pulmonary nodules and lung tumours move substantially during respiration with significant differences according to the tumour localization .
respiratory tumour motion is a big challenge for radiotherapy , especially for high precision techniques .
an additional target dose escalation which is deemed necessary for a curative approach of lung cancer is limited as so far as tumour motion is not sufficiently quantified and integrated into modern treatment planning .
thus , the risk of side effects within the surrounding normal tissue is unacceptably high .
radiation pneumonitis and the development of lung fibrosis are the physiological reactions of the lung which are not acceptable in severely ill patients .
thus modern techniques of adaptive radiotherapy will greatly benefit from devices which take lung and tumour motion into account in radiotherapy planning .
the final objective is an additional increase in the radiation dose within the tumour volume together with maximum protection of the adjacent normal lung parenchyma .
a basic requirement is the accurate measurement and quantification of the mobility of pulmonary tumours . but also for follow - up examinations of patients with pulmonary tumours , a confident registration of the respiratory level is important in order to achieve easy identification and valid measurements .
integration of the knowledge about lung and lung tumour motion is essential for planning of high precision radiotherapy .
four - dimensional imaging using computed tomography ( 4d - ct ) is under development for optimization of radiotherapy by integration of motion [ 24 ] .
these developments also include online imaging with a linac - integrated cone beam ct . the particular advantages of magnetic resonance imaging ( mri ) are the high soft - tissue contrast , differentiation of tumour and atelectasis , the possibility to choose the optimal plane for motion quantification and the possibility to integrate further dynamic parameters , e.g. lung perfusion or o 2 
 quantification , into one imaging session .
the temporal resolution is already reasonably high for monitoring lung tumour motion in real - time . 

novel mri techniques allow for non - invasive acquisition of the motion of lung and pulmonary tumours   during the respiratory cycle with high spatial and temporal resolution [ 8 , 9 ] .
continuous improvements and new developments in mr scanner technology , such as high performance gradient systems and pulse sequences , have made this possible and address the inherent challenges of mri of the chest , such as low proton density with an unfavourable signal to noise ratio and short t2   relaxation times with 
 substantial susceptibility artefacts .
parallel imaging techniques make use of the different spatial sensitivities for the receiver coils in order to employ them for the simultaneous acquisition of image data , significantly reduce the phase encoding steps and achieve a marked improvement in spatial and temporal resolution .
thus , important requirements were met to measure the respiratory movement of the lung quantitatively .
fast - low - angle - shot ( flash ) sequences with a temporal resolution of one image per second demonstrate chest wall motion per se and postoperative changes of respiratory motion , i.e. changes of the cranio - caudal distance before and after lung volume reduction surgery .
modified true - fast - imaging - with - steady - state - precession ( truefisp ) sequences , widely used in cardiac mri , were successfully applied in volunteers ( fig .
1 ) . by adding a mathematical model , a continuous quantitative measurement of lung volume was achieved , which again showed a high correlation with spirometry . 


after successful tests in volunteers , dynamic mri was transferred to patients with solitary pulmonary tumours in order to visualise lung tumour motion by mri in two dimensions ( fig .
quantification was performed by measuring the cranio - caudal distance and the diaphragmatic length over time .
again , the results showed good correlations with the forced expiratory capacity in 1 s .
the diaphragmatic length was particularly useful in the assessment of respiratory mechanics and the respective effects of pulmonary tumours .
in such studies the truefisp sequence was superior to a flash sequence due to its higher t2 signal .
the respiratory motion of patients with a pulmonary tumour showed a significant difference between the tumour bearing and the non - tumour bearing hemithorax .
in addition , the location of the nodule had a marked effect on the regional mobility of the lung .
thus , mri is capable of providing important complementary quantitative regional information , which might be especially useful during follow - up . depending on size and location of the tumour , differences in motion
were observed in the three axes ( x- , 
 y - and 
 z - direction ) . for this assessment
pulmonary nodules were grouped as cranial , middle and caudal according to giraud et al . 

especially , the cranio - caudal motion ( y -direction ) showed a significant dependence on tumour size and location .
small tumours in the caudal lung area can 
 move up to 5 cm during maximum respiration . but also during shallow breathing
, nodules in the lower lung zone exhibited a mean dislocation of 9.5 mm in the cranio - caudal direction and 6.6 mm in the anterior  posterior direction ( fig .
theoretical calculations for 20 patients with pulmonary nodules demonstrated that this information for treatment planning might already lead to a marked increase of the target dose .
a sufficient safety margin might not be larger than 3.4 mm for the upper , 4.5 mm for the middle , and 7.2 mm for the lower lung zone .
this is substantially smaller than the conventional safety margin of 5 or 10 mm , respectively , and might allow for an additional dose escalation especially in the upper lung zone .
but it has to be mentioned that an individual quantification of the tumour motion is recommended as the interindividual differences in tumour motion are high .
simultaneous application of markers on the external chest wall demonstrated correlation and influencing factors between internal and external motion .
further developments in parallel imaging technology , especially view sharing techniques , have resulted in a further improvement in temporal resolution .
this advantage can be applied to establish a three - dimensional flash sequence for a continuous volumetric visualisation of respiratory motion of lung and tumours . in 
 fig .
4 , the 3d - flash sequence is compared to the 2d - truefisp sequence .
a 3d - flash data set contains 52 slices with isotropic 3 mm voxels and a temporal resolution of one data set per second .
motion of pulmonary nodules can be visualised and quantified in all three axes in order to be transferred in potential treatment concepts .
segmentation of such data sets , however , is still demanding and time - consuming and can not be recommended for clinical use , yet . 
  besides the mere information about tumour motion , three - dimensional techniques also allow for quantification of changes in tumour volume during the respiratory cycle .
changes in tumour volume are associated with deformations and changes of the main rotation axes of the nodules .
although this is not real rotation , the rotation of the axes can add up to 30 and impressively 
 reflects the degree of deformation .
however , this information seems to be highly important , e.g. for further improvements in position and adaptation of collimators in radiotherapy .
in addition , the knowledge about deformation of nodules and changes in volume during the respiratory cycle has substantial impact on the accuracy of volume measurement during the follow - up of pulmonary nodules and attempts to calculate nodule doubling times . 

radiotherapy very quickly leads to a significant , often temporary reduction of motion of the treated hemithorax when compared to the situation prior to radiotherapy .
this ipsilateral reduction is compensated for by an increased motion range of the contralateral lung .
obviously , such changes between right and left lung can not be detected by global lung function tests .
further studies are warranted to demonstrate which patients might benefit from preventive treatment , e.g. to prevent clinical aggravation . 

additional split lung analysis is feasible revealing important insights into differences between right and left lungs undetectable by global pulmonary function tests .
motion of pulmonary tumours can be measured in two or three dimensions , effects of respiration on volume and shape can be calculated .
however , presently the evaluation of such data sets is extremely time - consuming . 

substantial portions of this paper and the figures were originally published in plathow c , meinzer hp , kauczor h - u .
respiratory cycle from maximum expiration to maximum inspiration within 1 s. top row shows a healthy volunteer with synchronous movement of both lungs .
bottom row shows a patient with lung cancer on the left and subsequently reduced motion range of the left lung . 
 
lung motion is reflected by measuring the distance from the apex to the top of the diaphragm ( l ) .
cranio - caudal lung tumour motion is measured with regard to the intervertebral space t 6/7 ( 1 ) ; antero - posterior motion with regard central anterior margin of the spine ( 2 ) ; and medio - lateral motion with regard to the centre line of the spinal processus ( 3 ) . 
  maximum lung tumour motion ( mm ) during shallow breathing in cranio - caudal ( cc ) , medio - lateral ( ml ) and antero - posterior ( ap ) direction depending on tumour localization in the upper , middle or lower lung zone . 
 
top row , 2d - truefisp sequence ; middle row , 3d - flash sequence with a temporal resolution of one volume data set per second ; bottom row , volume segmentation of lung and tumour from the 3d - flash data set . 

 novel technology has made dynamic magnetic resonance imaging ( mri ) of lung motion and lung tumour mobility during continuous respiration feasible . 
 this might be beneficial for planning of radiotherapy of lung tumours , especially when using high precision techniques . 
 this paper describes the recent developments to analyze and visualize pulmonary nodules during continuous respiration using mri . besides recent dynamic two - dimensional approaches to quantify motion of pulmonary nodules during respiration novel three - dimensional techniques are presented . beyond good correlation to pulmonary function tests mri also provides regional information about differences between tumour - bearing and non - tumour bearing lung and the restrictive effects of radiotherapy as well as the compensation by the contralateral lung . 
 
Introduction
MRI techniques
Visualisation of lung tumour motion
Therapy monitoring
Summary
Acknowledgement
Figures and Tables