Radiation Risk Due to Shunted Hydrocephalus and the Role of MR Imaging–Safe Programmable Valves

Radiation Risk Due to Shunted Hydrocephalus and the Role of MR Imaging–Safe Programmable Valves Diagnostic radiology is a double-edged sword: While providing critical information that forms the basis of treatment, it adds to the risk associated with cumulative radiation given to the patient. Shunted hydrocephalus exemplifies this conundrum. Hydrocephalus is a common neurosurgical condition that affects individuals of all ages, and the most common method for managing hydrocephalus is the surgical implantation of a shunt system to divert the flow of CSF from the ventricles. More than 125,000 shunts are implanted every year in the United States at a cost of US $2 billion. Nearly half of this cost is associated with shunt revisions. Although most cases of hydrocephalus have clinical improvement with the insertion of a shunt, it is rare for the device to last a lifetime without complications. Shunts can be obstructed and infected, and tubing may get disrupted, resulting in recurrence of symptoms. In 1 study, the shunt failure rate in children was reported to be 31% within the first year and 4.5% per year thereafter; the failure rate in adults was found to be comparable with that in children. In another study, the overall shunt survival in pediatric patients was 62% at 1 year, 52% at 2 years, 46% at 3 years, and 41% at 4 years. In a third study, the probability of shunt malfunction after 12 years was 81%. The high incidence of device problems and the potential for serious consequences as a result, combined with patients who have cognitive problems expressing their symptoms, predicts frequent visits to emergency departments and urgent care centers. The integrity of the tubing is checked by a series of x-rays of the head, chest, and abdomen; the size of the ventricles is assessed by CT of the head. CT is often the preferred technique because of its wide accessibility, ease of use, and brief imaging period. Initial scans focus on finding abnormal pathologies, while subsequent scans are oriented toward assessment of the shunt, determination of stability of ventricular volume, and identification of related complications. This need for confirming the suspicion of a shunt malfunction by diagnostic radiology increases the risk for longterm effects of ionizing radiation. The effective doses for x-rays are 0.1 (skull), 0.1 (chest), and 0.7 (abdomen) mSv, respectively; and for CT of the head, it is 2.0 mSv. In other words, a visit to the emergency department will result in nearly the same amount of radiation that any healthy individual gets from background radiation (estimated at 3 mSv) during a year. Despite this diligence in managing shunt problems, 2 of 3 patients who are investigated are not found to have shunt malfunction. Excessive exposure to radiation is of greater concern in children because rapidly dividing cells in children are more radiosensitive than those in adults. Additionally, a longer lifetime for children allows the manifestation of radiation injuries, which have a long latency period before they become apparent in patients. The National Council on Radiation Protection and Measurements estimated that during the past 2 decades the total exposure of the US population to ionizing radiation has nearly doubled. Studies have shown that patients most prone to harm from diagnostic radiation are children and young adults; individuals with medical conditions sensitive to radiation, such as diabetes mellitus and hyperthyroidism (which are possible risk factors associated with normal pressure hydrocephalus); and individuals receiving multiple doses with time. From the 72 million CT scans performed in the United States during 2007, 1 study estimated that 29,000 future cancers and 14,500 deaths could result from radiation (assuming the cancer incidence to be 0.04%). The radiation doses that an organ receives from a typical CT study involving 2–3 scans are in the range of direct statistical significance for increased cancer risk. There are significant associations between the estimated radiation doses provided by CT scans to red bone marrow and brain and subsequent incidence of leukemia and brain tumors. Assuming typical doses for scans done after 2001 in children aged younger than 15 years, cumulative ionizing radiation doses from 2–3 head CTs could almost triple the risk of brain tumors and 5–10 head CTs could triple the risk of leukemia. In 2002, the International Commission on Radiologic Protection stated, “The absorbed dose to tissue from CT can often approach or exceed the levels known to increase the probability of cancer.” Although some studies may rely on unproven scientific assumptions or have not finished collecting data, they illustrate an important consideration for maintaining diagnostic radiation exposure at a minimum. The use of MR images can reduce the amount of ionizing radiation exposure to patients with shunts, as opposed to the use of x-rays and CT scans. Reducing radiation delivered to patients could lessen the incidences of long-term effects of radiation, most notably cancer, because the risk of all solid cancers increases linearly with increasing radiation doses up to 2.5 Sv. Fast TSE T2 sequences are commonly used in rapid brain MR imaging. Despite their utility, at least 2 limitations have been described. One is the lack of sensitivity in identification of extra-axial and parenchymal blood products that can result from overdrainage. The other is decreased catheter delineation compared with CT. Rapid steady state gradient recalled echo scanning has been advocated to eliminate the problems associated with rapid brain MR imaging by using fast TSE T2 sequences. A common concern with shunt function is the overor underdrainage resulting from the mismatch of opening pressure of the valve to the needs of the patient. To address this issue of mismatch without the need for reoperation, programmable valves that allow clinicians to change the setting of the opening pressure were designed. These programmable valves are noninvasively adhttp://dx.doi.org/10.3174/ajnr.A3334