T2-weighted imaging, fat suppression, and gadolinium contrast
enhancement in both the early (less than 5 min) and delayed (greater than 10-15 min) phases. Each of these tools is reviewed in this chapter. Examples of how different tissue characterization methods can be used to assess cardiac masses, myocardial viability, and arrhythmogenic right ventricular displasia are then considered (with full protocols provided at the end of the chapter). The assessment of ischemic myocardium is discussed in more detail in Chapter III-7
provide anatomic assessment of the myocardium and great vessels, help characterize lesions with short T1 relaxation
times, and assess contrast
enhancement. T1 weighting can be achieved using spin echo
or spoiled gradient echo sequences. To improve conspicuity of myocardial and pericardial anatomy and pathology, black-blood images are often desired. Consequently, spin echo
or fast spin echo
) sequences with blood-nulling pulses are usually preferred over bright blood spoiled gradient echo sequences (Figure III5-1
In addition to providing anatomic images of the heart and chest, T1-weighted sequences may also help to characterize some tissues, particularly those that have short T1 relaxation
times and are therefore hyperintense on T1-weighted images
. Tissues with short Tl times are listed in Table III5-1
. With respect to detecting cardiac masses, the most common sources of confounding signal are fat and hemorrhage. It is important to recall that time-of-flight effects can also give rise to high signal on T1-weighted imaging and can result in a potential pitfall of interpretation.
For cardiac diagnoses, Tl-weighted spin echo
or fast spin echo
sequences are ECG
-gated. Since the TR must be set to one R-R interval
, image contrast
depends on the subject’s heart rate. If the heart rate is slow, the TR may approach or exceed 1000 msec
, reducing Tl weighting. The TE should be as short as possible for T1 weighting, typically under 50 msec
FIGURE III5-1. Short axis
T1-weighted (a) fast spin echo
and (b) spoiled gradient echo imaging.
[right half black circle] TABLE III5-1
Tissues Bright on T1-Weighted Images
Lipid (fat and sebaceous material)
Some forms of hydrated calcification
Gadolinium, manganese, magnesium, copper, and certain other metals
Flow or time-of-flight effects (pitfall)
T2-WEIGHTED AND T2*-WEIGHTED IMAGING
T2-weighted images of the heart are valuable in the detection and characterization of pathology with long T2 times, which yield high signal intensity
on these images. They are generally performed with spin echo
or fast spin echo
sequences. Depending on the subject’s heart rate, the TR is usually defined to be at least 2 R-R interval
s with a TE of 80-100 msec
. To detect high signal intensity
pathology, blood nulling is desirable. With fast spin echo
sequences, fat is high in signal intensity
, so fat suppression will improve lesion conspicuity.
Pericardial effusion (arrows) is high in signal intensity
on a steady-state free precession
gradient echo sequence because of its high T2/T1 ratio.
Occasionally, it may be desirable to obtain T2*
-weighted images that are sensitive to susceptibility effects.
For example, for the diagnosis of hemochromatosis, a long-TE spoiled gradient echo sequence will demonstrate decreased signal intensity
because of the susceptibility effects of iron in the myocardium (Figure III5-3
Low signal intensity
in the myocardium on spoiled gradient echo imaging, caused by iron deposition that is due to primary hemochromatosis.
A mass in the interatrial septum has high signal intensity
on Tl-weighted imaging (left, arrow). With a frequency
-selective fat-suppression pulse (right), the mass becomes uniformly hypointense, indicating that the lesion is a lipoma.
Fat suppression may be useful in cardiac MRI
to help distinguish whether lesions that are high in signal intensity
on Tl-weighted images contain fat. It is also used to improve lesion conspicuity on T2-weighted fast spin echo
images, where fat has high signal intensity
Two approaches can be employed to achieve fat suppression: a frequency
-selective fat suppression prepulse
or an inversion recovery prepulse
with short inversion time set to null tissues with short T1, such as fat (Figure III5-4
). For tissue characterization, the frequency
-selective method is more specific.
When fat suppression is needed to enhance lesion conspicuity with T2-weighted images, inversion recovery sequences are frequently used. To ensure blood nulling as well as fat suppression on T2-weighted FSE
images, triple inversion recovery sequences may be used, as described in Chapter III-3
Periaortic cystic lymphangioma in the retroperitoneum shows uniform low signal intensity
on T1-weighted gradient echo image (left) and high signal intensity
on T2-weighted fast spin echo
image (middle). Following contrast
administration, no enhancement is seen in the cyst on a coronal fat-suppressed T1-weighted gradient echo image (arrows, right).
-enhanced Tl-weighted imaging can also help to characterize pathology in the heart and pericardium. Most commonly, contrast
material is useful to differentiate enhancing masses, such as cardiac tumors, from nonenhancing pathology, such as cysts (Figure III5-5
) and thrombi. To assess the vascularity of a lesion, the delay time between injection and imaging is typically less than 5 minutes.
enhancement is assessed with black-blood ECG
-gated T1-weighted spin echo
sequences. Alternatively, fat-suppressed T1-weighted spoiled gradient echo imaging can be used (Figure III5-5
DELAYED CONTRAST ENHANCEMENT
One of the most active and still growing areas of cardiac MRI
is the use of delayed contrast
to characterize myocardial infarction or, alternatively, to assess myocardial viability. Following intravenous injection of extracellular contrast
agents such as gadolinium chelates, areas of infarct and fibrosis in subacute and chronic infarcts demonstrate delayed enhancement and delayed washout relative to viable myocardium. Delayed enhancement has also been demonstrated in acute infarcts and other diseases listed in Table III5-2
, although its patterns of distribution vary for different etiologies.
[right half black circle] TABLE III5-2
Causes of Delayed Contrast Enhancement
Subacute or chronic myocardial infarct
Acute myocardial infarct
Arrhythmogenic right ventricular dysplasia
About 10-30 min after injection, delayed washout of contrast
material from infarcted myocardium will cause it to appear slightly hyperintense relative to uninfarcted myocardium. However, this difference may be difficult to detect on routine Tl-weighted imaging (and with other modalities such as contrast
-enhanced computed tomography, CT
). To enhance the conspicuity of infarcted tissue, an inversion-recovery gradient echo sequence is used whereby the inversion time is selected to null signal from uninfarcted myocardium (Figure III5-6
). When the uninfarcted myocardium is nulled, hyperintense signal from the infarcted or abnormal myocardium becomes dramatically more obvious.
The pulse sequence
is illustrated in Figure III5-7
. With this ECG
-triggered inversion recovery sequence, the 180° inversion pulse is applied about 150-200 msec
after the R wave so that imaging occurs during diastole. Then after an appropriate inversion time, when the uninfarcted myocardium is crossing its null point
, the gradient echo acquisition is performed. Either a spoiled gradient echo sequence or a steady-state free precession
sequence can be used with k-space
segmentation. The inversion time varies across subjects and also depends on factors such as contrast
agent dose and time after injection. Selection of the optimal inversion time is critical to image quality.
Improved conspicuity of delayed enhancement of subendocardial myocardial infarct (arrows) with inversion recovery methods (right) compared with T1-weighted fast spin imaging (left). Both images were obtained 10 min following intravenous contrast
material, but the inversion recovery sequence was performed using an inversion time selected to null uninfarcted myocardium.
Inversion recovery sequence achieves nulling of viable myocardium by careful selection of the inversion time centric filling of k-space
ensures optimal suppression. Data are acquired every other heartbeat to allow recovery of longitudinal magnetization
FIGURE III5-8. Selected short (top row) and long axis (bottom row) views of the left ventricle to assess myocardial viability show extensive subendocardial infarction (some, but not all regions are marked with arrows).
To assess left ventricular viability or infarct, the inversion recovery sequence is performed for a series of short axis
images from the left ventricular base to the apex. Horizontal and vertical long axis
planes confirm short axis
findings (Figure III5-8
Choosing an Inversion Time
The selection of inversion time to null the uninfarcted myocardium can critically affect the diagnostic value of the infarct images. To understand why, recall from Chapter I-7
) that most MR
images are magnitude image
s. Magnitude image
s do not distinguish between signal from protons that are flipped below the xy plane and signal from protons that have recovered above the xy plane. By relying on magnitude data alone, patterns of tissue contrast
vary dramatically with different inversion times (Figure III5-9
). For example, at the null point
of viable myocardium, infarct will have signal intensity
is higher than that of blood. However, at the inversion time of the infarcted myocardium, image contrast
will be inverted, and the viable myocardium will have the greatest signal intensity
FIGURE III5-9. Signal intensity
of cardiac tissues following an inversion pulse, with darker lines depicting the magnitude or absolute value of the magnetization. Image contrast
at time points preceding the null point
can be confusing.
Selection of the inversion time that nulls normal myocardium is commonly performed by successive approximations. Inversion times range from 200 to 350 msec
for most subjects when imaged 10-20 minutes after administration of about 0.1 to 0.2 mmol per kg of body weight of gadolinium chelate. It is common for the nulling inversion time to change slightly over the course of the examination.
Inversion Time Mapping Sequence
Only gold members can continue reading. Log In