You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
radiotherapy planning relies on visualization of the morphology of tumours and the surrounding normal tissue .
the basic workhorse for radiotherapy planning is ct because of its well - known ability to demonstrate structure with high spatial resolution and important information about the radiodensity of the tissues .
recent and current developments in radiotherapy such as intensity - modulated and adaptive techniques would greatly benefit from image - based 
magnetic resonance imaging ( mri ) provides numerous techniques for image - based surrogates of different facets of function .
the following pairs of topics and techniques will be covered in this review : angiogenesis  perfusion mri ; metabolism  mr spectroscopy ; tissue at risk and tumour cellularity  diffusion weighted imaging ; and motion4d mri .
such techniques have been applied to many different tumours , such as cancers of the breast , prostate , cervix , rectum and liver , as well as gliomas , pulmonary nodules and multiple myeloma . from signal intensity time
curves descriptive parameters such as lag time , amplitude , slope and area under the curve ( wash - out ) can be calculated .
the results can be improved and perfusion can be quantified by the introduction of pharmacokinetic compartmental models .
the models provide results for amplitude and exchange rate and finally lead to quantification for regional blood volume , regional blood flow and mean transit time .
they can take the different properties of the contrast agents and the tissue of interest into consideration . depending on indication t1- or t2 -weighted sequences will be preferred .
it has been demonstrated that the amplitude of the signal correlates with the microvascular density of the tumour whereas the exchange rate points towards an increased permeability of the tumour vasculature ( fig .
1 ) . these parameters are also helpful to assess the angiogenic potential of tumours and are well - suited to follow - up particular therapies with anti - angiogenic compounds such as thalidomide for the treatment of multiple myeloma .
perfusion mri is also capable of predicting outcome in patients with cerebral metastases who have already undergone stereotactic radiotherapy at 6 week follow - up .
perfusion can also be assessed by non - contrast techniques such as arterial spin labelling ( fig .
mr spectroscopy ( mrs ) can provide metabolic information about tumour cells and the surrounding tissue .
shifts in the distribution of certain metabolites provide important hints towards both differential diagnosis and the tumour biological behaviour ( fig .
2 ) . clinical indications are mainly suspicious intracranial masses and prostate cancer . in the brain
, mrs is an important adjunct in the diagnosis of gliomas , e.g. separating them from metastases .
after radiotherapy , it is very helpful in differentiating recurrence from radionecrosis , where the appearance of lipid peaks is related to necrosis . in the prostate
the cancerous region typically does not contain citrate which is a metabolic product of the normal prostate gland .
the loss of citrate or an increase in the choline / citrate ratio is an important indicator for prostate cancer ( fig .
1 ) . since citrate is also present in benign hyperplastic nodules these two entities can be separated .
diffusion weighted imaging allows for localization and visualization of axonal tracts using 3d fibre reconstruction called tractography .
the displacement of important neural structures , e.g. the pyramidal tract , by a large mass can be exactly localized , and taken into account when the planned target volume is defined ( fig .
3 ) . in a study involving 20 patients exact knowledge of the course of the axonal tracts led to a change of radiotherapy planning in 71% . in this way , the volume receiving less than 15 gy could be significantly reduced , while the volume encompassed by the 80% isodose was unchanged .   as an increase in tumour cellularity due to rapid growth leads to a decrease in apparent diffusion constant ( adc ) 2d depiction of fibre integrity , e.g. fractional anisotropy , at high b - values can be used to measure tissue vitality . in primary brain tumours ,
the delineation of the target volume is impeded due to incomplete depiction of the tumour border in conventional mri .
it is known that these tumours grow along the axonal projections , in part displacing , in part infiltrating these structures .
thus , measures from diffusion weighted imaging , such as fractional anisotropy , can be used to assess fibre integrity and have been postulated as sensitive markers for overall tumour extent ; however , research has so far been hampered by limitations in data analysis .
current research is evaluating the role of dti in predicting tumour extent and shows promising initial results for improved tumour front delineation in patients with primary brain tumours .
dynamic three - dimensional ( 4d ) mri is an attractive means of imaging organ motion and the mobility of tumours over time .
this information is especially important for respiratory motion and lung tumours when high precision radiotherapy is used ( fig .
pilot studies of lung tumour mobility used 2d sequences with a temporal resolution of approximately 300 ms .
the mobility of lung nodules in x- , y- and z - directions was significantly affected by tumour size and location . obviously , mobility was greatest for small nodules in the caudal lung regions and cranio - caudal direction during deep breathing .
benefiting from new mr developments such as parallel imaging and view sharing the temporal resolution of a three - dimensional data set could be reduced to 1 s. this makes the 3d registration of lung nodules during continuous breathing feasible .
theoretical calculations show that the integration of such information into radiotherapy planning will result in a substantial increase of the dose that can be delivered to the planned target volume .
parallel assessment of chest wall motion detected by external mr markers and internal tumour motion followed by mri represents one way of triggering radiotherapy .   
besides mere volumetric visualization of morphology and structure , mri is also capable of providing  functional information which can be used to define individual biological target volumes .
prostate cancer . ( a ) parameter image of perfusion and permeability with the cancer showing increased values given as light colours representing gross tumour extension at pathology .
( b ) mr spectroscopy shows elevated choline and decreased citrate and creatine indicating the adenocarcinoma seen as a hypointense area in the t2-weighted planning sequences .
this area also corresponds to the area of increased perfusion and permeability ( a ) .  
( a ) t2-weighted turbo - spin - echo image shows the tumour in the left parietal lobe .
( b ) perfusion map obtained by arterial spin labelling , a non - contrast technique , shows tumour blood flow not elevated compared to grey matter .
( c ) single voxel spectroscopy from tumour tissue shows an increase of cho indicating increased cellularity and cell membrane synthesis .
( d ) colour - coded map of the ratio of n - acetyl - aspartate ( naa)/creatine superimposed on the t2-weighted tse image shows reduced naa signal suggesting loss of normal neural elements .  
( a ) tractography calculated from diffusion weighted imaging and registered on an axial flair image shows the avm with flow voids ( arrow ) and fibres of the corona radiata , including the cortico - spinal tract , which abuts on the avm .
( b ) sagittal view of the complete result of the tractography which superimposes the avm ( arrow ) .   lung cancer .
( a ) inspiratory image from a dynamic truefisp sequence obtained with a temporal resolution of three images per second shows the tumour in the right upper lobe with a small clip artifact within the tumour .
( b ) expiratory image from the same series shows the displacement of the tumour and also indicates the lack of chest wall infiltration .
morphology as demonstrated by ct is the basis for radiotherapy planning . 
 intensity - modulated and adaptive radiotherapy techniques would greatly benefit from additional functional information allowing for definition of the biological target volume . 
 mri techniques include several which can characterize and quantify different tissue properties and their tumour - related changes . 
 results of perfusion mri represent microvascular density and permeability ; mr spectroscopy depicts particular metabolites ; diffusion weighted imaging shows tissue at risk and tumour cellularity ; while dynamic 3d acquisition ( 4d mri ) shows organ motion and the mobility of tumours within them .