Every year in the United States, more than three quarters of a million people have a stroke, and approximately every 3 minutes someone dies from a stroke. A significant portion of stroke victims are young, and left with a devastating handicap for the rest of their lives. The monetary and societal costs of stroke represent a major economic challenge to the healthcare system.  With stroke – as with heart attack – rapid treatment is essential to limit the extent of irreversible brain injury (“time-is-brain”), and rapid determination of the cause and degree of existing brain injury can be critical in deciding treatment.

CT perfusion imaging is a quick, widely available test that displays information about blood flow to the brain that can help diagnose, treat, and predict outcome in stroke patients.  When MRI is not readily available or contraindicated, CT perfusion imaging provides the best possible estimate of brain tissue likely to die without urgent, advanced therapies, including arterial “clot busting” drugs and blood clot retrieval devices.  CT perfusion imaging can also help classify reversible brain injury (“transient ischemic attacks”) that – like cardiac angina – may not require such immediate, aggressive treatment, as well as evaluate brain injury caused by arterial spasm due to bleeding from aneurysm rupture.

Published protocols for performing CT perfusion imaging at “as low a radiation dose as reasonably achievable” – a principle endorsed by the American College of Radiology and American Society of Neuroradiology – have circulated in the medical community for over a decade.  Strict protocol rules and oversight radiation protection personnel at most medical centers ensure that optimal image quality is maintained with a total radiation exposure often considerably lower than the current FDA recommended maximum dose.  Indeed, in an early, highly quoted study that compared different scanning protocols, it was shown that image quality is actually improved when CT perfusion is obtained at a lower average X-ray beam energy than is standard for routine CT imaging.

In all of medicine – and especially for stroke – the potential risks of any diagnostic test or therapeutic procedure (however rare) must be weighed against the very real benefits of preventing death or severe disability.  We believe, and the medical literature supports, that CT perfusion imaging, when appropriately performed, is justified and provides safe, valuable information that can substantially contribute to the management of acutely ill patients in an emergency setting.  Recent advances in scanner hardware and software, and the ongoing efforts of industry, offer the promise of further, significant reductions in CT radiation dose. The radiology community is committed to work hard towards this goal of reducing CT radiation dose, and continuing to offer the best imaging care to our patients.

References:

Janet C Miller, D. Phil., et al. CT Perfusion Imaging of the Brain. Radiology Rounds: A Newsletter for Referring Physicians from the Massachusetts General Hospital Department of Radiology. Volume 8, Issue 6, June 2010. http://www.mghradrounds.org/index.php?src=gendocs&ref=2010_june

Wintermark M, Lev MH. FDA investigates the safety of brain perfusion CT. AJNR Am J Neuroradiol. 2010 Jan;31(1):2-3.

Latchaw RE, Alberts MJ, Lev MH, Connors JJ, Harbaugh RE, Higashida RT, Hobson R, Kidwell CS, Koroshetz WJ, Mathews V, Villablanca P, Warach S, Walters B; American Heart Association Council on Cardiovascular Radiology and Intervention, Stroke Council, and the Interdisciplinary Council on Peripheral Vascular Disease. Recommendations for imaging of acute ischemic stroke: a scientific statement from the American Heart Association. Stroke. 2009 Nov;40(11):3646-78.

Wintermark M, Rowley HA, Lev MH. Acute stroke triage to intravenous thrombolysis and other therapies with advanced CT or MR imaging: pro CT. Radiology. 2009 Jun;251(3):619-26.

Wintermark M, Maeder P, Verdun FR, Thiran JP, Valley JF, Schnyder P, Meuli R. Using 80 kVp versus 120 kVp in perfusion CT measurement of regional cerebral blood flow. AJNR Am J Neuroradiol. 2000 Nov-Dec;21(10):1881-4.

Broad expert consensus on the minimum requirements for CT perfusion scan acquisition can be found in Table 2 (page E25) of the following paper, which can be freely downloaded from PubMed:

Wintermark M, Albers GW, Alexandrov AV, Alger JR, Bammer R, Baron JC, Davis S, Demaerschalk BM, Derdeyn CP, Donnan GA, Eastwood JD, Fiebach JB, Fisher M, Furie KL, Goldmakher GV, Hacke W, Kidwell CS, Kloska SP, Köhrmann M, Koroshetz W, Lee TY, Lees KR, Lev MH, Liebeskind DS, Ostergaard L, Powers WJ, Provenzale J, Schellinger P, Silbergleit R, Sorensen AG, Wardlaw J, Wu O, Warach S. Acute stroke imaging research roadmap. AJNR Am J Neuroradiol. 2008 May;29(5):e23-e30.

One of my colleagues has complained about this change, stating that lesion conspicuity will be reduced by using thinner slices above the tent. I personally prefer the uniform and thinner slice thickness but cannot find anything in the literature to support either argument – there is lots about acquiring as thinly as possible in the posterior fossa but nothing regarding the optimum slice thickness for the cerebral hemispheres.

Can anybody help?

In a timely fashion that only electronic publication allows, Drs. Max Wintermark and Mike Lev, experts on CT perfusion, have put together a wonderful editorial (to appear in the print edition of AJNR, too) and an informative Special Collection on radiation exposure-related articles. Our series of Special Collections is biannual but when we believe that our readers and society constituency need further information, we can rapidly act and deploy educational materials that will keep us all well versed and up-to-date.

Additionally at www.ajnr.org you will be able to find our first podcast.  Our podcast editor is Dr. Doug Phillips from Cornell University.  This podcast is a group discussion regarding radiation exposure and CT scanning.  The group participants are Drs. Wintermark, Lev, Schaefer and Sanelli all experts in this field.  Before this podcast was recorded I invited all editorial board members of AJNR to send us their questions in order to assure a balanced point of view.  The discussants did a great job trying to address the most important issues.

Listen to the Podcast (opens as a mp3).

View the Special Collection.

The quantification of the perfusion parameters in neck cancer is valuable as the quantitative results may facilitate an objective disease monitoring in the same institution and, under certain circumstances, an interchangeability across different institutions. Nowadays, theoretical models deliver quantitative information (of course under certain inevitable assumptions concerning the relationship between MR signal and contrast agent concentration) which are obviously superior to heuristic (semi-quantitative) DCE parameters, such as peak enhancement, maximum upslope, time-to-peak enhancement, and washout slope. In the future, DCE-MRI should be besides CT a major player in this field and combined with diffusion-weighted sequences and spectroscopy may face equally the PET/CT.

Kim et al. focused on the nodal disease, which is a rather unattended aspect in the DCE imaging of neck cancer. The authors found significantly elevated baseline Ktrans in responders, which presumably has led to a better distribution of the chemotherapeutics than in the non-responders who had lower Ktrans values. This seems logical but on the other hand we should bear in mind that high Ktrans may imply severe neoangiogenesis, which, in turn, implies a more aggressive tumor with possibly higher microvascular density. Lower Ktrans may also be the result of necrotic areas thus, poor oxygenation and poor response to radiotherapy. Apparently, the interpretation of the microcirculatory parameters should not be one-sided and in a concomitant chemoradiation setting is definitely difficult to separate the effects of each therapy and draw easily logical conclusions. The authors could not demonstrate except of any significant association between ve, τi and response to therapy. This is not necessarily a drawback of the method but may reflect the heterogeneity of the volume of the target node as well as the different induction chemotherapy regimens across the patients. In a point of view what actually play the most important role are not the baseline microcirculatory parameter values themselves but how they shift during the therapy and after its completion. Obviously, the nodal and tumor response to therapy may have different time points, which are crucial for the further treatment planning. As expected, patients with small nodes in the present study [1] were complete responders after the preoperative chemoradiation, however, some of them had distant metastasis after 6 months. In other words, the 6-month follow-up is more reliable time point for deciding the predictive value of the DCE-imaging parameters. Furthermore, Kim et al. by demonstrating the feasibility of their method posed a very important question: how shall we analyse the nodal disease by means of DCE-MRI? Shall we calculate the microcirculation parameters on a single target node, on a node-to-node basis, or shall we average them?

The results in the presented paper are initial and in a small patient population, thus, far from drawing definite thresholds, cut-off values and significant predictive parameters. Ideally, the work of Kim et al. should trigger DCE-MRI studies that would: 1) compare the microcirculation parameters of histologically confirmed metastatic and reactive lymph nodes, 2) investigate the alteration of microcirculation parameters during the course of chemoradiation, defining the optimal time points for monitoring, and 3) compare the microcirculation parameters of tumoral and nodal residual disease/recurrence and chemoradiated neck tissue. Only under these premises, we would be able to use DCE-MR imaging as a diagnostic clinical tool and, thus, estimate the real predictive value of the microcirculation parameters.

References:

1. Kim S, Loevner LA, Quon H, Kilger A, Sherman E, Weinstein G, Chalian A, Poptani H. Prediction of Response to Chemoradiation Therapy in Squamous Cell Carcinomas of the Head and Neck Using Dynamic Contrast-Enhanced MR Imaging. AJNR Am J Neuroradiol. 2009 Oct 1. [Epub ahead of print]
2. Cao Y, Popovtzer A, Li D, Chepeha DB, Moyer JS, Prince ME, Worden F, Teknos T, Bradford C, Mukherji SK, Eisbruch A. Early prediction of outcome in advanced head-and-neck cancer based on tumor blood volume alterations during therapy: a prospective study. Int J Radiat Oncol Biol Phys. 2008;72:1287-90.
3. Bisdas S, Baghi M, Wagenblast J, Vogl TJ, Thng CH, Koh TS. Gadolinium-enhanced echo-planar T2-weighted MRI of tumors in the extracranial head and neck: feasibility study and preliminary results using a distributed-parameter tracer kinetic analysis. J Magn Reson Imaging. 2008;27:963-9.

Fall is upon us and so is the lecturing season! Like years before, this Fall I will be giving my brachial plexus lecture several times and the most commonly asked question by the audience is: “where can I get your MRI protocol?” For this reason I am posting it here.  Posting of protocols seems to gather considerable attention; Dr. Wiggins post on MRI and CT protocols has been viewed over 1900 times!  Caveats regarding this post: our BP protocol was designed to satisfy the needs of our clinicians here at UNC, your referring physicians may be expecting different information and you will have to adjust it to meet their needs.  There is more “than one way to skin a cat”: I suggest looking at the way others image the BP.  Dr. Ken Maravilla and his group at the University of Washington use MR neurography, a technique that we do not have but produces lovely images of the peripheral nerves.  They have published extensively about it.  Brian Bowen from the University of Miami has also written several articles on imaging of the BP and uses a very nice protocol which is different than ours.

University of Utah MRI Protocols (.doc)

University of Utah CT Protocols (.doc)