MR Safety: GU OtherHow about other GU devices like nephrostomy tubes and stents?
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Nephrostomy Tubes
Percutaneous nephrostomy catheters drain urine externally from the kidney in the setting of urinary tract obstruction. Nearly all marketed catheters are made of polyurethane and are thus MR Safe. However, some nephostomy (and suprapubic) catheters made by Cook have a locking cam mechanism (MacLoc®) on the hub containing a small amount of metal; these are thus (minimally) MR Conditional.
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Ureteral Stents
Stents are placed in the ureters and other portions of the GU tract to allow continued passage of urine in the setting of infection, stone disease, and tumor. The double-J (also known as the double pigtail) stent, pictured right, is the most common type deployed. Although sometimes intended to be permanent, many stents are removed within a few days or weeks after placement as the underlying disease regresses.
Before 2000 nearly all GU stents were exclusively made of plastic/polyurethane and considered MR Safe. Over the last 20 years, however, all-metal and metal-coil reinforced stents have been developed to better maintain shape and improve patency in the setting of obstructing stones and tumor infiltration. These are all MR Conditional.
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Double-J ureteral stent placed for stone disease (Source: Wikipedia)
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I am aware of only one ureteral stent that is considered MR Unsafe — the Magnetic Black-Star (Urotech). This stent has a small rare-earth magnetic connected at its tip within the bladder. (This allows a magnetic retriever device to be inserted to capture the stent and remove it without the need for cystoscopy.)
"MR Conditional" Cook Resonance® metallic ureteral stent
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"MR Unsafe" Magnetic Black-Star ureteral stent with magnetic retriever
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Prostatic Implants
Prostatic/Urethral Stents are much less common than ureteral stents and may be temporary or permanent. They are typically made plastic (polyurethane), metal (stainless steel, Nitinol), or both. Those containing metal are typically MR Conditional. I am aware of only one of these, the Spanner® Temporary Prostatic Stent (SRS Medical) is that has never undergone ASTM testing and according to manufacturer's literature should be removed prior to MRI. However, because this device has a similar construction to other steel-wire reinforced polyurethane stents I would expect it could be rated MR Conditional if testing were performed.
Serving a similar function to prostatic stents is the newly approved UroLift® System for benign prostatic hypertrophy. The lobes of the gland are held apart by 4 UroLift® implants inserted endoscopically. The ends of each implant are made of stainless steel and Nitinol (Ni-Ti alloy). The UroLift® System is MR Conditional at fields up to 3.0T.
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"MR Conditional" expandable metallic prostate stent
"MR Conditional" UroLift® system
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Gold seed (arrow) and multiple brachytherapy seeds
"MR Conditional" brachytherapy seeds
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Gold Seeds are small nonradioactive fiducial markers implanted in the prostate in patients undergoing external beam radiotherapy for prostate cancer. They assist with identifying the exact location of the prostate which may change with bladder distention and other factors between radiation therapy sessions.
Brachytherapy Seeds, by contrast, are radioactive. These "seeds" are actually small titanium capsules, each about the size of a grain of rice, and typically contain either Palladium-103 or Iodine-125.
Prostatic Transponders are slightly larger than seeds and consist of a glass capsule containing passive electrical components. Three are usually implanted. When stimulated by a plate-like electromagnetic source array placed over the patient's pelvis, they respond by emitting a unique signal in the 300-500 kHz range allowing them to be accurately localized.
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Gold seeds, transponders, and brachytherapy seeds are all rated as MR Conditional up to 3.0T.
References
Aoun F, Marcelis Q, Roumeguère T. Minimally invasive devices for treating lower urinary tract symptoms in benign prostate hyperplasia: technology update. Research and Reports in Urology 2015; 7:125–136. [DOI LINK]
Hendlin K, Korman E, Monga M. New metallic ureteral stents: improved tensile strength and resistance to extrinsic compression. J Endourol 2012; 26:271-274. [DOI LINK]
Rassweiler M-C, Michel M-S, Ritter M, Honeck P. Magnetic ureteral stent removal without cystoscopy: a randomized controlled trial. J Endourol 2017; 31:262-766. [DOI LINK]
Wilkstrom J, Isacsson U, Johansson, Lennernäs B. Magnetic resonance compatibility of a transponder aimed for radiotherapy positioning — a phantom study. Antican Res 2017; 37:4993-4996. [DOI LINK]
Aoun F, Marcelis Q, Roumeguère T. Minimally invasive devices for treating lower urinary tract symptoms in benign prostate hyperplasia: technology update. Research and Reports in Urology 2015; 7:125–136. [DOI LINK]
Hendlin K, Korman E, Monga M. New metallic ureteral stents: improved tensile strength and resistance to extrinsic compression. J Endourol 2012; 26:271-274. [DOI LINK]
Rassweiler M-C, Michel M-S, Ritter M, Honeck P. Magnetic ureteral stent removal without cystoscopy: a randomized controlled trial. J Endourol 2017; 31:262-766. [DOI LINK]
Wilkstrom J, Isacsson U, Johansson, Lennernäs B. Magnetic resonance compatibility of a transponder aimed for radiotherapy positioning — a phantom study. Antican Res 2017; 37:4993-4996. [DOI LINK]
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