Ultrafast 3D MRA including subsecond 3D MRA

An additional method for time resolved imaging is to optimize the basic T1-weighted fast 3D gradient echo pulse sequence used for conventional high spatial resolution 3D CE MRA for faster acquisition times instead of highest spatial resolution. With newer high performance gradients, 3D MRA can be performed every 5-7 sec [8] on many commercial scanners enabling multi-phase 3D CE MRA during a single breath hold (Fig. 3). The main compromise is that of spatial resolution, as a reduction in phase-encoding (ky) and slice-encoding (kz) steps are generally required.

More recently, Finn et al [1] introduced a "sub-

second" time-resolved 3D MRA technique ("subsecond angiography" or "freeze-frame angiogra-phy") that uses an ultrafast 3D spoiled gradient echo pulse sequence with markedly reduced TR (e.g. 1.6 msec) and TE (e.g. 0.8 msec) times whereby individual 3D acquisition times are between 400 and 900 msec (i.e."sub-second" 3D MRA). Tempo-

Fig. 3. Single breath-hold, multi-phase 3D CE MRA of the abdomen. Ultrafast 3D Multiphase CE-MRA (A-E, 5 phases, each 6.3 s; TR=3.2 ms, TE=1.1 ms, 40 degree flip angle) performed within a single breath hold after injection of 0.1 mmol/kg Gd-BOPTA (Brac-co Diagnostics, Milan, Italy). Note the distinct vascular phases (B-E) with the arterial, late arterial, portovenous and delayed venous phases shown (Reprinted and modified with permission from [3])

Fig. 3. Single breath-hold, multi-phase 3D CE MRA of the abdomen. Ultrafast 3D Multiphase CE-MRA (A-E, 5 phases, each 6.3 s; TR=3.2 ms, TE=1.1 ms, 40 degree flip angle) performed within a single breath hold after injection of 0.1 mmol/kg Gd-BOPTA (Brac-co Diagnostics, Milan, Italy). Note the distinct vascular phases (B-E) with the arterial, late arterial, portovenous and delayed venous phases shown (Reprinted and modified with permission from [3])

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Fig. 4a-c. Sub-second 3D MRA in a patient with a Type B aortic dissection and extralumi-nal leak. Sagittal trueFISP image (a) demonstrates the true (T) and false (F) channels and in-timal tear (arrow) distal to the origin of the left subclavian artery. There is also a small region of bright signal (arrowhead) posterior to the aorta suspicious for extra-luminal blood, but its source is not clearly determined. On sagittal dynamic sub-second 3D MRA (b, 6 selected images; c, single phase from dynamic study), the true and false channels as well as the intimal tear (arrow) are once again noted. On time resolved imaging, the aforementioned rounded area (arrowhead) is noted to enhanced at roughly the same time as the descending aorta consistent with a persistent communication with the aortic lumen

Arterial Dissection Mra

Fig. 4a-c. Sub-second 3D MRA in a patient with a Type B aortic dissection and extralumi-nal leak. Sagittal trueFISP image (a) demonstrates the true (T) and false (F) channels and in-timal tear (arrow) distal to the origin of the left subclavian artery. There is also a small region of bright signal (arrowhead) posterior to the aorta suspicious for extra-luminal blood, but its source is not clearly determined. On sagittal dynamic sub-second 3D MRA (b, 6 selected images; c, single phase from dynamic study), the true and false channels as well as the intimal tear (arrow) are once again noted. On time resolved imaging, the aforementioned rounded area (arrowhead) is noted to enhanced at roughly the same time as the descending aorta consistent with a persistent communication with the aortic lumen ral resolution was further facilitated with the implementation of partial Fourier and fractional FOV k-space schemes. The technique is typically obtained in coronal or sagittal orientation and is coupled with automated on-line image subtraction that can provide rapid generation of time resolved maximum intensity projections (MIPs) of the subtracted 3D CE MRA data sets for 3D MR DSA.

Most often it is advantageous to perform dynamic sub-second 3D MRA prior to conventional 3D CE MRA of higher spatial resolution. Time resolved imaging using sub-second 3D MRA can provide not only bolus timing information but also a wealth of information concerning the underlying anatomy and its flow pattern (Fig. 4). However a few key concepts must be remembered, especially if the sub-second angiogram is utilized as a replacement for timing runs prior to the 3D MRA.

First, sub-second angiography due to its inherently poor through plane resolution is usually best suited for interrogation of larger vessels, namely the aorta and pulmonary artery and their first order branches. If desired two consecutive sub-second MRAs may be performed in separate orientations or projections (e.g. coronal and sagittal). Both acquisitions can occur virtually back-to-back without risk of image compromise.

Second, the sub-second angiogram as originally implemented uses a 6 mL Gd-chelate contrast bolus injection followed by an 18-20 mL saline flush, both injected at 6 mL/sec. One can tailor these parameters slightly in certain situations, like reducing contrast amount to 4 mL or decreasing injection rate to 4 mL/sec, however image quality will likely suffer, especially in vessels with slow flow or in areas with a large intravascular blood volume (e.g. aneurysms). Increasing rates beyond 6 mL/sec or using higher total volumes of contrast plus flush (>25 mL) are not advisable as most power injector and intravenous tubing setups are not capable of accurately delivering faster flow rates [9]. Furthermore, delivering higher total volumes at a rapid rate will likely increase risk for venous extravasation. Anecdotally, in thousands of cases at Northwestern University there have been very few, if any, complications related to these low-volume, high-rate power injections. The injections are typically administered safely via an 18-gauge venous catheter placed in the antecubital fossa or a large vein in the forearm proximal to the wrist, preferably of the right arm. Injection into the back of the hand is discouraged due to the size of veins and more importantly the risk of closed space extravasations and complications.

Time Resolved imaging of Contrast KirieticS

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Fig. 5. K-space acquisition scheme for TRICKS [Image courtesy of Dr. F. Korosec, University of Wisconsin, Madison, WI; and Dr. T. Carroll, Department of Radiology, Northwestern University, Chicago, IL, USA]

Strengths of sub-second angiography include, as its name implies, the extremely high temporal resolution achievable (400-500 milliseconds), the very small 4-6 mL contrast dose necessary, and relative insensitivity to minor motion such as when patients are unable to perform a breath-hold. Disadvantages include the need for relatively high performance gradients with slew rates > 100mT/m/ msec. Images obtained are of low through-plane resolution and are therefore essentially projection-al images. Hence, sub-second MRA often benefits from performance in two separate orientations or projections.

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