Abstract:
Disclosed herein is a magnetic resonance imaging calibration assembly in particular, for dynamic contrast—enhanced magnetic resonance imaging. An exemplary magnetic resonance imaging calibration assembly according to the present disclosure can comprise a subject receptacle for receiving at least a portion of a subject. The exemplary magnetic resonance imaging calibration assembly can further comprise a plurality of phantom compartments, each of which can contain a calibration phantom with a predetermined known T relaxation time. The plurality of phantom compartments can be attached to the subject receptacle in different ways. For example, according to some exemplary embodiments of the 10 present invention, the phantom compartments are separate compartments attached or fixed onto the subject receptacle. According to other exemplary embodiments, the phantom compartments can be formed at least partially by the subject receptacle. The phantom can be for a T1 calibration making use of its known T1.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to magnetic resonance imaging, in particular dynamic contrast-enhanced magnetic resonance imaging. 
       BACKGROUND OF THE INVENTION 
       [0002]    In Dynamic Contrast-Enhanced MRI (DCE-MRI), a contrast agent containing a substance which can be detected via magnetic resonance imaging is injected into a subject. For example, gadolinium containing compounds may be injected into a patient&#39;s blood stream, and a time series of magnetic resonance imaging images is made using a T1-weighted protocol. The time series, typically started before injection and continuing for several minutes, shows the spread of contrast agent by means of the changed T1 caused by the Gadolinium. 
         [0003]    DCE-MRI is very useful in diagnosing certain medical conditions or in evaluating the effectiveness of a therapy. If a time series of magnetic resonance images are made using a T1-weighted protocol, gadolinium based compounds may be used to illustrate the evaluate or measure vascularization of a region of a subject. For example, the technique may be used to show neuvascularization caused by tumor growth. 
       SUMMARY OF THE INVENTION 
       [0004]    The invention provides for a magnetic resonance imaging calibration assembly, a magnetic resonance imaging system, a computer program product and a computer-implemented method in the independent claims. Embodiments are given in the dependent claims. 
         [0005]    A quantitative analysis of the spread of the contrast agent through a subject however is made difficult by the fact that the signal change in a voxel due to the contrast agent is not a simple (e.g. linear) function of the concentration of contrast agent in that voxel. 
         [0006]    One area of application of this invention is MRI of the breast. Increased uptake of contrast agent in breast cancer tissue shows increased blood flow and/or capillary permeability which is indicative of, amongst others, breast cancer. The example of breast magnetic resonance imaging is used here, however embodiments of the invention are not limited to breast magnetic resonance imaging. 
         [0007]    There are presently two main approaches towards analysis of DCE-MRI, and several hybrid methods. The first is phenomenological. Here, clinicians simply observe the signal intensity as a function of time. Based on the structures seen as well as heuristics on the MRI signal as a function of time, radiologists draw conclusions on what is seen. Physicians typically observe that, after an initial uptake (measured 2-3 minutes after contrast injection), the signal may increase further, stabilize or decrease (called types 1, 2 and 3, respectively), where type 3 is strongly correlated with malignancy and type 1 is somewhat correlated with benign conditions. Later publications try to put thresholds, both on the initial uptake and on the distinction between the types, but these do not generalize from one scanner and protocol to another. Present state of the art is to leave it to the observer to pick thresholds for his or her scanner and scanner protocol, hence the total lack of quantitative recommendations in Bi-Rads. This is particularly cumbersome for sites that have scanners of different manufacturers, because they may need different thresholds for these scanners. This is also a problem for Breast MRI CAD systems, which contain thresholds on relative enhancement which depend on the scanner and protocol. 
         [0008]    The second approach to quantitative DCE-MRI is pharmacokinetic modeling. Here, an attempt is made to estimate contrast concentrations in vessels and tissue from signal intensities and subsequently to derive parameters in a tissue model from these concentrations. This approach has the promise of being more quantitative, but imposes special requirements on the scanner protocol. For instance, a reference scan is required to measure the T1 of the tissue without contrast. A high temporal resolution of the scan (better than one image every 40-60 seconds) is required to measure blood flow and the arterial input function, which is required to estimate the tissue parameters accurately. Current clinical practice for the breast does not meet these requirements (no reference scan, one image every 1-2 minutes). 
         [0009]    It is therefore advantageous to develop a way to describe signal intensity as a function of time in DCE-MRI quantitatively, i.e. that is independent of scanner and scanner protocol. In particular, we want to describe the contrast uptake of DCE-MRI of the breast in a way that is independent of scanner and scanner protocol. 
         [0010]    As described above, the two current approaches are:
   (1) Phenomenological, which requires the user to interpret the curves and compare them to other curves that were acquired with the same protocol. Breast MRI CAD systems, like Confirma&#39;s CADStream and Invivo&#39;s DynaCAD, require thresholds to be set on relative enhancements measures. These thresholds today are dependent on the scanner type and protocol.   (2) Pharmacokinetic modeling, might offer a way around this, but imposes requirements on the scanning protocol: high temporal resolution, T1 calibration scan.   
 
         [0013]    The phenomenological approach, described above, suffers from arbitrary thresholds that users have to choose. What is more, these thresholds are different between one scanner and scanner protocol and another. 
         [0014]    Pharmacokinetic modeling requires scans with a temporal resolution that is significantly higher than today&#39;s clinical practice. Also, a special scan is required to estimate the pre-contrast T1 of the tissue. 
         [0015]    Embodiments of the invention may address these or other technical problems by providing a phantom near the breast that has several compartments, each containing a different, known contrast agent concentration, dissolved in a known medium, e.g. water or air-bubble free agar. The use of a calibration phantom during the acquisition of magnetic resonance data may allow the calculation of contrast agent concentration maps. The actual concentration of the contrast agent is calculated empirically as opposed to simply examining intensity in an image. Furthermore such a technique does not require the high temporal resolution that is required for Pharmacokinetic modeling. 
         [0016]    During implementation of an embodiment of the method the intensity values for the compartments of the phantom is obtained: 
         [0017]    A simple way to get these is to have the user draw regions of interest manually. Intensity values and standard deviations can then be derived by averaging the pixel values in each ROI. 
         [0018]    Software can also be used to detect the phantom automatically, an embodiment of an algorithm is: 
         [0019]    Detect and exclude the body from the scan, e.g. by thresholding any of the acquired volumes and doing a propagation from the posterior side, invert and multiply with the original image. 
         [0020]    Detect the remaining objects, e.g. by thresholding the remaining data and measuring size, shape and position of the resulting objects and comparing with model data. 
         [0021]    Remove spurious detections by testing the objects size, shape and position against a model. 
         [0022]    The advantage of this method is that contrast agent uptake can now be measured in a scanner and scan protocol independent way. This is in particular useful for Breast MRI CAD systems, where thresholds are set on relative enhancement, which result in a classification of pixels. Presently, these thresholds depend on the scanner type and protocol. Using this method, these thresholds only need to be found once for all scanner types and protocols. 
         [0023]    Clinically, the patient may be scanned with the phantom present. The scanning protocol is exactly the same protocol that would have been used without the phantom. If the proton density of the phantom is different than that of the subject, a fast scan to measure proton density in some cases. 
         [0024]    For other calibration scans—including T1 measurements could be included. In case of T1 measurements (using a variable flip angle (VFA) approach or otherwise) the phantoms could be used to refine the T1 measurements, for example—the VFA T1 data from the phantom could be used to correct the flip angle that is used. 
         [0025]    A computer-readable storage medium as used herein encompasses any tangible storage medium which may store instructions which are executable by a processor of a computing device. The computer-readable storage medium may be referred to as a computer-readable non-transitory storage medium. The computer-readable storage medium may also be referred to as a tangible computer readable medium. In some embodiments, a computer-readable storage medium may also be able to store data which is able to be accessed by the processor of the computing device. Examples of computer-readable storage media include, but are not limited to: a floppy disk, a magnetic hard disk drive, a solid state hard disk, flash memory, a USB thumb drive, Random Access Memory (RAM) memory, Read Only Memory (ROM) memory, an optical disk, a magneto-optical disk, and the register file of the processor. Examples of optical disks include Compact Disks (CD) and Digital Versatile Disks (DVD), for example CD-ROM, CD-RW, CD-R, DVD-ROM, DVD-RW, or DVD-R disks. The term computer readable-storage medium also refers to various types of recording media capable of being accessed by the computer device via a network or communication link. For example a data may be retrieved over a modem, over the internet, or over a local area network. 
         [0026]    Computer memory is an example of a computer-readable storage medium. Computer memory is any memory which is directly accessible to a processor. Examples of computer memory include, but are not limited to: RAM memory, registers, and register files. 
         [0027]    Computer storage is an example of a computer-readable storage medium. Computer storage is any non-volatile computer-readable storage medium. Examples of computer storage include, but are not limited to: a hard disk drive, a USB thumb drive, a floppy drive, a smart card, a DVD, a CD-ROM, and a solid state hard drive. In some embodiments computer storage may also be computer memory or vice versa. 
         [0028]    A computing device as used herein refers to any device comprising a processor. A processor is an electronic component which is able to execute a program or machine executable instruction. References to the computing device comprising “a processor” should be interpreted as possibly containing more than one processor. The term computing device should also be interpreted to possibly refer to a collection or network of computing devices each comprising a processor. Many programs have their instructions performed by multiple processors that may be within the same computing device or which may even distributed across multiple computing device. 
         [0029]    A ‘user interface’ as used herein is an interface which allows a user or operator to interact with a computer or computer system. A user interface may provide information or data to the operator and/or receive information or data from the operator. The display of data or information on a display or a graphical user interface is an example of providing information to an operator. The receiving of data through a keyboard, mouse, trackball, touchpad, pointing stick, graphics tablet, joystick, gamepad, webcam, headset, gear sticks, steering wheel, pedals, wired glove, dance pad, remote control, and accelerometer are all examples of receiving information or data from an operator. 
         [0030]    ‘Magnetic Resonance (MR) data’ as used herein encompasses the recorded measurements of radio frequency signals emitted by atomic spins by the antenna of a Magnetic resonance apparatus during a magnetic resonance imaging scan. A Magnetic Resonance Imaging (MRI) image is defined herein as being the reconstructed two or three dimensional visualization of anatomic data contained within the magnetic resonance imaging data. This visualization can be performed using a computer. 
         [0031]    In one aspect the invention provides for a magnetic resonance imaging calibration assembly. The magnetic resonance imaging calibration assembly comprises a subject receptacle for receiving at least a portion of a subject. The magnetic resonance imaging calibration assembly further comprises a plurality of phantom compartments. Each of the plurality of phantom compartments contains a calibration phantom with a predetermined T1 relaxation time. The plurality of phantom compartments is attached to the subject receptacle. The plurality of phantom compartments may be attached to the subject receptacle in several different ways. In some embodiments the phantom compartment are separate compartments that are attached or fixed onto the subject receptacle. In other embodiments the phantom compartments are formed at least partially by the subject receptacle. 
         [0032]    In other words the magnetic resonance imaging calibration assembly comprises a subject receptacle for holding or supporting at least a portion of the subject and multiple phantom compartments. Each of the phantom compartments may contain a calibration phantom that has a different predetermined T1 relaxation time. This embodiment is advantageous because by holding calibration phantoms with different predetermined T1 relaxation times the magnetic resonance imaging calibration assembly can be used for calibrating T1 weighted magnetic resonance images. This may be particularly useful for calibrating magnetic resonance images acquired before and after a T1 relaxation time contrast agent has been injected into a subject. 
         [0033]    In another embodiment each of the plurality of phantom compartments has a distinct cross section. Another way of wording this is that each of the plurality of phantom compartments has a cross section which is distinguishable or identifiable with respect to the other cross sections. This is advantageous because if a T1 weighted magnetic resonance image is constructed each of the phantom compartments will be easily identifiable in the magnetic resonance image purely by the cross sections of the phantom compartments. The various profiles may be detected using image recognition; several different techniques may be used: computing the area, perimeter, number of corners, or by template matching. 
         [0034]    In another embodiment in at least one of the plurality of phantom compartments comprises a tube. This is advantageous because a tube may be filled with a calibration phantom and wrapped around or mounted on the subject receptacle. 
         [0035]    In another embodiment the at least one of the plurality of phantom compartments contains at least two sub-compartments. At least one sub-compartment is not filled with the T1 relaxation time calibration phantom. This is advantageous because the identification of sub-compartments that are not filled with T1 relaxation time calibration phantom may provide a means of identifying each of the phantom compartments. 
         [0036]    In another embodiment each of the tubes forms a closed circuit that may be advantageous for location in multiple slice magnetic resonance imaging data. If a phantom compartment is not continuous to perform a closed circuit then there may be slices where the phantom compartment is not visible in the magnetic resonance imaging image. 
         [0037]    In another embodiment the subject support further comprises a radio frequency coil for acquiring magnetic resonance data. This embodiment may be advantageous because incorporating the radio frequency coil into the subject support may save space and allow easier integration of the magnetic resonance imaging calibration assembly into a magnetic resonance imaging system. 
         [0038]    In another embodiment the magnetic resonance imaging calibration assembly further comprises a biopsy apparatus for performing a biopsy of a biopsy zone of the subject. The biopsy apparatus has a known geometry relative to the plurality of phantom compartments. This embodiment may be advantageous because when a magnetic resonance imaging image is constructed the anatomy of the subject relative to the phantom compartments is known. Likewise, if the biopsy apparatus is integrated into the magnetic resonance imaging calibration assembly then the geometry of the biopsy apparatus may be known relative to the phantom compartments also. For instance the biopsy apparatus may have a needle which is inserted into a subject using a mechanism. 
         [0039]    In another embodiment the predetermined T1 relaxation time is equivalent to a known T1 contrast agent concentration. For instance if a T1 relaxation time contrast agent is injected into a subject the phantom compartments may contain different concentrations of that particular contrast agent. However in other embodiments the T1 relaxation time of the calibration phantom is caused by a different T1 relaxation time contrast agent. 
         [0040]    In another aspect the invention provides for a magnetic resonance imaging system. The magnetic resonance imaging system comprises a magnet for creating a magnetic field for orienting the magnetic spins of nuclei of a subject located within an imaging volume. The magnetic resonance imaging system further comprises a radio frequency transceiver adapted for acquiring magnetic resonance data using a radio frequency coil. It is understood herein that a reference to a radio frequency transceiver also refers to separate radio frequency transmitter and radio frequency receiver. Likewise the reference to a radio frequency coil also refers to separate transmit and receive radio frequency coils. 
         [0041]    The magnetic resonance imaging system further comprises a subject support for receiving a magnetic resonance imaging calibration assembly. The magnetic resonance imaging calibration assembly comprises a subject receptacle for receiving at least a portion of the subject. The magnetic resonance imaging calibration assembly further comprises a plurality of phantom compartments. Each of the plurality of phantom compartments contains a T1 relaxation time calibration phantom with a predetermined T1 relaxation time. The plurality of phantom compartments is located within the imaging volume. The magnetic resonance imaging system further comprises a magnetic field gradient coil adapted for spatial encoding of the magnetic spins of nuclei within the imaging volume. The magnetic resonance imaging system further comprises a magnetic field gradient coil power supply adapted for supplying current to the magnetic field gradient coil. 
         [0042]    The magnetic resonance imaging system further comprises a computer system comprising a processor. The computer system is adapted for controlling the magnetic resonance imaging system. For instance the computer system may be interfaced to send and receive control signals to the various components of the magnetic resonance imaging system. The computer system is equivalent to a control system for the magnetic resonance imaging system. The magnetic resonance imaging system further comprises a memory containing machine-readable instructions for execution by the processor. 
         [0043]    Execution of the instructions causes the processor to acquire T1-weighted magnetic resonance data using the radio frequency coil. The processor may use the computer system to send control signals to the radio frequency transceiver and the magnetic field gradient coil power supply and in this way received data from the radio frequency transceiver which comprises the magnetic resonance data. Execution of the instructions further causes the processor to reconstruct a T1-weighted magnetic resonance image from the T1-weighted magnetic resonance data. Using Fourier techniques that are well known magnetic resonance data may be reconstructed into a magnetic resonance image. Execution of the instructions further cause the processor to determine a T1 calibration by identifying each of the plurality of phantom compartments in the T1-weighted magnetic resonance image. Each of the plurality of phantom compartments contains a calibration phantom that has a predetermined T1 relaxation time. By identifying the location of the phantom compartments in the magnetic resonance image a calibration can be constructed directly by comparing the intensity at the location of the calibration phantom with the predetermined or known T1 relaxation time. 
         [0044]    It is understood herein that references to a magnetic resonance image may also refer to multiple magnetic resonance images. For instance the magnetic resonance data may contain volumetric data. During the reconstruction process the magnetic resonance data may be reconstructed into multiple magnetic resonance images which represent slices of the volume from which the magnetic resonance data was obtained. It should also be noted that as Fourier techniques are used to reconstruct the magnetic resonance images signals from outside of the imaging volume or a specific region of interest may also construct to the reconstruction of a particular image. 
         [0045]    In another embodiment each of the plurality of phantom compartments has a distinct cross section. The plurality of phantom compartments is identified at least partially by identifying the distinct cross section in the T1-weighted magnetic resonance image. To accomplish this in some embodiments simple shape recognition or pattern recognition may be used. Since the distinct cross section has a different member of the corners or edges the phantom compartments may be readily identified by known image recognition techniques. 
         [0046]    In another embodiment at least one of the phantom compartments comprises a tube. The at least one of the plurality of phantom compartments contains at least two sub-compartments. At least one sub-compartment is not filled with the calibration phantom. The plurality of phantom compartments identified at least partially by detecting the at least one sub-compartment that is not filled in the T1-weighted magnetic resonance image. Again it is noted with reference to the T1-weighted magnetic resonance image may actually refer to multiple images. For instance, if the magnetic resonance data was for a volume which was then later reconstructed into multiple slices or images. This embodiment is advantageous because the sub-compartments which are not filled with the calibration phantom allow a spatial encoding of the various calibration phantoms. This spatial encoding allows simple recognition of the different calibration phantoms. 
         [0047]    In another embodiment the plurality of phantom compartments are identified at least partially by the relative position and/or intensity in the T1-weighted magnetic resonance image. When the magnetic resonance imaging calibration assembly is constructed the T1 relaxation time of the plurality of phantom compartments is known. Also the relative location of the various phantom compartments is a known quantity. The magnetic resonance imaging calibration assembly is a mechanical component with the plurality of phantom components fixed to the subject receptacle. Since these geometries are fixed the relative position of the different phantom compartments along with their predetermined T1 relaxation times is known. This knowledge may be used at least partially to identify the location of each of the plurality of phantom compartments. Similarly since the T1-weighted magnetic resonance image will show a different intensity for different phantom compartments depending upon the T1 relaxation time this difference in intensity can also be used to identify the phantom compartments properly. The predetermined T1 relaxation time is known for each of the plurality of phantom compartments. Image recognition software can identify the location of the phantom compartments and then it may be possible to assign the T1 value to each of the phantom compartments by sorting the intensity in the T1-weighted magnetic resonance image. 
         [0048]    In another embodiment the instructions further cause the processor to acquire proton-weighted magnetic resonance data. The acquisition of the proton-weighted magnetic resonance data is useful for comparing the calibration phantoms with the magnetic resonance data acquired from the subject. The difference in the proton density can be used in constructing a calibration. The instructions further cause the processor to reconstruct a proton-weighted magnetic resonance image. The instructions further cause the processor to construct a T10 map in accordance with the proton-weighted magnetic resonance image, the T1-weighted magnetic resonance image, and the T1 calibration. The T10 map is essentially a starting or initial T1 map that is used for calibration purposes. 
         [0049]    The instructions further cause the processor to acquire post-contrast-agent-T1 -weighted magnetic resonance data. The post-contrast-agent-T1-weighted magnetic resonance data is magnetic resonance data that is acquired after the T1-weighted magnetic resonance data. The post-contrast-agent-T1-weighted magnetic resonance data may for instance be acquired after a T1 contrast agent has been injected into the subject. In some embodiments this may be accomplished automatically by using a delay. For instance after a physician or healthcare professional has injected the subject the physician or healthcare provider may activate a button or control on a graphical user interface on the computer system which starts a timer. In other embodiments the processor may trigger the acquisition after receiving a command from a physician or a healthcare provider for instance through a graphical user interface. 
         [0050]    The instructions further cause the processor to reconstruct a post-contrast-agent-T1-weighted magnetic resonance image in accordance with the post-contrast-agent-T1-weighted magnetic resonance data. The instructions further cause the processor to construct a contrast agent concentration map in accordance with the post-contrast-agent-T1-weighted magnetic resonance image, the T10 map and the proton-weighted magnetic resonance image. This embodiment may be extremely advantageous because the contrast agent concentration map which has been constructed may be independent of the scanning system or MRI system which is used. This may be advantageous to simply acquiring pre and post-contrast agent T1-weighted magnetic resonance images and subtracted them. 
         [0051]    In another aspect the invention provides for a computer program product comprising machine executable instructions for execution by a processor of a magnetic resonance imaging system according to an embodiment of the invention. Execution of the instructions causes the processor to acquire T1-weighted magnetic resonance data using the radio frequency coil. Execution of the instructions further causes the processor to reconstruct a T1-weighted magnetic resonance image from the T1-weighted magnetic resonance data. Execution of the instructions further cause the processor to determine a T1 calibration by identifying each of the plurality of phantom compartments in the T1-weighted magnetic resonance image. The computer program product may for instance be stored on a computer-readable storage medium. As such embodiments of the invention also provide for a computer-readable storage medium containing the computer program product. 
         [0052]    In another aspect the invention provides for a computer-implemented method of determining a T1 calibration. Execution of the method by a magnetic resonance imaging system according to an embodiment of the invention comprises the step of acquiring a T1-weighted magnetic resonance data using the radio frequency coil. The method further comprises the step of reconstructing a T1-weighted magnetic resonance image from the T1-weighted magnetic resonance data. The method further comprises the step of determining the T1 calibration by identifying each of the plurality of phantom compartments in the T1-weighted magnetic resonance image. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0053]    In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which: 
           [0054]      FIG. 1  shows a flow diagram which illustrates a method according to an embodiment of the invention; 
           [0055]      FIG. 2  shows a flow diagram which illustrates a method according to a further embodiment of the invention; 
           [0056]      FIG. 3  shows a flow diagram which illustrates a method according to a further embodiment of the invention; 
           [0057]      FIG. 4  illustrates an example of a magnetic resonance calibration assembly according to an embodiment of the invention; 
           [0058]      FIG. 5  illustrates an example of a magnetic resonance calibration assembly according to a further embodiment of the invention; 
           [0059]      FIG. 6  shows examples of profiles that may be used for phantom compartments; 
           [0060]      FIG. 7  illustrates the spatial encoding of phantom compartments using empty and filled sub-compartments of the calibration phantom; 
           [0061]      FIG. 8  illustrates an example of a magnetic resonance calibration assembly according to a further embodiment of the invention; 
           [0062]      FIG. 9  illustrates an example of a magnetic resonance imaging system according to an embodiment of the invention; 
           [0063]      FIG. 10  shows T1 and T2 weighted magnetic resonance images using a magnetic resonance calibration assembly according to an embodiment of the invention; and 
           [0064]      FIG. 11  shows a comparison of subtraction images and contrast agent concentration maps. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0065]    Like numbered elements in these figures are either equivalent elements or perform the same function. Elements which have been discussed previously will not necessarily be discussed in later figures if the function is equivalent. 
         [0066]      FIG. 1  shows a flow diagram which illustrates a method according to an embodiment of the invention. In step  100  T1-weighted magnetic resonance data is acquired. In step  102  a T1-weighted magnetic resonance image is reconstructed from the T1-weighted magnetic resonance data. Then in step  104  a T1 calibration is determined by identifying each of the phantom compartments in the T1-weighted magnetic resonance image. 
         [0067]      FIG. 2  shows a flow diagram which illustrates a method according to a further embodiment of the invention. In step  200  proton-weighted magnetic resonance data is acquired. In step  202  a proton-weighted magnetic resonance image is reconstructed. In step  204  T1-weighted magnetic resonance data is acquired. In step  206  a T1-weighted magnetic resonance image is reconstructed from the T1-weighted magnetic resonance data. In step  208  a T1 calibration is determined by identifying each of the phantom compartments in the T1-weighted magnetic resonance image. In step  210  a T10 map is constructed. The T10 map is constructed using the proton-weighted magnetic resonance image, the T1-weighted magnetic resonance image and the T1 calibration. In step  212  a post-contrast-agent-T1-weighted magnetic resonance data is acquired. In step  214  a post-contrast-agent-T1-weighted magnetic resonance image is reconstructed using the post-contrast-agent-T1-weighted magnetic resonance data. Finally in step  216  a contrast agent concentration map is constructed. The contrast agent concentration map is constructed using the post-contrast-agent-T1-weighted magnetic resonance image, the T10 map and the proton-weighted magnetic resonance image. 
         [0068]      FIG. 3  shows a flow diagram which illustrates a further embodiment of the invention. In block  300  a series of dynamic contrast enhanced MRI images are acquired. These may be images for instance acquired at various times after a T1 relaxation time contrast agent has been injected into a subject. In block  302  an image acquired with a spoiled gradient echo sequence (SPGE) is acquired with a low tip angle. The data from blocks  300  and  302  are combined in block  304  with data obtained from a calibration phantom  306  according to an embodiment of the invention. In block  304  there is an empirical correction for the proton density and a T10 map is constructed. Block  308  after block  304  represents the T10 map. In block  310  there is an empirical conversion to concentration of the calibration. In block  312  further magnetic resonance imaging data is acquired and the empirical calibration is used to direct a series of concentration maps which map the concentration of the contrast agent within a subject over a period of time. 
         [0069]    The relevant steps that are illustrated in  FIG. 3  are: 
         [0000]    1. Detection of the phantom, and phantom compartments.
 
We obtain intensity values at each time instance in the dynamic scan, for each of the compartments of the phantom, either interactively or supported by an algorithm.
 
2. Computation of proton density.
 
The dynamic images are corrected for proton density. For this purpose, a proton weighted additional scan is made (e.g. a spoiled gradient echo acquisition acquired with a low flip angle). The dynamic images can be corrected using:
 
         [0000]    
       
         
           
             
                 
             
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         [0000]    In an embodiment, we could use the phantom to calibrate proton density, e.g. by using the phantom compartments as a gold standard for 100% proton density. Calibrated proton density maps may have diagnostic value.
 
3. Computation of T10 maps.
 
The pre-contrast dynamic images are converted into T1 maps (hence T10 maps). We make use of the following relationship:
 
         [0000]        S·T   1 ≈constant,
 
         [0000]    which, when we compare a voxel with a reference tissue, leads to the relationship: 
         [0000]    
       
         
           
             
               
                 
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         [0000]    which, using the phantom as a reference tissue of known T1, allows us to compute T10 maps. This approach works well for low contrast agent concentrations (&lt;1 mM), as typically found in tissue. For higher concentrations it becomes less accurate.
 
4. Computation of a contrast agent concentration map.
 
We now have T10 maps that show the initial T1 of the tissue. When can use the other images of the dynamic scan to compute T1 maps after the contrast agent has been administered. We can then compute the change in relaxivity (R=1/T1) and use this equation:
 
         [0000]        ΔR   1 ( t ) =R   1 ( t ) −R   10   =r   1   ·C ( t ), 
         [0000]    to compute the contrast agent concentration. In this equation, which r1 (mM -1 s -1 ) is the longitudinal relaxivity and C(t) (mM) the contrast agent concentration. 
         [0070]    In an embodiment, instead of using a linear relationship in steps 3 and 4, we can fit a curve to the signal versus contrast relationship in the various compartments of the phantom. 
         [0071]      FIG. 4  shows an embodiment of a magnetic resonance imaging calibration assembly  400  according to an embodiment of the invention. The magnetic resonance imaging calibration assembly comprises a subject receptacle  402 . In this case the subject receptacle  402  is a cup-shaped plastic piece. Surrounding the subject receptacle  402  is a collection of phantom compartments  404 ,  406 ,  408 ,  410 ,  412 ,  414 . Each of the phantom compartments  404 ,  406 ,  408 ,  410 ,  412 ,  414  is a tube which forms a closed circuit and is filled with distilled water solutions containing various concentrations of the T1 relaxation phantom Gd-DTPA manufactured by Omniscan. The concentration in phantom compartment  404  is a 0.5 mM concentration. The concentration in phantom compartment  406  is a 0.4 mM concentration. The concentration in the phantom compartment  408  is a 0.3 mM concentration. The concentration in phantom compartment  410  is a 0.2 mM concentration. The concentration in phantom compartment  412  is a 0.1 mM concentration. The concentration in phantom compartment  414  is a 0.0 mM concentration. 
         [0072]      FIG. 5  shows a diagram with a first magnetic resonance imaging calibration assembly  500  and a second magnetic resonance imaging calibration assembly  502 . Both the first magnetic resonance imaging calibration assembly  500  and the second magnetic resonance imaging calibration assembly  502  are located within a subject support  504 . Also shown in the Fig. is a subject  506  which has a first breast  508  and a second breast  510 . The first breast  508  is shown as being at least partially within the first magnetic resonance imaging calibration assembly  500 . The second breast  510  is shown as being within at least partially the second magnetic resonance imaging calibration assembly  502 . The first magnetic resonance imaging calibration assembly  500  has a first subject receptacle  512 . The second magnetic resonance imaging calibration assembly  502  has a second subject receptacle  514 . The first breast  508  is partially located within the first subject receptacle. The second breast  510  is located within the second subject receptacle  514 . 
         [0073]    Surrounding the first subject receptacle  512  is a plurality or a collection of phantom compartments  516 . In this embodiment the phantom compartments  516  are tubes which surround the first subject receptacle  512  horizontally. 
         [0074]    The second magnetic resonance imaging calibration assembly  502  shows an alternative embodiment. In the second magnetic resonance imaging calibration assembly there are two groups of phantom compartments  518 ,  520 . First there is a vertical group of phantom compartments  518  which are tubes which are arranged vertically. Adjacent to the vertical phantom compartments  518  are a collection of horizontal phantom compartments  520 . 
         [0075]      FIG. 6  shows a collection of cross sections  600  which may be used to distinguish different phantom compartments. Amongst the cross section  600  is a square  602 , a circle  604 , a triangle  606 , a hexagon  608 , and a plus shape  610 . These are examples of shapes which may be easily identifiable in a magnetic resonance image. It will be noted that each of these shapes has a different number of corners. If the magnetic resonance imaging slice goes through the cross section at an oblique angle then the shapes will be distorted. However, the distortion would not affect many image recognition algorithms. For instance an algorithm could simply count the number of corners and distinguish all of these shapes. The shapes shown in  FIG. 6  are illustrative and do not form a complete set of distinct cross sections. One skilled in the art will recognize that other shapes are also possible. 
         [0076]      FIG. 7  shows a collection of phantom compartments  700 . Each of the phantom compartments  700  is divided into three sub-compartments  701 . Shaded sub-compartments represent a filled sub-compartment  702 . A filled sub-compartment  702  is a sub-compartment filled with a calibration phantom with a predetermined T1 relaxation time. There are also un-shaded sub-compartments  704  which represent empty sub-compartments  704 . Empty sub-compartments are not filled with a calibration phantom. Dividing the phantom compartments  700  into individual sub-compartments  701  has the advantage that there can be a spatial encoding of the individual phantom compartments. An example of such a code can be developed by examining  FIG. 7 . For instance if the filled compartments  702  represent 1 and the empty compartments represent a 0 a code can be developed. For instance phantom compartment  706  has three filled compartments. The code for this would then be the binary code  111 . Phantom compartment  708  has a first sub-compartment which is not filled and then two filled compartments. The binary code would then be  011 . Following this example the code for phantom compartment  710  would be  101 . The code for phantom compartment  712  would be  110 . Finally the code for phantom compartment  714  would be 010. By examining one or more magnetic resonance imaging images the spatial code for a particular phantom compartment could be deduced. This could be used to identify or partially identify a phantom compartment in a magnetic resonance image or in a series of magnetic resonance images. 
         [0077]      FIG. 8  shows a further embodiment of a magnetic resonance imaging calibration assembly  800 . This magnetic resonance imaging calibration assembly  800  comprises a subject receptacle  802 . Within the subject receptacle  802  there is a first phantom compartment  804 , a second phantom compartment  806 , a third phantom compartment  808 , and a fourth phantom compartment  810 . The view shown in  FIG. 8  is a cross sectional view. The first phantom compartment has a circular cross section. The second phantom compartment  806  has a triangular cross section. The third phantom compartment  808  has a square cross section. The fourth phantom compartment  810  has a pentagonal cross section. In this embodiment there is a hole  812  at the bottom of the subject receptacle  802 . Located below the hole  812  is a biopsy needle  814  which is connected to a mechanism  816  which is able to actuate the biopsy needle  814 . The biopsy needle  814  has a tip  818 . Also shown is a subject  820  which has a breast  822  within the subject receptacle  802 . Within the breast  822  is a biopsy zone  824 . The biopsy zone  824  is a zone for which a physician or healthcare professional would like to perform a biopsy using the biopsy needle  814 . 
         [0078]    The dashed box  826  represents an imaging zone  826  of a magnetic resonance imaging system. The Fig. shown in  FIG. 8  illustrates how the magnetic resonance imaging calibration assembly  800  can be used to guide the biopsy needle  814 . After a magnetic resonance image is acquired the biopsy zone  824  may be located by a medical or healthcare professional in a magnetic resonance image. The position of the biopsy zone  824  is known relative to the phantom compartments  804 ,  806 ,  808 ,  810 . The location of the tip of the biopsy needle  818  is also known relative to the phantom compartments  804 ,  806 ,  808 ,  810 . This is because both the phantom compartments  804 ,  806 ,  808 ,  810  and the mechanism  816  and the biopsy needle  814  form a known mechanical assembly. The location of the phantom compartments  804 ,  806 ,  808 ,  810  relative to the tip  818  of the biopsy needle  814  can be used to send instructions to the mechanism  816  to guide the tip  818  of the biopsy needle  814  to the biopsy zone  824  to perform the biopsy. 
         [0079]      FIG. 9  shows an example of a magnetic resonance imaging system  900  according to an embodiment of the invention. A cross sectional view of the magnet  902  is shown. Within the bore of the magnet there is a magnetic field gradient coil  904 . It is understood that the magnetic field gradient coil  904  represents three sets of magnetic field gradient coils for encoding in three different spatial dimensions. Connected to the magnetic field gradient coil is a magnetic field gradient coil power supply which supplies current for energizing the magnetic field gradient coil. Within the bore of the magnet  902  is an imaging zone  826  which is a region which has a magnetic field uniform enough for acquiring magnetic resonance imaging data. Within the imaging zone are shown a radio frequency coil  908  for acquiring magnetic resonance data. The radio frequency coil is connected to a radio frequency transceiver  910 . Also within the bore of the magnet  902  is a subject support  909 . On the subject support there is a subject  920 . A breast  822  of the subject  820  is located within the subject receptacle  802  of a magnetic resonance imaging calibration assembly  800 . The magnetic field gradient coil power supply  906  and the radio frequency transceiver  910  are connected to the hardware interface  912  of a computer system  913 . The computer system  913  also comprises a processor  914  which is connected to the user interface  912 . The processor is also connected to a user interface  916 , computer storage  918  and computer memory  920 . 
         [0080]    In some embodiments the radio-frequency coil  908  may be integrated into the magnetic resonance imaging calibration assembly  800 . In some embodiments the magnetic resonance imaging calibration assembly  800  and the subject support  909  may be integrated into a single component. In other embodiments the magnetic resonance imaging calibration assembly  800  may be removable from the subject support  909 . 
         [0081]    The storage  918  is shown as containing T1-weighted magnetic resonance data  922 , T1-weighted magnetic resonance image  924 , a T1 calibration  926 , a proton-weighted magnetic resonance data  928 , a proton-weighted magnetic resonance image  930 , a post-contrast-agent-T1-weighted magnetic resonance data  932 , a post-contrast-agent-T1-weighted magnetic resonance image  934 , a contrast agent concentration map  936 , and an T10 map. The computer memory  920  is shown as containing computer executable code for operating and controlling the magnetic resonance imaging system  900 . The computer memory is shown as containing a magnetic resonance imaging system control module  938 . The magnetic resonance imaging system control module  938  contains computer executable code for controlling the operation and functioning of the magnetic resonance imaging system. 
         [0082]    The computer memory is also shown as containing a magnetic resonance image reconstruction module  940 . The magnetic resonance image reconstruction module contains computer executable code which is able to reconstruct magnetic resonance data into a magnetic resonance image. For instance the magnetic resonance reconstruction module  940  is able to reconstruct the T1-weighted magnetic resonance data  922  into the T1-weighted magnetic resonance image  924 . Likewise module  940  can reconstruct the proton-weighted magnetic resonance data  928  into the proton-weighted magnetic resonance image  930 . The magnetic resonance image reconstruction module  940  is also able to reconstruct the post-contrast-agent-T1-weighted magnetic resonance data into the post-contrast-agent-T1-weighted magnetic resonance image  934 . 
         [0083]    Also shown within the computer memory is the phantom compartment recognition module  942 . Depending on the type of phantom compartments  804 ,  806 ,  808 ,  810  the phantom compartment recognition module  942  may be able to recognize different types of phantom compartments. If different cross sections are used the phantom compartment recognition module may be able to recognize the cross sections. If the phantom compartments are spatially encoded the phantom compartment recognition module  942  may be able to detect the spatial encoding to recognize the phantom compartments. The computer memory  920  is also shown as containing a T1 calibration module  944 . The T1 calibration module  944  is able to use the phantom compartment recognition module  942  and the T1-weighted magnetic resonance image  924  to construct the T1 calibration  926 . The memory is also shown as containing a T10 map construction module  946 . The T10 map construction module  946  is able to use the proton-weighted magnetic resonance image  930 , the T1-weighted magnetic resonance image  924  and the T1 calibration  926  to construct the T10 map  937 . Also shown with the memory is a contrast agent concentration map construction module  948 . The contrast agent concentration map construction module  948  is able to construct the contrast agent concentration map  936  using the post-contrast-agent-T1-weighted magnetic resonance image  934 , the T10 map  937  and the proton-weighted magnetic resonance image  930 . 
         [0084]      FIG. 10  shows a T2-weighted image  1000  and a T1-weighted image  1002 . Within both images a breast  1004  is visible and also images of the phantom compartments  1006 . The phantom illustrated in  FIG. 4  was used to generate these images. The difference in intensity of the phantom compartments  1006  is visible in  FIG. 10 . 
         [0085]      FIG. 11  shows two time series of images on the left the images  1100  show DCE-MRI image constructed using the classical intensity subtraction images. The images on the right are contrast agent concentration maps  1102  calculated from the same data. The images are at different times. The images marked  1104  are at the initial time t=0 seconds. The images marked  1106  are at the t=121 seconds. The images marked  1108  are at the time t=186 seconds. The images marked  1110  are at the time t=251 seconds. These figures show that both the subtraction images  1100  and the contrast agent concentration maps  1102  show similar data. The contrast agent concentration maps  1102  have the advantage that they will be independent of the magnetic resonance imaging system used. In addition the contrast agent concentration maps  1102  show empirically calibrated contrast agent concentrations. 
         [0086]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
         [0087]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           400  magnetic resonance imaging calibration assembly 
           402  subject receptacle 
           404  phantom compartment 0.5 mM concentration 
           406  phantom compartment 0.4 mM concentration 
           408  phantom compartment 0.3 mM concentration 
           410  phantom compartment 0.2 mM concentration 
           412  phantom compartment 0.1 mM concentration 
           414  phantom compartment 0.0 mM concentration 
           500  first magnetic resonance imaging calibration assembly 
           502  second magnetic resonance imaging calibration assembly 
           504  subject support 
           506  subject 
           508  first breast 
           510  second breast 
           512  first subject receptacle 
           514  second subject receptacle 
           516  phantom compartments 
           518  vertical phantom compartments 
           520  horizontal phantom compartments 
           600  cross sections 
           602  square 
           604  circle 
           606  triangle 
           608  hexagon 
           610  plus shape 
           700  phantom compartments 
           701  sub compartments 
           702  filled sub compartment 
           704  empty sub compartment 
           706  phantom compartment 
           708  phantom compartment 
           710  phantom compartment 
           712  phantom compartment 
           714  phantom compartment 
           800  magnetic resonance imaging calibration assembly 
           802  subject receptacle 
           804  first phantom compartment 
           806  second phantom compartment 
           808  third phantom compartment 
           810  fourth phantom compartment 
           812  hole 
           814  biopsy needle 
           816  mechanism 
           818  tip of biopsy neele 
           820  subject 
           822  breast 
           824  biopsy zone 
           826  imaging zone 
           900  magnetic resonance imaging system 
           902  magnet 
           904  magnetic field gradient coil 
           906  magnetic field gradient coil power supply 
           908  radio-frequency coil 
           909  subject support 
           910  radio frequency transceiver 
           912  hardware interface 
           913  computer system 
           914  processor 
           916  user interface 
           918  storage 
           920  memory 
           922  T1-weighted magnetic resonance data 
           924  T1-weighted magnetic resonance image 
           926  T1 calibration 
           928  proton-weighted magnetic resonance data 
           930  proton-weighted magnetic resonance image 
           932  post-contrast-agent-T1-weighted magnetic resonance data 
           934  post-contrast-agent-T1-weighted magnetic resonance image 
           936  contrast agent concentration map 
           937  T10 map 
           938  magnetic resonance imaging system control module 
           940  magnetic resonance image reconstruction module 
           942  phantom compartment recognition module 
           944  T1 calibration module 
           946  T10 map construction module 
           948  contrast agent concentration map construction module 
           1000  T2-weighted image 
           1002  T1-weighted image 
           1004  breast 
           1006  phantom compartments