Recent advances in medical physics

Authors: Willi A Kalender Harald H Quick

Publish Date: 2010/12/05

Volume: 21, Issue: 3, Pages: 501-504

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Abstract

Some of the major interests in medical physics over the last few years have concerned the technical advances in Computed Tomography and high field Magnetic Resonance Imaging This review discusses the introduction of Dual Source CT and explains how it can not only offer faster data acquisition but also operate with lower radiation doses This provides enormous benefits for all patients but for cardiac and pediatric examinations in particular The advances in MRI at 7 T esla are also impressive with better signal to noise cardiac and musculoskeletal applications are discussed technical improvements are workinprogress for other applicationsDiagnostic imaging is a field of research and development which has strongly grown in importance over the last decades in general but in particular also for medical physics While xray imaging and related dose issues were the traditional issues for medical physics for nearly a century the focus now is on modern threedimensional imaging This trend well reflected in European Radiology has little to do with basic or pure medical physics It mostly concerned work regarding the physical principles related technologies the testing of new approaches and apparatus in cooperation with radiologists Xray computed tomography CT and magnetic resonance imaging MRI dominated as topics Below we will refer to highpitch dualsource CT and to 7 T MRI as examples which both offer challenges to medical physics and radiology and are of great interest with respect to their potential for clinical radiology“What are the limits of ‘scan’ speed in CT” This question was not in fashion 20 years ago at that time the assumption prevailed that CT was “dead” and that it would in time be substituted largely by MRI The introduction of multislice spiral CT changed the situation completely as was covered well over the years by original articles in European Radiology Highpitch dualsource CT which emerged around 2008 was also first presented in European Radiology What has come from that promiseDualsource CT DSCT was introduced to clinical radiology around 2005 1 2 The principal innovation may appear trivial Instead of one a DSCT system simply houses two xray tubes and two detectors under its cover To get DSCT to work was above all an engineering effort but image quality issues mostly due to scattered radiation originating from two sources proved to be a major additional challenge But now there are many hundreds of clinical DSCT installations worldwide that work reliably and have set a new standard for highspeed CTA primary motivation for DSCT was to provide shorter CT data acquisition times for cardiac imaging and at the same time to provide the necessary xray power The solution provided is straightforward the effective CT data acquisition time is cut in half since two acquisition systems work in parallel the xray power is doubled because there are two xray tubes DSCT has proven to offer superior diagnostic results in numerous cardiac studies 3 4 Effective CT data acquisition times of below 100 ms per image are now standard for cardiac CT The next challenge was to reduce the total examination timePitch is defined as the ratio of table feed per rotation to the detector coverage p = d/NT Whenever pitch is increased for a spiral CT examination the total CT data acquisition is reduced accordingly In singleslice CT a maximum pitch = 2 is possible for multislice CT this value is reduced to typically 15 DSCT allows pitch can be doubled to about 3 If a slightly reduced field of measurement is acceptable as for example in cardiac or pediatric CT values up to typically 34 are allowed For 64slice CT systems with a rotation time of 03 s or less table feed values of 40 cm/s or slightly higher result Of course there initially is the concern if this technique has possible limitations with respect to image quality Ertel Lell et al were able to show by phantom experiments supported by motion robots that the two critical parameters temporal resolution and spatial resolution were unimpaired 5 Tacelli RemyJardin et al confirmed these results by clinical studies 6 Both these studies have meanwhile been confirmed numerous timesHighpitch dual source spiral CT 026 s scan of the coronary arteries at pitch 32 and an effective dose of 08 mSv The temporal acquisition scheme is illustrated in the top left quadrant a 3D and multiplanar views in quadrants two to four Images are a courtesy of S Achenbach Erlangen Germany 8Since around the year 2006 the number of worldwide installations of 7 T esla 7 T highfield MR systems for human wholebody examination is steadily increasing The driving motivation behind this trend clearly is the gain in signaltonoiseratio SNR associated with higher field strength that can be considered “the currency” of MR High SNR potentially can be traded in improved spatial and/or temporal resolution for detailed image quality and rapid image acquisition times Also a number of physical effects such as changing tissue relaxation times increased magnetic susceptibility and increased chemical shiftto name only a fewprovide the basis for dramatically further improving the inherently excellent soft tissue contrast of MR with high field strengthThe continuously rising number of installed 7 T highfield MR systems is also reflected in the significant number of scientific papers dedicated to 7 T highfield MR that have been published in European Radiology in the years 2009 and 2010 All of these papers show early clinical applications of this field strength Due to the enormous physical and technical challenges of MR imaging at 7 T that are associated with a short radiofrequency RF excitation wavelength the initial application of human 7 T MR often stays limited to “bits and pieces” and imaging of body parts small enough to fit into the vendor provided RF head coils Against this backdrop we have recently seen examples of early 7 T Neuro applications 9 10 11 7 T Musculoskeletal applications 12 as well as high resolution 7 T highfield MR of ex vivo specimen of axillary lymph nodes 13 in European RadiologyCustombuilt 8channel radiofrequency RF transmit/receive body coil for 7 T cardiac imaging Image a shows numeric simulations on an anatomic body model b Photograph of the realized body coil prototype c–e First 7 T in vivo human cardiac images acquired on a healthy volunteer showing short axis c twochamber view d and fourchamber view e Imaging parameters were FLASH 2D cine sequence flip angle 40° FOV 340 × 306 mm resolution 14 × 14 × 40 mm3 GRAPPA R = 2 20 phases per RR interval acquisition time 11 s/20 phases All images courtesy of Erwin L Hahn Institute for MRI University of DuisburgEssen Essen Germany

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