Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-09-18
2001-05-15
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S309000
Reexamination Certificate
active
06233475
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention disclosed and claimed herein generally pertains to magnetic resonance (MR) angiography, i.e., to MR imaging of an artery or like vessel carrying blood or other fluid. More particularly, the invention pertains to a method of the above type wherein an amount of contrast agent, or bolus, is inserted into the vessel to enhance contrast between blood flowing through the vessel, and adjacent stationary tissue or other structure. Even more particularly, the invention pertains to a method of the above type for closely determining the arrival time of the bolus at a site or location of imaging.
It is now a well known practice in MR angiography to insert a volume of contrast agent, such as gadolinium chelate, into blood flowing along a vessel. The volume or mass of contrast agent is referred to as a bolus, and has the effect of shortening the T1 time of the blood. Thus, an MR image of the blood, acquired by a fast gradient echo or like technique, will show up very well with respect to adjacent stationary tissue of the vessel structure. These agents have been found to be very effective, particularly when used with three-dimensional (3D) MR angiographic techniques. However, if imaging occurs some minutes after the administration of contrast material, complex images are created in which distinction between target vessels (usually arterial) and other vasculature is difficult. Time-dependent leakage of contrast material into adjacent tissue increases background signal intensity, which adds a further hindrance to image interpretation. At present, there is increasing interest in imaging arteries by trying to capture first-pass arterial enhancements, resulting from use of contrast material, by coordinating the onset of a 3D MR angiographic sequence with injection of the contrast material. This approach is often referred to as “dynamic contrast material-enhanced 3D MR angiography”, and aims at imaging arteries during first-pass arterial enhancement, prior to the onset of venous enhancement. Arteries targeted with this approach include the descending aorta and the mesenteric, renal, and hepatic arteries.
There are several basic approaches to capturing first-pass arterial enhancement. In the fixed transit time approach, imaging is initiated after a fixed time interval after injection. In the test bolus approach, a small test bolus of contrast is used to determine a priori the transit time of contrast from the time of injection at the injection site to the time of arrival at the imaging site. This information is then used to coordinate the initiation of a 3D MR angiographic sequence after the subsequent injection of a full bolus of contrast. In the automated trigger approach, only a full bolus of contrast is injected, and after detection of its arrival at the imaging site, a 3D MR angiographic sequence is initiated. In the latter two approaches, a method of determining the arrival of contrast at the imaging site is required.
While many studies use the fixed transit time approach, the true transit time of contrast material can vary on the order of tens of seconds from patient to patient, depending on each patient's cardiovascular status. For instance, typical transit times to the liver have been found to vary from 8 to 32 seconds. Even more important, the time window between the onsets of arterial and venous enhancement is usually just seconds in duration, and is therefore shorter than the imaging time in a typical 3D MR angiographic sequence. In the case of the liver, this time window has been noted to be as short as 8 seconds and to average approximately 16 seconds. Data collection from lower order k-space needs to occur during this time window, in order for final images to demonstrate only arterial enhancement. The shorter the time window, the more likely it becomes that the use of a fixed time delay will lead to suboptimal images that miss first-pass arterial enhancement prior to venous enhancement, and the greater the necessity for an accurate estimate of the transit time.
In applying dynamic contrast-enhanced 3D MR angiography to the carotid arteries, two important features have been noted. First, there is a very short optimal time window for imaging, typically 5-10 seconds, during which contrast material is within the arteries and the cranial circuit but has not yet reached the veins of the neck. Second, the blood-brain barrier prevents absorption of gadolinium-based contrast material, which creates a particularly strong venous signal during venous enhancement that complicates assessment of the arteries. For these two reasons, it is essential in dynamic contrast-enhanced 3D MR angiography of the carotid arteries to have accurate measurement or estimation of the patient-dependent transit time of contrast material, from injection site to imaging site.
SUMMARY OF THE INVENTION
The invention is generally directed to a method for determining the arrival time of selected contrast material along an artery or other vessel, between an injection site and a site of MR imaging. The method includes injecting the contrast material into blood or other fluid flowing through the vessel at the injection site. Coincident in time with injection of the contrast material, acquisition of a succession of MR images commences, each image being directed to the same section taken through the vessel, proximate to the imaging site. Excitation pulses are applied to first and second zones to selectively saturate MR signals passing therethrough, the first and second saturation zones being positioned on opposing sides of the imaging section, and respectively extending along the vessel. The MR images are monitored as they are respectively acquired, to detect the first of such images to indicate arrival of the contrast material at the imaging site.
In a preferred embodiment of the invention, each of the MR images is acquired during a brief time period, such as a period of approximately 1 second, by means of a fast sequence such as a two-dimensional gradient recalled echo sequence. The imaging section comprises a section taken through the vessel which is oriented in substantially perpendicular relationship to the direction of fluid flow. Each of the saturation zones is in abutting relationship with the section, and the excitation pulses applied to the saturation zones comprise RF pulses of selectively low flip angle, such as 18°. A series of pulses, such as five, produces a steady state MR environment in the saturation zones.
In a useful embodiment, the vessel comprises a carotid artery, and the injection site is at a venous site in the antecubital fossa. In this embodiment, a test bolus of contrast is employed to determine transit time as described above. Thereafter, a full bolus, comprising a dose of contrast material having a substantially greater volume than the test bolus, is injected into the vessel. After a delay time equal to the transit time, following full bolus injection, a 3D MR angiography scan is commenced at the imaging site. If the time window for imaging is very short, centric view ordering is usefully employed, to ensure collection of as much lower order k-space data as possible.
In another useful embodiment, that of an automated trigger examination, using a similar intravenous set-up, the contrast material comprises a full bolus, and after detection at the imaging site, a 3D MR angiography scan is immediately commenced at the imaging site. Again, if the time window for imaging is very short, centric view ordering is usefully employed.
In view of the above, important purposes of the invention include providing a method for more effectively coordinating commencement of a 3D MR angiographic sequence with injection of contrast material into an associated artery or vessel, and capturing first-pass arterial contrast enhancement, i.e., imaging when a contrast material first arrives at the imaging site, and prior to the onset of venous enhancement. Other purposes include determining transit time of a bolus between an injection site and an imaging site with substantial acc
Farb Richard I.
Kim Jae K.
Wright Graham A.
Jenkens & Gilchrist P.C.
Lateef Marvin M.
Mantis Mercader Eleni
Synnybrook Health Science Center
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