Jens Edmund

A review of substitute CT generation for MRI-only radiation therapy.

Radiotherapy based on magnetic resonance imaging as the sole modality (MRI-only RT) is an area of growing scientific interest due to the increasing use of MRI for both target and normal tissue delineation and the development of MR based delivery systems. One major issue in MRI-only RT is the assignment of electron densities (ED) to MRI scans for dose calculation and a similar need for attenuation correction can be found for hybrid PET/MR systems. The ED assigned MRI scan is here named a substitute CT (sCT). In this review, we report on a collection of typical performance values for a number of main approaches encountered in the literature for sCT generation as compared to CT. A literature search in the Scopus database resulted in 254 papers which were included in this investigation. A final number of 50 contributions which fulfilled all inclusion criteria were categorized according to applied method, MRI sequence/contrast involved, number of subjects included and anatomical site investigated. The latter included brain, torso, prostate and phantoms. The contributions geometric and/or dosimetric performance metrics were also noted. The majority of studies are carried out on the brain for 5–10 patients with PET/MR applications in mind using a voxel based method. T1 weighted images are most commonly applied. The overall dosimetric agreement is in the order of 0.3–2.5%. A strict gamma criterion of 1% and 1mm has a range of passing rates from 68 to 94% while less strict criteria show pass ratesu2009>u200998%. The mean absolute error (MAE) is between 80 and 200 HU for the brain and around 40 HU for the prostate. The Dice score for bone is between 0.5 and 0.95. The specificity and sensitivity is reported in the upper 80s% for both quantities and correctly classified voxels average around 84%. The review shows that a variety of promising approaches exist that seem clinical acceptable even with standard clinical MRI sequences. A consistent reference frame for method benchmarking is probably necessary to move the field further towards a widespread clinical implementation.

The prognosis of perforating eye injuries

In every eye trauma and especially if the bulb itself is affected the most imp6rtant problem will be the resulting injury of the visual function. Even if the prognoses of more serious contusions are not apparently superior to those of perforations, it is, however, especially the perforating lesion that is the most ambiguous, owing to various, partly primary, partly secondary complications in the eye affected. The aim of this investigation has been the re-examination of some cases of perforating eye injuries in order to elucidate the relations of certain of the above-mentioned complications to the final visual result through a description of the course of the disease. The material, the number and distribution of which are shown in table 1, comprising all the cases of ocular perforations hospitalized at the University Clinic, Rigshospitalet, Blegdamsvej, Copenhagen during the years 1958 1965 incl. The total number of cases is 263, of which 93 per cent were males while only 7 per cent were females.The preponderance of male cases is a well-known

FAMILIAL RETINAL DETACHMENT

That heredity plays a role in so-called spontaneous idiopathic detachment of the retina has been known for a long time. As early as 1885 Lang submitted a family in which brother and sister were afflicted. A collected account of the problem was not, however, published until 1936 when R i c h e r reported on 36 families with retinal detachment. In the course of time, authors from a number of different countries have reported on single families with inherited cases. In Denmark, Brendstrup (1941) has submitted 5 cases occurring in 2 families. According to Frangois the heredity is generally recessive, but cases of irregular autosomal dominance have been reported (Waardenburg), and in one family sex-linked dominance (Vogt). In the great majority of cases the disease has been found in families and in individuals with severe or excessive myopia. A few cases of familial retinal detachment in non-myopic persons have been reported by Zur Nedden and Frangois. The relationship between myopia and retinal detachment is generally known, although the number of cases occurring in non-myopic subjects appears to be on the increase. The infantile traumatic cases and in particular the senile degenerative cases usually belong to this group of non-myopic cases. Myopic retinal detachment, on the other hand, appears to occur in younger adults, between 30 and 50. Spontaneous retinal detachment appears to be caused by certain degenerative changes of the eyeground occurring with particular frequency in myopia. These myopic retinal changes do not differ from ordinary senile degeneration; only, they occur at an earlier age (Vogt). Myopia thus favours retinal detachment which, however, does not differ from other forms of retinal detachment. Vontobel demonstrated that the degenerations do not occur until the myopia has stopped progressing. H e advanced the view that the retinal changes are not due to mechanical causes related to the myopia, but to a special inherited trait. This has been confirmed in studies on mono-

A criterion for the reliable use of MRI-only radiotherapy.

BackgroundMRI-only radiotherapy will eliminate the systematic registration errors introduced when transferring MRI information to the CT. However, challenges concerning the missing information on electron density, necessary for dose calculation and patient setup on bony anatomy are introduced. This study presents a possible statistical approach to evaluate, if deviations based on MRI-only radiotherapy as compared to the CT based radiotherapy are acceptable.Methods18 head-and-neck, 21 prostate, 10 vesica and 8 pelvic patients were included in the study. Data from each patient contained a CT and a T2-weighted MRI scan, a structure set and a clinically approved CT based treatment plan, which was re-calculated with identical parameters on the density corrected MRI scans. A statistical analysis including a 95% confidence interval was performed in clinically relevant DVH points.ResultsThe mean differences in the investigated DVH points were in the order of 1.5% for the PTV and up to 4.2% for organs at risk. In addition, a proposed criterion of 2% dose difference in the PTV coverage for 95% of the patients was fulfilled for all diagnostic groups for a bulk segmented MRI in the DVH points, Dmedian and D2%, while only head-and-neck and prostate further fulfilled the criterion in D98%.ConclusionHere, we suggested a method for establishing a reliable use of MRI-only radiotherapy. A population-based study comparing CT based dose calculations with those obtained on a suggested segmentation of MRI should be initiated and acceptable deviations in clinically relevant DVH points should be established. Such a population-based approach could form a part of the clinical commissioning of MRI-only radiotherapy.

Retinal detachment in the aphakic eye.

In this investigation of 2,091 eyes the occurrence of retinal detachment in the aphakic eye has been studied. Two groups are compared as to the incidence and type of cataract. Among 187 cases of aphakic retinal detachment in the second group, the relation between type of cataract, type of tear, cataract surgery and reattachment is recorded and the surgical procedure of the detachment discussed. The significant features and the course of the aphakic detachment are outlined and the predisposition to cataractous eye is emphasized.

THE CLINICAL PICTURE AND PROGNOSIS OF RETINAL DETACHMENT.

The clinical picture of a disease is the appearance that the disease shows us, when we are faced with it, and by which we recognize it. But frequently this picture may present itself in the form of a mosaic in which some of the pieces are missing. In that event, the complete picture has to be deduced from the available pieces. Not until we have seen numerous cases can we assemble the missing pieces to form an entity to guide us in assessing the individual case. To form the background of a clinical description of retinal detachment, therefore, I have used an investigation of a large clinical series. By analysing this series, I have tried to set up some guidance for improved diagnosis and treatment, and thereby also better results. The material, derived from the University Eye Clinic, Rigshospitalet, Copenhagen, comprises a total of 900 cases admitted to the department as retinal detachment during the period 1934-1959. The sex ratio and total cases for each year are recorded in Fig. 1. It will be seen that the annual number of cases has been increasing, especially from 1945 to 1959, and incidentally this increase continues. Whether it is real or merely a consequence of the general increase in the population and longevity cannot be concluded from the present material. Investigations by Michaelson (1961) in Israel appear to show that the curve is rising. Certain findings indicate that in this country too the increase is real. For the period 1934-1945 incl. the average number of cases is 12.7, while for the period 1946-1959 it is 20.9. Even more interest attaches to the sex ratio. While in all previous series there has been a male preponderance in the ratio 60 : 40

Cone beam computed tomography guided treatment delivery and planning verification for magnetic resonance imaging only radiotherapy of the brain

ABSTRACT Background. Radiotherapy based on MRI only (MRI-only RT) shows a promising potential for the brain. Much research focuses on creating a pseudo computed tomography (pCT) from MRI for treatment planning while little attention is often paid to the treatment delivery. Here, we investigate if cone beam CT (CBCT) can be used for MRI-only image-guided radiotherapy (IGRT) and for verifying the correctness of the corresponding pCT. Material and methods. Six patients receiving palliative cranial RT were included in the study. Each patient had three-dimensional (3D) T1W MRI, a CBCT and a CT for reference. Further, a pCT was generated using a patch-based approach. MRI, pCT and CT were placed in the same frame of reference, matched to CBCT and the differences noted. Paired pCT-CT and pCT-CBCT data were created in bins of 10 HU and the absolute difference calculated. The data were converted to relative electron densities (RED) using the CT or a CBCT calibration curve. The latter was either based on a CBCT phantom (phan) or a paired CT-CBCT population (pop) of the five other patients. Results. Non-significant (NS) differences in the pooled CT-CBCT, MRI-CBCT and pCT-CBCT transformations were noted. The largest deviations from the CT-CBCT reference were < 1 mm and 1°. The average median absolute error (MeAE) in HU was 184 ± 34 and 299 ± 34 on average for pCT-CT and pCT-CBCT, respectively, and was significantly different (p < 0.01) in each patient. The average MeAE in RED was 0.108 ± 0.025, 0.104 ± 0.011 and 0.099 ± 0.017 for pCT-CT, pCT-CBCT phan (p < 0.01 on 2 patients) and pCT-CBCT pop (NS), respectively. Conclusions. CBCT can be used for patient setup with either MRI or pCT as reference. The correctness of pCT can be verified from CBCT using a population-based calibration curve in the treatment geometry.

VISUAL EVOKED POTENTIAL AS A PROGNOSTIC FACTOR FOR VITRECTOMY IN DIABETIC EYES

To determine whether or not the electrophysiological status of the eye can give information as to the result of vitreous surgery, the visual evoked potential to flash stimulation was measured prior to vitreous surgery. Seventy‐four eyes were submitted to surgery. Latencies of flash evoked potentials showed a distribution suggestive of a Gaussian curve with an abnormal extension. Dividing the material into 2 groups based on a latency longer or shorter than 100 millisec revealed a highly significant difference in visual improvement after surgery between patients with long latency and patients with short latency (P < 0.001). With a latency longer than 100 millisec the patient has an almost equal chance of visual improvement or visual reduction by surgery. The frequency of operative complications was twice as high in patients with latency longer than 100 millisec compared to patients with a latency shorter than 100 millisec.

A patch-based pseudo-CT approach for MRI-only radiotherapy in the pelvis.

PURPOSEnIn radiotherapy based only on magnetic resonance imaging (MRI), knowledge about tissue electron densities must be derived from the MRI. This can be achieved by converting the MRI scan to the so-called pseudo-computed tomography (pCT). An obstacle is that the voxel intensities in conventional MRI scans are not uniquely related to electron density. The authors previously demonstrated that a patch-based method could produce accurate pCTs of the brain using conventional T1-weighted MRI scans. The method was driven mainly by local patch similarities and relied on simple affine registrations between an atlas database of the co-registered MRI/CT scan pairs and the MRI scan to be converted. In this study, the authors investigate the applicability of the patch-based approach in the pelvis. This region is challenging for a method based on local similarities due to the greater inter-patient variation. The authors benchmark the method against a baseline pCT strategy where all voxels inside the body contour are assigned a water-equivalent bulk density. Furthermore, the authors implement a parallelized approximate patch search strategy to speed up the pCT generation time to a more clinically relevant level.nnnMETHODSnThe data consisted of CT and T1-weighted MRI scans of 10 prostate patients. pCTs were generated using an approximate patch search algorithm in a leave-one-out fashion and compared with the CT using frequently described metrics such as the voxel-wise mean absolute error (MAEvox) and the deviation in water-equivalent path lengths. Furthermore, the dosimetric accuracy was tested for a volumetric modulated arc therapy plan using dose-volume histogram (DVH) point deviations and γ-index analysis.nnnRESULTSnThe patch-based approach had an average MAEvox of 54 HU; median deviations of less than 0.4% in relevant DVH points and a γ-index pass rate of 0.97 using a 1%/1 mm criterion. The patch-based approach showed a significantly better performance than the baseline water pCT in almost all metrics. The approximate patch search strategy was 70x faster than a brute-force search, with an average prediction time of 20.8 min.nnnCONCLUSIONSnThe authors showed that a patch-based method based on affine registrations and T1-weighted MRI could generate accurate pCTs of the pelvis. The main source of differences between pCT and CT was positional changes of air pockets and body outline.

RETINAL DETACHMENT SURGERY. I. SURGERY ON THE VITREOUS BODY

Although surgical procedures have been tried right from the time that retinal detachment was first recognized, at about the same time that the ophthalmoscope was invented, rational therapy had to await Gonin’s description of the importance of retinal rupture in the genesis and treatment of the disease. In spite of the amazingly good results of Gonin’s ignipuncture, coagulating and later diathermy treatment alone proved insufficient. Paufiyue and Shapland modified the scleral shortening originally introduced by Lindner to lamellar excision, but this technique too soon showed its limitations. Schepens and Custodis introduced the buckling procedure, circular and localized. The former was later simplified by Arruga, who, by his nylon suture cerclage introduced the quickest and simplest method, but that is not saying the best. By his construction of the photocoagulator Meyer-Schwieckerath made it possible to treat retinal rupture from the inside, and the next therapeutic advance in the more complicated cases was in the same direction. It has always been realized that the vitreous body is an important factor in the pathology and pathogenesis of retinal detachment, and therapeutic attempts attacking this tissue are of old date. However, all were disappointing, and these therapeutic attempts were not revived until the advent of Shafer’s human vitreous replacement and later Cibis’ experiments using injection of silicone oil. Thus, innumerable attempts have been made to improve the operative treatment of retinal detachment. But the more polymorphous the picture the more do we have to admit that the ideal treatment has not yet been devised. The results of these