Magnetic Resonance Imaging and Arteriography

Basic Principles

Magnetic resonance imaging (MRI) is performed using a large external magnetic field, magnetic field gradients, and an applied oscillating magnetic field known as the radiofrequency field (RF). The combination of these three applied magnetic fields produces signals from inside tissue that can be used to create MR images.

The details of the external magnetic field, gradients, and RF determine many of the characteristics of MR images. The external magnetic field magnetizes the subject by making the protons align parallel with the external field. The physical characteristics of the protons and the size of the external magnetic field define the resonance frequency. If the external magnetic field is uniform, all the protons will resonate at the same frequency. Most MRI machines operate at an external field strength of 1.5 or 3 Tesla (T); potential advantages of 3T magnetic resonance angiography (MRA) are discussed later in the chapter. Lower field systems tend to produce images at a slower rate or at a lower resolution than needed for MRA. Recently, 7-Tesla systems have been approved for clinical use in the head and extremities, but they are not currently approved for angiographic imaging.

Magnetic field gradients alter the uniform external magnetic field in a linear fashion in any of three directions. The gradients ramping on and off produce the noise heard when an MR image is being made.

The gradient fields cause protons to resonate at a frequency that is a function of the magnetic field gradient’s position, analogous to a radio station’s frequency corresponding to a specific position on the dial. The speed and strength of the gradients determine the size of the images and may be a limiting step in imaging speed.

Resonant coils placed either within the bore of the system or adjacent to the region of interest produce the RF. These fields are tuned to match the resonant frequency of the protons inside the patient. The applied RF in combination with the gradients are used to manipulate the protons inside the patient to produce a signal. This signal is detected by a receiver RF coil, also tuned to the resonant frequency of the protons. Once detected, the signal is sent to an amplifier and receiver, where it is digitized and processed with a mathematical algorithm known as a Fourier transform to produce the MR image.

Characteristics of MR Images

Contrast in MR images depends on characteristics of the object being imaged and specifics of the acquisition process. Images are typically referred to as either T1 weighted or T2 weighted. T2-weighted images display simple fluids such as urine, bile, or cerebrospinal fluid as bright and other tissues as lower signal. T2-weighted imaging is one of the basic sequences for imaging of tumors but is not typically used for angiographic imaging. MRA and MR venography (MRV) are generally performed with T1-weighted image sequences. Objects that are bright on T1-weighted images, including fat, methemoglobin, flow effects, and MRI contrast, will often be bright on MRA sequences.

Magnetic Resonance Pulse Sequences

An MR pulse sequence is a combination of RF and gradients that are used to create an image. There are many types, but most are variations of spin-echo or gradient-echo sequences. Spin-echo sequences use RF alone to produce the MR signal, whereas gradient-echo sequences use RF and applied gradients. As a rough generalization, spin-echo sequences are used to produce T1- or T2-weighted images while gradient-echo sequences can only create T1-weighted images.

Pulse Sequence Parameters

The tissue characteristics that determine its MR appearance are the T1 and T2 parameters of the tissue. Some tissues are bright on T1-weighted images whereas others are bright on T2-weighted images. All pulse sequences have fundamental parameters known as echo time (TE) and repetition time (TR) that determine image contrast. T2-weighted images have a longer TE, in the range of 80 ms or greater, and a longer TR, in the range of several seconds. Given the long TE and TR, these sequences are slower and not appropriate for contrast-enhanced MRA. T1-weighted images, however, have a very short TR and TE, with a TE of 1 ms or less and a TR from hundreds of milliseconds to less than 10 ms for MRA sequences. Fast T1-weighted gradient-echo sequences are used for most contrast-enhanced MRA imaging.

Other parameters include the field of view (FOV) and image matrix. FOV is the size of the imaged region. In a two-dimensional (2D) image, FOV may be 40 × 30 cm with a slice thickness of 5 mm. The resolution of the image in the 5-mm slice depends on the number of pixels within the 40 × 30-cm region. Most MRA imaging, however, is performed with a three-dimensional (3D) sequence and a FOV specified with three dimensions, such as 40 × 30 × 30 cm. An image matrix of 256 × 192 × 64 will result in a voxel (or 3D pixel) size of 1.56 × 3.26 × 4.7 mm. Post-processing is usually performed to make the last dimension twice as small as specified by the pulse sequence. In this case, yielding a voxel size of 1.56 × 3.26 × 2.35 mm.

MR Angiography and Venography

Noncontrast-Enhanced MR Angiography

Noncontrast-enhanced MRA, such as time-of-flight (TOF) imaging, has largely been supplanted by contrast-enhanced methods. However, the risk of nephrogenic systemic fibrosis (NSF) in patients with poor renal function who have received gadolinium-based contrast agents (GBCA) ^, (see “Contrast-Enhanced MR Angiography” below for further details) has led to renewed interest in noncontrast techniques.

TOF angiography uses a rapid T1-weighted pulse sequence in sequentially acquired 2D slices or a 3D imaging slab, causing loss of signal within the slice or slab. Fully magnetized protons in vessels flow into the slice or slab, producing higher signal than stationary tissue, resulting in an image in which flowing blood appears much brighter than surrounding tissue. For example, in an axial slice through the mid abdomen, the aorta and inferior vena cava (IVC) as well as the mesenteric arteries and veins would appear bright. To remove either the arteries or veins, a special RF pulse is applied either above or below the slice to eliminate signal from that tissue. If an inferior saturation pulse were used, only the protons flowing into the slab from above (the aorta and mesenteric arteries) would appear bright, while with a superior pulse, only protons flowing from below (inferior vena cava) would appear bright. A series of such images obtained sequentially produces a 2D TOF abdominal MRA.

Additional noncontrast MRA techniques include electrocardiogram (ECG) gated steady-state free precession (SSFP), ECG gated arterial spin labeling (ASL), navigator gated SSFP, and half-Fourier fast spin-echo imaging (HASTE) with flow-spoiled gradients. These use different methods to accentuate signal of flowing blood and attenuate signal from nonmoving structures and tissue with signal characteristics different from blood. The SSFP technique, for example, provides rapid imaging of both large and small vessels, such as the aorta, ^, carotids, and renal arteries, without the need for a contrast agent.

Fresh blood imaging (FBI) relies on the signal difference between systolic and diastolic triggered acquisitions. The arterial signal in systole is dark due to spin-dephasing effects of fast arterial flow, whereas venous blood is bright throughout the cardiac cycle because of constant slow flow. By applying flow-spoiling pulses, arteries can be separated from veins in the peripheral run-off vessels. ^, Two alternative approaches using SSFP sequences include flow-sensitive dephasing (FSD) and quiescent-interval single-shot (QISS). QISS MRA is performed using a 2D ECG-gated single-shot SSFP acquisition; initial saturation pulses suppress in-plane background tissues and venous inflow. Advantages of QISS are short acquisition times and ease of use, allowing imaging of fast-flowing vessels such as the aorta and renal arteries, however this technique requires magnetic field homogeneity. Figures 30.1–30.3 show examples of noncontrast MRA acquired using different techniques. Evaluation is ongoing ; however, these techniques of image acquisition may replace contrast-enhanced MRA in many circumstances.

Figure 30.1, Oblique sagittal noncontrast-enhanced steady-state free precession MR angiogram of the thoracic aorta. The signal from flowing blood appears bright.

Figure 30.2, ( A ) Coronal MIP of time-of-flight MRA of the renal arteries. ( B ) Coronal MIP of noncontrast MRA acquired with a newer method that uses ECG gating to subtract diastolic from systolic images, leaving only arterial systolic flow. Note markedly improved visualization of accessory right renal arteries compared to TOF technique ( arrows ).

Figure 30.3, ( A ) Coronal MRA of the lower extremities acquired using the QISS technique. ( B ) Zoomed-in view of the calves demonstrates occluded right posterior tibial artery and collaterals around the ankle ( arrows ).

Contrast-Enhanced MR Angiography

MR contrast agents currently approved by the Food and Drug Administration (FDA) include several in which the rare earth element gadolinium is chelated with another substance to avoid release of toxic free gadolinium into the body. These agents shorten the T1 of protons in the vicinity, making them more conspicuous on T1-weighted imaging sequences. There are several important differences between MR contrast and iodinated contrast used for computed tomographic angiography (CTA) or standard angiography. First, as suggested, MRI is designed not to image the agent itself but its effect on protons in the surrounding water. One implication of this is that a very small amount of MR contrast material may be detected by its effect on multiple water molecules, whereas an equivalently small amount of iodinated contrast is simply not detectable directly by CTA or standard angiography. This is one reason that the volume of MR contrast can be much less than that of iodinated contrast used for CTA or angiography. Other advantages of gadolinium-based contrast include decreased nephrotoxicity and a lower incidence of contrast reactions.

Step-Table and Continuous-Table MR Angiography

For imaging a large volume, such as the entire aorta or peripheral arterial runoff, acquisition of the entire volume at once is impractical and may lead to decreased image quality. Image spatial resolution is inversely proportional to the matrix size, and matrix size directly impacts scan duration, so a higher resolution image requires a longer time to acquire. If the FOV is very large, the acquisition time required to achieve the necessary spatial resolution for visualization of the vessels of interest may be beyond the breath-holding capacity of the patient and can lead to respiratory motion artifact. In addition, intravenously administered contrast will flow from the aortic root to distal vessels in a time dependent on the rate and volume of injection, the patient’s cardiac output, and the presence of any proximal occlusive disease; thus, not all portions of the arterial system will enhance optimally at the same time. For these reasons, it is desirable to use a step-table acquisition in which portions of the anatomy of interest are imaged sequentially, with the scanner table moving between stations to place the specific anatomy of interest near the isocenter of the magnet.

A peripheral runoff imaging study may include four step-table stations – abdomen/pelvis, thighs, calves, and feet – each imaged with a coil, FOV, spatial resolution, and orientation optimized to the vascular anatomy in that station. For example, the abdominal and pelvic vessels may best be imaged in the coronal plane with a larger FOV and a large array coil, while smaller pedal vessels would optimally be acquired in an oblique plane oriented along the foot, with a small FOV, high spatial resolution, and smaller coils for an improved signal-to-noise ratio (SNR). Although step-table examinations are generally performed in a proximal-to-distal direction, there may be circumstances in which a “reverse step-table” protocol is preferable, for example, for a patient with known abdominal aortic pathology in whom there is less suspicion of thoracic aortic disease. In this case, the abdominal aorta could be imaged first, in the coronal plane, followed by the thoracic aorta, imaged in a more advantageous oblique sagittal plane. This would provide higher resolution imaging of both regions, in two shorter breath-holds, than possible with a single large-FOV acquisition, with imaging of the area of interest in a pure arterial phase and perhaps some venous filling for the less critical thoracic portion. Step-table MRA requires additional hardware, including a set of MRI coils for optimal imaging of each anatomic segment and optional software to control the automated table motion. As mentioned before, a disadvantage of the step-table technique may be a longer total acquisition time, which can lead to venous contamination in the later stations.

Three-Dimensional Image Processing

Contrast-enhanced MRA is performed as a 3D acquisition; the 3D volume can be viewed as a stack of 2D images, but the higher resolution allows multi-planar reformatting (MPR) to create images in multiple orientations that optimize visualization of the anatomy or pathology. Images can be viewed using many types of algorithms to display the 3D data to the best advantage. Because contrast within vessels is designed to have the highest intensity, an algorithm that displays the brightest voxels, known as maximum intensity projection (MIP), is commonly used for reformatting 3D MR angiograms. Volume rendering (VR) has been demonstrated to be equivalent or superior to MIP for both MRA and CTA of several vascular territories. ^,

Hardware and Software for MR Angiography

MRA is performed optimally on a 1.5 or 3T MR system with the fastest and strongest gradients available using multichannel receivers and coils with powerful image reconstruction computer systems. This will result in shorter acquisition times with higher spatial resolution and improved SNR as well as rapid availability of reconstructed images.

1.5- vs. 3-Tesla MRI

Because the degree of magnetization produced is linearly related to the magnetic field strength, a 3T magnet can increase image signal up to twice as much over the 1.5T magnet. This increased signal can then be traded for decreased imaging time or increased spatial resolution. Although the first applications of 3T use were in brain, spine and musculoskeletal imaging, the entire range of MRI is now being performed at 3T, including breast, body, and cardiac MRI, as well as MRA. T1 relaxation times are longer at 3T, which results in increased enhancement using the same amount of gadolinium contrast or the ability to use less contrast to obtain the same degree of enhancement. Preliminary studies have shown improved image quality and accuracy of 3T over 1.5T MRA for imaging of the renal, carotid, and infrapopliteal arteries.

One disadvantage of 3T is a fourfold increase in deposition of RF energy, known as the specific absorption rate (SAR). FDA-mandated limits on SAR are encountered more commonly using 3T than 1.5T. Although this is not an important problem for contrast-enhanced MRA, it can be an issue when using noncontrast-enhanced SSFP techniques and in applications such as cardiac MRI. Additional concerns include higher cost for 3T systems and an increase in image artifacts related to magnetic field inhomogeneity. Despite these difficulties, more MRA studies will be performed at 3T as radiology practices switch to this technology given its advantages for general MRI.

Other hardware and software

Dedicated coils for specific vascular beds are also useful and include at least a neurovascular coil for head and neck MRA and a peripheral angiography coil covering the pelvis to the feet for runoff studies. MRA of the thoracic, abdominal, and pelvic vessels is performed with multiple flexible phased-array coils that can be placed over the anatomy of interest; digital and pedal vessels can be imaged either with small local extremity coils or by placing both extremities in a dedicated head coil that is designed for high-resolution and SNR imaging.

Multichannel coils and RF receivers allow parallel imaging, which refers to a collection of MR acquisition and processing techniques used to accelerate image data acquisition. ^, As one of the effects of this acceleration is a loss of signal, parallel imaging is used mostly in high-SNR applications, including MRA, and most contrast-enhanced MRA examinations done on newer scanners are performed with parallel imaging.

Compressed sensing is a mathematical framework that provides reconstruction of data from highly under-sampled measurements in order to gain acceleration of acquisition time. This method is useful in pediatric imaging and where shorter breath-hold sequences are required. Compressed sensing approaches are now available from several manufacturers and will be incorporated into routine MRA protocols in the coming years.

An MR-compatible dual-chamber power injector is needed for rapid administration of contrast, followed by a saline bolus, which can decrease the amount of contrast used. MR- compatible patient monitoring equipment is also useful, especially when imaging hospitalized patients or when sedation is needed. MR-compatible blood pressure cuffs can be useful for reducing venous contamination in extremity MRA. For example, placing cuffs around both thighs while inflating to half to two thirds systemic arterial pressure effectively delays venous return and allows for improved visualization of calf arteries.

Additional software may be needed for step-table and parallel imaging, as well as newer noncontrast-enhanced and contrast-enhanced MRA techniques, so it is important when purchasing MR systems to investigate which sequences are included in the base MR system package and which are optional.

A post-processing workstation with multiplanar reformatting, maximum intensity projection, and volume rendering capability is the minimum necessary for visualization and interpretation of MRA studies. Additional software, such as automated vessel analysis with curved planar reformatting, vessel centerline extraction, automated segmentation, and bone removal, may also be useful for specific applications but is more relevant for CTA than for MRA.

Clinical Applications of Mr Angiograpy

Aortic Vascular Disease

Contrast-enhanced 3D MRA has been used to assess the aorta in patients with many different types of disease. Assessment of the aortic arch by contrast-enhanced MRA may be superior to any other technique. Several examples are shown in Figure 30.4 . Other diseases that are routinely evaluated by MRA include aortic dissection, ^, as shown in Figure 30.5 , and aneurysms. ^, MRA is also used to evaluate patients with other vascular diseases as shown in Figures 30.6 and 30.7 , such as connective tissue disorders including Marfan syndrome, and for vascular stent-graft placement planning, as well as for endoleak assessment in those with a nitinol-based stent-graft. ^,