Computed Tomography [CT]-guided aspiration cytology is valuable in the work-up of many deep lesions [1]. Since Magnetic Resonance [MR] imaging is rapidly replacing CT as the imaging study of choice for evaluating many areas of pathology, the ability to perform MR-guided aspiration cytology is becoming increasingly important. Attempts at MR-guided biopsy with conventional stainless steel CT needles result in images with unacceptably large image artifacts that obscure underlying anatomy. A needle for MR-guided biopsies of the liver first developed by Mueller et al. [2] has decreased the number of artifacts and is ideal for relatively thick MR sections of the liver. The decreased artifact of the liver needle necessary for thick section liver studies is still too large to locate small lesions in areas of complex head and neck anatomy where higher resolution and thinner MR sections are necessary. Other investigators are studying nonferrous plastic sheaths with larger diameters for MR-guided percutaneous drainage procedures [3]. Finally, new field-echo imaging techniques with narrow flip angles, which probably will be the most practical pulse sequences for MR-guided biopsy because of their speed, tend to accentuate magnetic susceptibility artifacts of all needle types because of the increased T2 sensitivity of gradient refocusing techniques [4]. A team of William Hanafee and Louis Teresi led by Robert Lufkin MD developed a new MR needle with fewer artifacts on both spin-echo and gradient-echo sequences that is ideal for MR guided aspiration cytology of lesions using high-resolution, thin-section MR imaging[Figure 1].
Robert Lufkin and his team tested variety of stainless steel needle alloys [Figure 2]. For each needle, the effects of length, composition, annealing, degree of beveling, internal and external diameter, and pulse sequence on the image artifact were studied. Standard spinal CT needles made of 304 stainless steel as well as the Mueller needle made of 31 6 stainless steel were also examined [5]. In an attempt to decrease the number of artifacts on MR, a new needle was developed from c-276 stainless steel that has four times the nickel content of the Mueller needle and five times the nickel present in the standard CT needle. The needles were suspended in a phantom made of 2.5 mM NiCI solution (Ti = 460 msec at 0.3 T) for imaging. MR scans were performed on a 0.3-T permanent magnet (Fonar Beta 3000, MeMlIe, NV) MR whole-body imager. With one signal excitation on a 256 x 256 matrix, 2DFT images were obtained. A 25.6-cm field of view yielded 1 .0 x 1 .0 mm pixels; 3.5-mm-thick image slices were Obtained. Field-echo sequences using a reduced flip angle of 300 standard spn-echo sequences were employed with an echo time (TE) of 14 msec and a repetition time (TR) of 400 msec. Imaging during an MR-guided clinical aspiration cytology was performed with the new needle.
Images of needles of comparable size that were made of the three stainless steel alloys described above. Artifacts produced by the standard stainless steel CT needle tended to obscure accurate needle tip localization. The Mueller liver needle produced fewer artifacts than the standard CT needle but still tended to obscure the needle tip on the thin-section images used. For each needle tested, the artifacts were increased on the field-echo sequences relative to spin-echo sequences. The new needle which, of the needles studied, produced the fewest artifacts with both spin-echo and field-echo se- quences. As noted by others, the degree of needle tip bevel, annealing, or length had no significant effect on image artifacts [2]. A minimal increase in artifacts occurred when the needle diameter was increased for each alloy.
The main difference between the new Lufkin needle and the other needles tested is the increased nickel content of the stainless steel alloy. Nickel reduces the ferromagnefic properties of the iron in the alloy and generally decreases magnetic susceptibility differences between the alloy and surrounding tissues [7]. The nickel works by changing the highly susceptible form of alpha iron into less susceptible gamma iron. Abrupt local changes in induced magnetic field (susceptibility) result in magnetic field inhomogeneities that interfere with gradient fields used in imaging. The result is local geometric image distortions oriented along the read gradient [6]. Alterations in image intensity also occur with a similar orientation. A combination of these geometric and brightness distortions can completely obscure underlying anatomy and pathology. The lack of the refocusing pulse allowing an extremely short TE and TA with gradient refocusing techniques also means that the imaging sequences are generally much more sensitive to magnetic inhomogeneities and susceptibility differences [4]. Therefore, the magnetic susceptibility artifacts produced by all the needles are accentuated by using gradient refocusing techniques. Because of the high speed of field echo imaging, this technique will be useful for MR-guided biopsies. Thus, MA needles with minimal differences in susceptibility are especially valuable. The overall trend toward higher resolution, thinner section imaging will also make needles with smaller artifacts increasingly important for MA guided biopsies throughout the body. This needle is widely available from a number of vendors today [fig 3].
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