## Mri Pulse Sequence Diagrams

■■ Mx + i ■ My ~ p (x, y) • exp[-ikxx] ° p[cos (kxx) + i ■ sin (kxx)]

Fig. 6. An array of magnetic moments stacked up along the horizontal axis. In this example, only the transverse component of the magnetization is shown as vectors, aligned initially with the horizontal axis, after application of a radiofrequency pulse. The phase <p of the transverse magnetization is measured here with respect to the horizontal axis. (A) Without application of a phase-encoding pulse, all magnetization vectors point in the same direction (center graph for Gp = 0). (B) Applying a phase-encoding gradient pulse in the direction of the horizontal axis will result in a variation of the phase of the transverse magnetization in the direction of the phase-encoding gradient. (C) Increasing the amplitude of the phase-encoding gradient will increase the phase warp. The transverse magnetization components Mx and My vary sinusoidally, such as Mx ^ cos(kxx) for a phase encoding pointing in the x direction.

proportional to the local spin density p(x,y) in the slice plane, and sinusoidal dependence on the phase angle q>(x):

The quantity kx is the spatial frequency at which the x and y components of the transverse magnetization (Mx and My) are modulated by application of the phase-encoding gradient in the x direction. We refer to the variation of the magnetization phase as phase warp.

For a 1D distribution of spins, as in the example of Fig. 6, the signal that is recorded after a phase encoding can be represented as a sum or integral, with each contribution weighted by a phase factor to account for the phase warp produced by a phase encoding gradient:

The index i is a label for the phase-encoding pulse. St is the ith Fourier component of the spin density distribution p(x). L denotes the spatial extent of the object to be imaged; that is, p(x) = 0 when |x| > L. The spin density distribution p(x) can be reconstructed if the phase encodings are repeated for a range of kx values to sample the spin density distribution at different length scales or spatial frequencies. The phase encodings are repeated N times to achieve a spatial resolution in the direction of the phase-encoding gradient of Ax = L/2N.

Fig. 6. An array of magnetic moments stacked up along the horizontal axis. In this example, only the transverse component of the magnetization is shown as vectors, aligned initially with the horizontal axis, after application of a radiofrequency pulse. The phase <p of the transverse magnetization is measured here with respect to the horizontal axis. (A) Without application of a phase-encoding pulse, all magnetization vectors point in the same direction (center graph for Gp = 0). (B) Applying a phase-encoding gradient pulse in the direction of the horizontal axis will result in a variation of the phase of the transverse magnetization in the direction of the phase-encoding gradient. (C) Increasing the amplitude of the phase-encoding gradient will increase the phase warp. The transverse magnetization components Mx and My vary sinusoidally, such as Mx ^ cos(kxx) for a phase encoding pointing in the x direction.

2.10. 2D Imaging With Phase Encoding and Readout Gradients

The combined use of phase-encoding gradients and readout gradients allows progress beyond the determination of 1D density profiles and imaging of the density of spins in a thin slice, with x and y denoting the two orthogonal axes in the slice plane. T1 and T2 relaxation effects are neglected in the following discussion for simplicity. The (relative) variation of the phase along the x direction, which was produced by a previous phase-encoding gradient pulse, is preserved if we now switch on a (readout) gradient in the orthogonal y direction. The readout gradient Gy in the y direction is left on while the signal is detected.

A 2D data matrix is built up by repeating the signal readouts for different kx values (i.e., phase-encodings). This means that the radiofrequency excitation, phase encoding pulse, and readout gradient pulses are repeated, and for each repetition, the phase-encoding gradient amplitude is incremented to cover a range of kx values centered around kx = 0. The time it takes to repeat this group of encoding steps is called the repetition time; it is one of the basic parameters that determines the total time for acquiring an image.

The final result of repeating the radiofrequency excitation, readouts, and phase encodings M times is a data matrix S(kx,ky),

Fig. 8. Pulse sequence diagram for an imaging sequence that uses gradient echoes for readout of the signal. The initial radiofrequency pulse generally has a flip angle much smaller than 90° to maximize the signal when the repetition time is relatively short (~2-10 ms) compared to repetition times of spin-echo imaging sequences. The gradient echo results from two gradient pulses on the read channel with opposite polarity. The maximum amplitude of the gradient echo occurs when the integral of the gradient waveforms is zero, that is, when the areas under the negative and positive polarity lobes of the gradient waveforms cancel out. PE, phase encoding; R.F., radiofrequency; TE, echo time.

Fig. 8. Pulse sequence diagram for an imaging sequence that uses gradient echoes for readout of the signal. The initial radiofrequency pulse generally has a flip angle much smaller than 90° to maximize the signal when the repetition time is relatively short (~2-10 ms) compared to repetition times of spin-echo imaging sequences. The gradient echo results from two gradient pulses on the read channel with opposite polarity. The maximum amplitude of the gradient echo occurs when the integral of the gradient waveforms is zero, that is, when the areas under the negative and positive polarity lobes of the gradient waveforms cancel out. PE, phase encoding; R.F., radiofrequency; TE, echo time.

which can be Fourier transformed to obtain the spin density p(x,y). The spatial resolution of the resulting image in the phase encoding Ax and readout directions Ay is determined by the number of readout samples N and the number of phase encodings that were applied: Ax = Lx/2N and Ay = Ly /2M. The field of view dimensions Lx and Ly are determined by the magnitude of the sampling steps in data space (Akx, Aky): LXJ = 2n/Akx>y.

### 2.11. Spin-Echo Imaging

In a spin-echo imaging sequence, the readout gradient is applied during the formation and decay of a spin-echo. A single encoding step starts with a slice-selective 90° pulse. The phase-encoding gradient can be applied immediately afterward; simultaneously, a "dephasing" gradient pulse can be applied along the readout axis. A (slice-selective) 180° pulse at time t = 2x causes the appearance of a spin-echo at t = x. A readout gradient pulse is centered on the echo center. The basic building block of the spin-echo sequence, shown in Fig. 7, is repeated N times for N phase-encoding steps.

A train of echoes can be produced by a string of 180° pulses that are spaced 2x apart and result in a considerable speedup of the image acquisition. This technique is therefore called fast spin-echo imaging to distinguish it from the single-echo variant described above.

### 2.12. Gradient Echo Imaging

Spin-echoes provide an effective means of refocusing the transverse magnetization. A similar, but nevertheless different, type of echolike effect can be achieved by applying two gradient pulses of opposite polarity instead of a 180° radiofrequency pulse. The first gradient pulse causes a rapid dephasing of the transverse magnetization. The second gradient pulse, of opposite polarity, can reverse this effect. An echo-type signal is observed and peaks at the point at which the phase warp produced by the first pulse is cancelled. This type of echo is called a gradient echo. A pulse sequence diagram for a gradient echo imaging sequence is shown in Fig. 8.

A train of gradient echoes can be created by consecutive pairs of dephasing and rephasing gradient waveforms. The acquisition of multiple phase-encoded gradient echoes after a single radiofrequency excitation is useful for very rapid image acquisition, but is limited by the T2* decay of the signal.

A variation of the gradient echo technique that reestablishes phase coherence to the best possible degree before application of the next radiofrequency excitation (i.e., the next phase-encoding step) can be used to produce a steady state. This allows the application of radiofrequency pulses with fairly large flip angles. Instead of relying on T1 relaxation to return

Fig. 9. Gradient echo images of phantom tubes filled with saline solution and different concentrations of a paramagnetic substance that reduces the Tj of saline in proportion to its concentration in the saline solution. The saline samples with the shortest Tj appear brightest because the saturation of a Tj-weighted signal with a gradient echo sequence is weakest for the short Tjs. The image to the left, acquired for a shorter repetition time (TR) than the image on the right, has the stronger Tj weighting. The signal intensity contrast between different saline samples is more pronounced for the image on the left, acquired with a shorter repetition time, because of the stronger Tj weighting. The images were acquired with a relatively short echo time (TE) of 4.8 ms to minimize contrast from differences in T2. The paramagnetic substance in the saline samples was an aqueous solution of gadopentetate dimeglumine, a W-methylglucamine salt of the gadolinium complex of diethylenetriamine pentaacetic acid, which is a magnetic resonance contrast agent routinely used in clinical practice.

the magnetization from the transverse plane to the B0 direction, the magnetization is toggled back and forth by the radiofrequency pulses between the z-axis and the transverse plane. The attainable signal-to-noise ratio with this approach is significantly higher than with "conventional" gradient echo imaging. This type of gradient echo imaging is referred to in the literature by various acronyms: "steady-state free precession imaging," "true FISP," or "balanced fast field echo imaging." In particular, for cardiac cine studies, this technique has led to a marked improvement of image quality. Steady-state free precession works best with very short repetition times, which in turn impose high demands on the gradient system of the MR scanner in terms of ramping gradients up and down.

### 2.13. Contrast Weighting

Biological tissues and blood have approximately the same density of jH nuclei, and spin density images show poor contrast to differentiate, for example, tissue from blood or fat from muscle. One of the most appealing aspects of MRI is the ability to manipulate the image contrast based on differences in the Tj or T2 relaxation times. For a gradient echo sequence, the Tj weighting is determined by the combination of flip angle and repetition time. Reducing repetition time or increasing the flip angle increases the Tj weighting in the image. The same applies to a spin-echo sequence (Fig. 9).

The T2* weighting of a gradient echo image is controlled by the time delay between the radiofrequency pulse and the center of the readout window, that is, the echo time TE. The T2 weighting of the spin-echo signal is similarly determined by the echo time. In a fast spin-echo sequence, multiple echoes can be used to read out the signal with different phase encodings for each echo. The echo chosen for reading out the phase encodings with low k values will determine the effective T2 contrast of the image. We note that the "coarse" features of an image and the image contrast are determined by the low spatial frequency components in the data matrix S(kx ,ky). If the first few echoes in an echo train are chosen for the low k value encodings, then the T2 weighting is less than if these phase encodings are instead moved to the later echoes in the echo train.

Controlling the Tj weighting through adjustment of repetition time and the flip angle imposes some limits that can be circumvented by applying an inversion pulse before the image acquisition and performing the image acquisition as rapidly as possible. The time between the inversion pulse and the start of the image acquisition controls the Tj contrast in this case. The image acquisition after the magnetization inversion is typically performed with a gradient echo sequence that uses small flip angles; that is, the Tj contrast is controlled by the prepulse and the delay after the prepulse, instead of the repetition time, and the flip angle a of the gradient echo image acquisition. Gradient echo imaging with a magnetization preparation in the form of a 180° or 90° radiofrequency pulse is often the method of choice for rapidly acquiring Tj-weighted images of the heart.

MR contrast agents provide a further means for controlling the image contrast by injecting a compound with paramagnetic ions that reduce the Tj of blood and tissue permeated by the agent. The local Tj reduction depends on: (j) delivery of contrast agent to the tissue region through the blood vessels, (2) the degree to which the contrast agent molecules can cross barriers such as the capillary barrier, and (3) the distribution volume of the contrast agent within the tissue. The contrast seen after injection of such an agent can be used to determine pathology, such as the breakdown of the cardiac cell membranes or an above-normal concentration of contrast agent in infarcted myocardium.

3. MRI OF CARDIAC ANATOMY,

FUNCTION, PERFUSION, AND VIABILITY

### 3.1. Cardiac Morphology and Tissue Characterization

Spin-echo techniques are the method of choice for imaging cardiac anatomy and for tissue characterization. Figure j0 shows an example of a T2-weighted fast spin-echo image of a canine thorax with an in-plane resolution of approx 1.2 mm and for a 4-mm thick slice.

### 3.2. Cardiac Function

MRI is now considered the gold standard for assessing the hemodynamics of the ventricles of the heart and measuring parameters such as ejection fraction, end-diastolic volume, end-systolic volume, stroke volume, and cardiac mass. Cine loops are acquired to follow the changes in ventricular dimensions

Fig. 10. Cardiac anatomy of a canine heart imaged with a T2-weighted fast spin-echo sequence and in-plane resolution of 1.2 mm. Cardiac structures such as the left ventricle (LV), right ventricle (RV), and aorta are labeled. The signal from blood in the ventricular cavities was nulled by a magnetization preparation consisting of radiofrequency inversion pulses. Furthermore, the use of an echo train, with seven spin-echoes in this case, and a long effective echo time also causes attenuation of the signal from moving blood. These so-called black-blood imaging techniques are very useful for anatomical imaging to avoid image artifacts from flowing blood.

Fig. 11. Illustration of the principle of segmented acquisitions of data as used for imaging multiple phases of the cardiac cycle in ventricular function studies. The image acquisition is synchronized to the cardiac cycle by triggering of the pulse sequence with the R-wave on the electrocardiogram (ECG). The total number of phase encodings is split into five groups or segments in this example. The same five phase encodings are performed during each phase of one cardiac cycle. During the next R-to-R interval, five other phase encodings are performed for each cardiac phase. The R-wave-triggered acquisition of phase encodings is repeated k times to obtain a total of k -5 phase encodings. The temporal extent of each cardiac phase is shown in the diagram by the boxes that contain the symbolic representations of the phase-encoded lines as vertical lines. The temporal resolution of the resulting cine loop is determined by the number of lines per segment (five in this example) and the repetition time for each phase-encoding step. Typical resolutions are on the order of40-50 ms for resting heart rates and higher during inotropic stimulation of the patient's heart. The image acquisition is performed while the patient holds his or her breath. In this example, the required duration of the breath hold would be k heartbeats, with k typically on the order of 10-20, depending on the heart rate. TR, repetition time.

over the entire cardiac cycle and assess cardiac function. The acquisition of each image in the cine loop is broken up into several "segments," and the image segments are acquired over consecutive heartbeats, as shown in Fig. 11.

The acquisition of such image segments for each cardiac phase is synchronized with the heart cycle by gating of the encoding steps with the patient's electrocardiogram. This technique works well as long as the subject has a regular heartbeat. The final result of the segmented acquisition is a series of images, one for each phase of the cardiac cycle. These images can be played as a cine loop (e.g., to assess ventricular function). The sharpest quality of images can be obtained by having the patient hold his or her breath during image acquisition.

The segmented data acquisition approach (3) always involves a tradeoff between temporal resolution (i.e., number of frames covering one R-to-R interval) and spatial resolution, because the image acquisition needs to be performed within a time short enough to allow for suspended breathing. With cardiac ultrasound, the image acquisition is rapid enough that it can be performed in "real-time" mode, something that only now is considered possible with MRI. To date, obtaining MRI realtime frame rates of 10 frames/s still involves significant compromises in terms of spatial resolution. In patients without a (regular) sinus rhythm, new real-time MRI techniques have started to offer a viable alternative.

### 3.3. Myocardial Tagging

Cine imaging of the heart can be combined with a series of magnetization preparation pulses that null the longitudinal magnetization along thin parallel stripes in the slice plane. The stripes appear as black lines on the MR images. This stripe pattern is created immediately after the R-wave and before acquisition of the segmented phase encodings (Fig. 11). The stripe lines visible in the resulting images are "embedded" in the tissue and are therefore distorted if any myocardial motion occurs. Thus, intramyocardial displacements can be tracked through monitoring visible motion of the tag lines.

Figure 12 shows an example of a myocardial stripe pattern laid down at end-diastole and, in a second frame, the same pattern at end-systole with evident distortion of the tag lines because of myocardial contraction. The tag lines, created right after the R-wave, tend to fade during the cardiac cycle because of T1 relaxation, but for normal resting heart rates (e.g., 60-70

Fig. 12. Images with spatial modulation of magnetization in the form of vertical stripes in a human volunteer. The vertical tag lines were created right after the R-wave of the electrocardiogram. The upper left panel shows a magnified view of the heart during this initial phase. A second image is shown on the upper right for an end-systolic phase, with the distortion of the tag lines caused by cardiac contraction clearly apparent. The tagging technique is equivalent to the implantation of intramyocardial markers. Tracking of the tag lines over the cardiac cycle allows determination of myocardial strains and has been shown to provide a sensitive method for assessing regional wall motion abnormalities.

Fig. 12. Images with spatial modulation of magnetization in the form of vertical stripes in a human volunteer. The vertical tag lines were created right after the R-wave of the electrocardiogram. The upper left panel shows a magnified view of the heart during this initial phase. A second image is shown on the upper right for an end-systolic phase, with the distortion of the tag lines caused by cardiac contraction clearly apparent. The tagging technique is equivalent to the implantation of intramyocardial markers. Tracking of the tag lines over the cardiac cycle allows determination of myocardial strains and has been shown to provide a sensitive method for assessing regional wall motion abnormalities.

beats/min), the tag lines can persist long enough to allow visualization of cardiac motion over nearly the entire R-to-R interval. Tag lines in the ventricular blood pool disappear very quickly because of the rapid motion and mixing of blood in the ventricle; this effect is useful for recognizing the endocardial border.

It has been shown that tagging techniques and analysis of myocardial strain patterns yield higher sensitivity compared to "conventional" cine MRI for the detection of mild wall motion abnormalities. Initial studies leading to these observations were confined to research studies, mostly in animal models with well-characterized levels of myocardial ischemia (4). The clinical application of MR tagging, particularly when it involves quantification of the myocardial strain patterns, is still hindered by the considerable efforts required for post-processing the images. Continuing research in this area has led to new approaches, such as the harmonic phase (HARP; 5,6) and displacement encoding with stimulated echoes (DENSE) (7) techniques, that may circumvent such bottlenecks and result in a more widespread clinical application of MR tagging techniques.

### 3.4. MRI Cine of the Heart During Stress

The use of inotropic agents such as dobutamine in combination with echocardiography is a common practice for detecting wall motion abnormalities. Catecholamines such as dobutamine increase the myocardial contractility and the heart rate, thereby causing an increased oxygen demand that may lead to acute ischemia in myocardial regions with compromised blood supply, fibrosis, or other progressive pathologies. In regions in which dobutamine induces ischemia, the endocardial excursion does not increase to the same extent as in nonischemic myocar-dial wall segments.

MR imaging under dobutamine stress is typically performed at several levels of inotropic stimulation. The dobutamine dosage is incremented at intervals of 3-4 min, starting with a dosage of 10 ^g/kg body weight per minute. The increase in dobutamine dosage is stopped at a maximum of 40 ^g/kg/min

Fig. 13. Example images for three different slice locations during different phases of contrast agent transit. Images for each slice location are arranged in horizontal rows. These images were acquired with a rapid T1-weighted gradient echo sequence (repetition time = 2.2 ms, echo time = 1.2 ms, flip angle 15°) in a patient with a perfusion defect in the lateral wall, highlighted by the arrow in the images. Cardiac motions appear frozen as the acquisition time for each image is short on the time-scale of a heart beat, and the image acquisition is synchronized to the heart rhythm by use of the R-wave on the electrocardiogram as a trigger signal for the scanner.

Fig. 13. Example images for three different slice locations during different phases of contrast agent transit. Images for each slice location are arranged in horizontal rows. These images were acquired with a rapid T1-weighted gradient echo sequence (repetition time = 2.2 ms, echo time = 1.2 ms, flip angle 15°) in a patient with a perfusion defect in the lateral wall, highlighted by the arrow in the images. Cardiac motions appear frozen as the acquisition time for each image is short on the time-scale of a heart beat, and the image acquisition is synchronized to the heart rhythm by use of the R-wave on the electrocardiogram as a trigger signal for the scanner.

or at a lower dosage if a wall motion abnormality becomes visible. Dobutamine stress testing requires rapid feedback from the images to avoid excessive stress to the patient that could result in a cardiac arrest or an excessive ischemic insult. Ideally, monitoring of ventricular function at different levels of dobuta-mine-induced stress should be accomplished with (near) realtime feedback from the cine images (8). Importantly, a recent comparison of wall motion studies performed in the same subjects with MRI and ultrasound by Nagel et al. (9) has demonstrated a significantly higher diagnostic accuracy of stress MRI in comparison to stress echocardiography.

### 3.5. Myocardial Perfusion

Rapid MR imaging of the heart during passage of an injected contrast agent bolus can be used to assess blood flow in the heart muscle (10-12). The term perfusion, as used in this context, refers to blood flow in the tissue through the coronary arterial network from the epicardial artery, through arterioles with diameters of 5-100 ^m, down to the capillaries, and through the venous network. The flow of blood is, under these circumstances, best described as the amount of a labeled substance that can traverse a unit volume of tissue per unit of time.

For MRI studies of tissue perfusion, a contrast agent is typically used as a tracer, thereby introducing a labeled substance with flow through myocardial tissue that is tracked by rapid serial imaging of the heart. The wash-in of contrast agent into the myocardium produces signal intensity changes in the images. Under normal conditions, the wash-in of an injected contrast agent takes only a couple of heartbeats. More specifically, following the time-course of contrast enhancement in the myocardium after injection of a contrast agent such as Gd-DTPA (gadolinium diethylenetriamine pentaacetic acid) provides a means to detect areas of ischemia in the heart.

To date, assessment of perfusion with MRI is generally accomplished by 2D T1-weighted imaging of multiple slices during each heartbeat. An example of the resulting images is shown in Fig. 13. Typically, these imaging protocols use a non-slice-selective 180° or 90° radiofrequency pulse for T weighting, followed by a rapid gradient echo readout of the image in less than 200 ms. The regional contrast enhancement should ideally be proportional to the contrast agent concentration. Such an approximately linear relationship between regional signal intensity and contrast agent concentration is only observed at lower contrast agent dosages, typically less than 0.05 mmol/kg of Gd-DTPA for fast, inversion recovery-prepared gradient echo sequences (repetition time less than 3 ms; echo time less than 2 ms) (13). A more marked contrast enhancement can be obtained with higher contrast agent dosages, but then the kinetics of the contrast agent and the correlation of contrast enhancement with tissue blood flow cannot be well assessed in a quantitative manner.

Rapid contrast agent administration is crucial for assessing myocardial perfusion with such agents, as this improves the

Fig. 14. Relation between myocardial blood flow measured during maximal vasodilation (e.g., with adenosine in a region downstream from an epicardial lesion) and the percentage lumen area reduction resulting from an epicardial lesion. Magnetic resonance perfusion studies were performed in patients who underwent coronary angiography after magnetic resonance imaging (MRI) exams. The myocardial blood flow was determined by quantitative analysis of the myocardial contrast enhancement (13). The gray curve was calculated from the model equation of Gould and Lipscomb (65), which related myocardial blood flow to the reduction of the maximal cross-sectional lumen area reduction.

Fig. 14. Relation between myocardial blood flow measured during maximal vasodilation (e.g., with adenosine in a region downstream from an epicardial lesion) and the percentage lumen area reduction resulting from an epicardial lesion. Magnetic resonance perfusion studies were performed in patients who underwent coronary angiography after magnetic resonance imaging (MRI) exams. The myocardial blood flow was determined by quantitative analysis of the myocardial contrast enhancement (13). The gray curve was calculated from the model equation of Gould and Lipscomb (65), which related myocardial blood flow to the reduction of the maximal cross-sectional lumen area reduction.

Fig. 15. Tj-weighted images acquired in a patient with a myocardial infarct; images were taken at 15 min after injection of 0.2 mmol/kg of Gd-DTPA (gadolinium diethylenetriamine pentaacetic acid) contrast agent. Infarcted myocardium appears brighter than noninfarcted myocardium. The Tl contrast is adjusted to null the signal intensity in normal myocardium because this leads to the most pronounced contrast between infarcted and noninfarcted myocardium. Images were acquired for short-axis views and for different levels. LV, left ventricle.

Fig. 15. Tj-weighted images acquired in a patient with a myocardial infarct; images were taken at 15 min after injection of 0.2 mmol/kg of Gd-DTPA (gadolinium diethylenetriamine pentaacetic acid) contrast agent. Infarcted myocardium appears brighter than noninfarcted myocardium. The Tl contrast is adjusted to null the signal intensity in normal myocardium because this leads to the most pronounced contrast between infarcted and noninfarcted myocardium. Images were acquired for short-axis views and for different levels. LV, left ventricle.

sensitivity for detecting changes of myocardial blood flow (14). Therefore, the goal is to ensure that the primary bottleneck to the rate of contrast enhancement is the rate of transport of contrast agent through myocardial tissue, and not the rate at which the contrast agent is injected. The presence of a narrowing in the coronary arteries is best detected when the resistance vessels downstream have been vasodilated. The relation between the degree of stenosis and blood flow reduction during maximal vasodilation is illustrated in Fig. 14.

### 3.6. Myocardial Viability

Extracellular contrast agents such as Gd-DTPA, commonly used for MRI perfusion studies, normally cross the cell membrane only after severe myocardial injury; thus, loss of myocardial viability has occurred (15-19). The distribution volume of an extracellular contrast agent is larger in injured than in normal myocardial tissue. Given sufficient time, an extracellular contrast agent reaches an approximate equilibrium distribution at which the contrast enhancement of tissue relative to the contrast enhancement in the ventricular blood pool is proportional to the distribution volume (20,21). Loss of viability and leakage of the contrast agent into the cell results in T1-weighted signal hyperenhancement, as shown in Fig. 15.

The timing for imaging of hyperenhancement is important. The time to reach 90% of equilibrium concentration depends on the distribution volume, but generally does not exceed 15 min (15). Larger infarcts can show a core zone that initially lacks enhancement even at 5 min or longer after contrast agent injection. This phenomenon, linked to microvascular obstruction, was shown to carry a graver prognosis for the patient than if the core no-enhancement zone was absent (22,23).

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