Automatic Passive Tracking of an Endorectal ProstateBiopsy Device Using Phase-Only Cross-Correlation

Andre de Oliveira,1 Jaane Rauschenberg,1 Dirk Beyersdorff,2 Wolfhard Semmler,1 andMichael Bock1*

MR-guided transrectal prostate biopsy is currently a time-con-suming procedure because the imaging slice is often manuallyrealigned with the biopsy needle during lesion targeting. In thiswork a pulse sequence is presented that automatically followsa passive marker attached to a dedicated MR biopsy deviceholder, thus providing an alternative to existing active trackingmethods. In two orthogonal tracking FLASH images of themarker the position of the needle axis is automatically identifiedusing a phase-only cross-correlation (POCC) algorithm. Theposition information is then used to realign a trueFISP imagingslice in real time. In phantom experiments the sensitivity of thistechnique to initial misalignments of the marker and to thesignal-to-noise ratio was evaluated. In several puncture exper-iments the precision of the needle placement was analyzed. ThePOCC algorithm allowed for a precise identification of themarker in the images even under severe initial misalignments ofup to 45°. At a frame rate 1 image/s a precision of the needleplacement of 1.5 � 1.1 mm could be achieved. Magn ResonMed 59:1043–1050, 2008. © 2008 Wiley-Liss, Inc.

Key words: passive device tracking; MR-guided prostate biopsy

Prostate cancer is one of the most common tumors in eldermen (1) and, despite a relatively low sensitivity (2), ultra-sound (US)-guided prostate biopsy is the gold standard fora definitive diagnosis. During the last 10 years severalgroups have used the excellent soft tissue contrast pro-vided by high-field MRI to improve the sensitivity of pros-tate biopsies. In a preliminary study, T2-weighted (T2w)MR imaging presented a higher sensitivity (83%, withpositive predictive value (PPV) � 50%) than transrectalUS (33%, with PPV � 57%) (3); however, the final diag-nosis can only be obtained by a biopsy. Based on thisresult, MR-guided prostate biopsies were performed usingvarious techniques.

Initial trials with a 0.5 T open magnet system demon-strated in two patients that prostate biopsies could suc-cessfully be performed under MR guidance using fast spinecho (FSE) images to observe the biopsy needle in realtime (4). Furthermore, an MR-compatible prostate biopsydevice (Invivo, Schwerin, Germany) was presented andsuccessfully tested in 12 patients to harvest biopsy sam-ples under MR-guidance in a 1.5 T closed-bore MR system(5). The device consists of a passive marker attached to amechanical holder, which defines the needle axis of thebiopsy device. In a similar set-up, using the same device, a

group of 37 patients with negative US-guided prostatebiopsies underwent MR-guided prostate biopsies (6). Inthis case, real-time imaging was used during needle posi-tioning to improve the biopsy precision. Recently, MR-guided prostate biopsies were performed in 27 men withelevated levels (�4.0 ng/mL) of prostate-specific antigen(PSA) and negative US-guided biopsies and prostate can-cer was detected in 55% of the patients (7). This endorec-tal biopsy device has no integrated endorectal coil. There-fore, external phased array coils are used to obtain goodSNR during interventions.

One of the main disadvantages of MR-guided prostatebiopsies with passive markers is the need for operators toperform time-consuming manual slice orientations. Theprostate biopsy device is localized in the images by re-peated manual adjustments of the slice position and ori-entation, which can take up to several minutes after eachmovement of the device. This procedure is also imprecise,since the operator visually assesses whether the slice isparallel to the needle direction.

Active tracking with small RF coils is an alternative topassive tracking. This technology, originally introducedfor catheter tracking, has found widespread use in in-terventional MRI for the automatic localization of de-vices (8,9). To measure the tracking coil position 1Dprojection data are acquired in all three spatial axesbetween the acquisition of two real-time MR images, andthe position information is used to automatically alignand orient the imaging slice. Compared to passive tech-niques, active tracking is technically more complex.Additionally, the electrically conducting structures inactive tracking coils can lead to potentially dangerousdevice heating (10,11).

Active tracking markers were used in an MR-compat-ible robotic manipulator (8) which was designed to po-sition the biopsy needle semiautomatically under re-mote control. This system uses three fiducial coils todetermine the 3D position of the robot and real-time MRimaging is used to direct the needle to suspect lesions(12–14). Additionally, an endorectal imaging coil is in-tegrated to improve image quality during the interven-tions (8). The system is also designed for brachytherapyof the prostate (9).

In this work a new passive tracking pulse sequence isproposed to reduce the duration of the intervention. Thissequence uses phase-only cross-correlation (POCC) (15) toautomatically determine the position of a prostate biopsypassive marker (PBPM). With this information the positionand orientation of an imaging slice is calculated automat-ically to enable real-time navigation in the prostate wherethe PBPM is used as a pointer similar to a US probe.

1Deutsches Krebsforschungszentrum, Heidelberg, Germany.2Charite, Berlin, Germany.*Correspondence to: Dr. Michael Bock, Deutsches Krebsforschungszentrum(dkfz), Abt. Medizinische Physik in der Radiologie (E020), Im NeuenheimerFeld 280, 69120 Heidelberg, Germany. E-mail: m.bock@dkfz.deReceived 9 March 2007; revised 3 August 2007; accepted 6 September 2007.DOI 10.1002/mrm.21430Published online in Wiley InterScience (www.interscience.wiley.com).

Magnetic Resonance in Medicine 59:1043–1050 (2008)

© 2008 Wiley-Liss, Inc. 1043

THEORY

Cross-correlation is a powerful tool that has been exten-sively used for feature extraction in medical applications(16–18). In this work cross-correlation is applied to mea-sure the position of a passive marker in an image and theposition information is used to automatically align theorientation of a real-time imaging slice with the marker.The robustness of cross-correlation methods can be im-proved using the phase information from the Fourier spec-tra (19). In the following, this POCC is briefly explained.

Let I(x,y) be an image that contains only a target object(e.g., a cross-section of the PBPM, which appears as a ringstructure, Fig. 1) at an arbitrary position (x0, y0) and letM(x,y) be a synthetic marker image containing the object atits center (Fig. 1b). In an MRI experiment M(x,y) is gener-ated using the known physical dimensions of the object(e.g., inner and outer ring diameter), and the protocolinformation (field of view (FOV) and matrix size) for scal-ing.

The POCC is calculated as a normalized convolution,which is more efficiently computed as a multiplication ofthe k-space representations of I and M:

POCC�kx,ky� �I�kx,ky�

�l�kx, ky���M�kx, ky�*�M�kx, ky��

[1]

Under ideal conditions one can assume that the measuredimage is a shifted version of the synthetic image with ashift vector (x0, y0). In k-space M and I thus differ only bytheir phase (Fourier shift theorem) and Eq. [1] can besimplified to:

POCC�kx,ky� �M�kx,ky� � e�i�kx x0�ky y0�

�M�kx,ky� � e�i�kx x0�ky y0���M�kx,ky�*�M�kx,ky��

��M�kx,ky�

2��M�kx,ky�

2� � e�i�kx x0�ky y0� � e�i�kx x0�ky y0� [2]

The result of this operation is a pure phase matrix. Theshift information can be extracted from the POCC by ap-plying an inverse Fourier transform which yields a deltafunction centered at (x0, y0) (Fig. 1c):

POCC�x,y� � FFT�1�POCC�kx,ky�� � ��x0,y0� [3]

In practice, other objects, noise, and artifacts are present inI(x,y) and the POCC image will not be a delta function butrather a spread distribution with a high signal intensity atthe position of the target object. Nevertheless, if the signalintensities of the other objects are not too high, the objectposition can still be found from the maximum of the POCCimage (Fig. 1d).

MATERIALS AND METHODS

In this section the endorectal biopsy device and the track-ing sequence are explained in detail. Then the influence ofinitial misalignments and SNR on the alignment errors arestudied independently under optimal conditions. Finally,the needle positioning precision is evaluated in a phantomthat emulates a clinical situation with surrounding tissuesignal.

Prostate Biopsy Device

The prostate biopsy device (4) was developed at Invivo(Schwerin, Germany) in cooperation with Charite (Berlin).The device allows harvesting biopsy samples via a trans-rectal route with the patient in a prone position. It consistsof a mechanical holder, which is attached to the patienttable of a conventional closed-bore MR system. The holderhas three degrees of freedom (Fig. 2a,b): two rotational andone translational (head-feet). The PBPM (Fig. 2c) consistsof a cylinder-shaped tube with an outer diameter of D1 �8.0 mm and a length of 80.0 mm which is filled withcontrast agent solution (Gd-DTPA/water: 1:100, T1/T2 �29.1 ms / 24.0 ms). The cylinder has an inner open diam-eter D2 � 4.0 mm that is used to insert the biopsy needle.Once properly placed in the patient’s rectum, the PBPM isconnected to the prostate biopsy device and positionedover a suspect region in the prostate gland, which typicallyappears as a low signal intensity region on T2w images.The biopsy needle has a fixed length of 15 cm, of which3 cm penetrate the prostate after the biopsy gun is shot.This length is enough for targeting the peripheral zone ofthe prostate, but a longer needle may be required to accessdeeper sites (5).

In an image plane orthogonal to the needle direction thePBPM appears as a ring structure. When observed in aplane parallel to the needle the device is seen as twoparallel lines, which can be used to visually confirm theneedle trajectory.

FIG. 1. To illustrate the theory, (a) a heavily T1-weighted cross-sectional image of the biopsy marker (white ring) held by a volun-teer, (b) synthetic marker image, (c) corresponding POCC imagewith the maximum signal intensity at the center of the marker, and(d) an anatomic image with overlaid cross delineating the marker’scentroid is shown.

1044 de Oliveira et al

Tracking Pulse Sequence

Before a biopsy sample can be retrieved in an MR-guidedprocedure the biopsy needle needs to be aligned with thedesired target. To automatically follow and track thePBPM device during target definition a dedicated trackingpulse sequence was implemented on a clinical 1.5 T MRsystem (Siemens, Symphony, Erlangen, Germany) that isequipped with a 30 mT/m gradient system (Quantum gra-dients). Using an initial localizer, two parallel trackingslices were positioned orthogonal to the PBPM axis (Fig.3). To take advantage of the relatively short T1 of thecontrast solution in the PBPM, T1w spoiled gradient echoimages (FLASH) with short TR are acquired (TR/TE �4.5 ms / 3.0 ms, � � 45°, FOV: 128 256 mm2, matrix size:128 256, partial Fourier: 4/8, acquisition time: 288 ms)providing a strong contrast between the PBPM and otherstructures.

At the beginning, two tracking images are acquired,which are positioned about 5 cm from each other, and thepulse sequence is paused for 100 ms. During this time thenew slice position is calculated by the specially modifiedimage reconstruction program. A vector defining the nee-dle direction is calculated from the position of the devicein the two images. This information is sent via an internalcomputer network to the hardware controller to reorientthe slice position of the subsequent trueFISP image acqui-sition (TR/TE � 4.5/3.0, � � 45°, FOV: 128 256 mm2,matrix size: 128 256, partial Fourier: 4/8, acquisitiontime: 288 ms) with the PBPM main axis. In this slice thereadout direction is aligned with the PBPM main axis andthe last degree of freedom that defines the phase encodingdirection is selected by the operator in the user interface asan angle varying between 0° and 180°. The estimated tra-jectory of the needle is overlaid onto the image for bettervisualization. In this case, needle bending was not consid-ered due to the fact that the biopsy needle advances onlyabout 1 cm, with an additional 2 cm after the biopsy gun isshot.

The pulse sequence (Fig. 4) is then repeated continu-ously, and, based on the position measured in the previous

tracking images, the tracking FLASH images are reposi-tioned so that they are always perpendicular to the needleaxis and centered over the estimated PBPM position tohelp prevent the ring-shaped marker escaping the FOV.

Position Calculation

To automatically identify the PBPM ring structure in thetracking images the POCC algorithm was implemented inC�� using the standard image reconstruction environment(ICE, Image Calculation Environment) of the MR system.Initially the image reconstruction program receives theprotocol information (FOV and matrix size) from the MRsystem and creates an artificial mask image based on aphysical model of the object (ring structure with outerdiameter D1, inner diameter D2). Once the mask is pre-pared the position of the PBPM is obtained with the POCCalgorithm.

A subpixel interpolation method was implemented toimprove the precision of the POCC algorithm (20). There-fore, a region of interest (ROI) was automatically placed atthe highest peak of the POCC and the position (xr,yr) wascalculated using a center-of-mass algorithm:

�xr,yr�

� � �j��n

n �i��n

n

xiPOCC�xi,yj�

�i��n

n �j��n

n

POCC�xi,yj�

,

�j��n

n �i��n

n

yj POCC�xi, yj�

�i��n

n �j��n

n

POCC�xi, yj� � [4]

Here, 2n�1 is the size of the ROI in pixels. In all calcula-tions n was set to 3. The POCC is only calculated forimages acquired perpendicular to the needle axis. Duringtheir online reconstruction the position of the PBPM isobtained and sent to the gradient hardware via the internalcomputer network.

FIG. 2. Lateral (a) and frontal (b) view of the prostate biopsy holdingdevice showing its degrees of freedom. At the distal end of theholder the passive marker and the biopsy gun (c) are attached. (d)The marker appears as a ring in a FLASH image acquired perpen-dicular to the needle main axis.

FIG. 3. Automatic passive tracking. a: Initial slice positioning on alocalizer image where the two FLASH marker images (yellow boxes)are intentionally misaligned against the needle axis. b,c: Positiondetection in the two FLASH marker images using POCC (whitecross); note the elliptic shape of the marker cross-section. d: Sub-sequent real-time trueFISP image showing the estimated biopsyneedle direction (dotted line). e: Based on the POCC position infor-mation the tracking slices are repositioned.

Automatic Passive Tracking 1045

Initial Misalignment

To assess whether initial misalignments affect the robust-ness of the automatic slice positioning due to the change ofshape of the marker from a circular to an ellipsoidal ring,the PBPM was intentionally rotated against the trackingslice normal. Therefore, the PBPM was placed at isocenterand rotated from �45° to 45° in steps of 1° with respect tothe needle/cylinder axis using a dedicated holder thatallowed for an angular precision of 1°. Two protocols wereused: TR/TE � 4.5 ms / 3.0 ms, � � 45°, FOV: 128 256 mm2, acquisition time: 576 ms with matrix size of128 256 and 256 512. To analyze errors originatingfrom spatial resolution, both the pixel and subpixel reso-lution results were computed with the POCC algorithm.For each angulation, tracking images were acquired andthe estimated PBPM direction was compared to the origi-nal rotation. During this experiment the computation timeof the POCC reconstruction was also measured using soft-ware timers (time stamps).

SNR

To assess the influence of SNR on the precision of thelocalization the PBPM was placed at isocenter parallel tothe main axis of the scanner. A set of 50 FLASH imageswere acquired (TR/TE � 7.2 ms / 4.0 ms, � � 35°, FOV:256 256 mm2, matrix size: 256 256 mm2) with thehead coil. The SNR was changed by varying the bandwidth(BW) from 100 to 1000 Hz/pixel in steps of 100 Hz/pixel.For each image series the position of the PBPM was cal-culated using the POCC algorithm. To achieve very lowSNRs of two or less, three additional image sets wereacquired with BW � 500/600/700 Hz/pixel, using a lowerflip angle (� � 5°). The SNR was calculated in each imageas the ratio of the average intensity value of the ROI placedin the PBPM to the average intensity value of the ROIplaced in the surrounding empty space. The position cal-culation was performed in all 50 images of the series andthe standard variation of the calculated position was mea-sured.

Precision Experiments

The precision of the needle positioning was measured in aphantom with vitamin E capsules (ellipsoids with major/

minor axis � 10 mm /5 mm) as targets. The phantom wasprepared with five vitamin E capsules placed under a thinlayer of gelatin in a cylindrical glass vial (diameter:135 mm, height: 20 mm). The vitamin pills were placed ina cross pattern under a paper sheet (Fig. 5a). On the sheetthe center of each vitamin E capsule was marked and thecontours of the capsules were drawn (Fig. 5b). The prostatebiopsy device was then mounted on the patient table andthe targets were placed under the PBPM. To obtain a goodSNR, a flexible loop coil and spinal array coils were usedfor signal reception. During imaging with the passivemarker tracking sequence the prostate biopsy device wasmanually aligned with the targets (Fig. 5c). Once a desiredposition was obtained, two perpendicular images with thereadout direction parallel to the PBPM main axis wereacquired to confirm the needle trajectory. The prostatebiopsy device was locked and an MR-compatible 16G bi-opsy needle with asymmetric bevel (double-shot fully au-tomatic biopsy gun; MRI devices/DAUM) was inserted inthe vitamin E capsules, leaving a mark on the paper (Fig.5d). This procedure was repeated three times and thepositioning errors were obtained for each vitamin E cap-sule from the distance between the holes and the center ofthe targets.

Tracking Experiments in a Prostate Biopsy Phantom

To evaluate the pulse sequence in a realistic setup an USprostate biopsy phantom (CIRS, Norfolk, CT) was used.The phantom contains an artificial prostate of 5.0 cmlength with three spherical lesions of 1.0 cm diameter,which present slightly lower signal intensity on T2w im-ages. Two bottles (Siemens, Erlangen) filled with phantomsolution ((1.25 g NiSO4 6H2O � 5 g NaCl)/1000 g H2O)were placed on both sides of the phantom to simulatesignal from the patients legs that could lead to aliasingartifacts.

The PBPM was introduced into the artificial rectum andthe three lesions were targeted using the automatic slicepositioning sequence. Once a lesion was found, two per-pendicular images were acquired in order to obtain theneedle trajectory. Finally, the device was locked and theneedle was positioned using trueFISP images as a refer-ence (TR/TE � 4.5 ms / 3.0 ms, � � 45°, FOV: 128 256 mm2, matrix size: 128 256, partial Fourier: 4/8,

FIG. 4. Diagram showing thesequence blocks: two FLASHimages are acquired and usedto determine the PBPM posi-tion. During a short pause thescanners digital signal proces-sors receive and prepare newslice positions using the newslice positions, a trueFISP im-age is acquired, and the se-quence continues until the in-tervention is finished.

1046 de Oliveira et al

acquisition time: 288 ms). After needle insertion the posi-tion was confirmed with a T2w FSE sequence (TR/TE �4100 ms / 115 ms, FOV: 200 250 mm2, matrix size: 192 256 mm, acquisition time: 230 ms).

RESULTS

The results of the misalignment experiment are shownin Fig. 6. The average absolute error of rotation estima-tion was 1.6° 1.1° / 1.2° 0.9° for 1.0 mm / 0.5 mmresolution without interpolation and 1.0° 0.8° / 0.8°

0.7° for 1.0 mm / 0.5 mm resolution with subpixelinterpolation. The time needed by the POCC reconstruc-tion program was 16.0 ms/image and 33.0 ms/image formatrix sizes of 128 256 and 256 512. As expected,a larger angulation error is seen for the lower spatialresolution. The mean error of the POCC with subpixelinterpolation was 0.6°/0.4° smaller for the matrix sizesof 128 256 and 256 512 than that of the POCCwithout interpolation. An ascendant error tendency wasobserved for rotations higher than 20°. In this experi-ment a sharp oscillatory behavior of up to 3° was mea-

FIG. 5. a: Precision experiment setup. b: 3D surface rendering showing the position of the vitamin E capsules. c: Paper showing needle marks.d: trueFISP images during real-time PBPM alignment. The dashed line shows expected needle trajectory. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]

FIG. 6. Rotation estimation error.The graphs show the rotation ex-periment results for pixel size of1.0 mm (left) and 0.5 mm (right)with single pixel (top) and sub-pixel (down) precision.

Automatic Passive Tracking 1047

sured. This behavior was less pronounced for highermatrix size and for subpixel precision.

The SNR experiment result is shown in Fig. 7. Thestandard deviation (SD) of the estimated position wassmaller than 0.1 mm for SNR values higher than 4. The SDwas less than 0.5 mm for SNR values between 2 and 4, and,as SNR decreased, a significant increase in the SD of up to55 mm was observed.

The precision experiment shows that the average dis-tance between the needle hole and the vitamin E capsulescenter was 1.5 1.1 mm. All targets were successfullyperforated by the biopsy needle. In this experiment, loss oftracking was observed once when the device was rotated

rapidly, forcing the operator to stop the sequence and startit again.

Figure 8 shows images acquired during the trackingexperiment with the prostate biopsy phantom. The totalexperiment duration was 30 min. During experiment setupit was observed that when the signal originating from thebottle phantoms was very high due to its proximity to thebody array coils it led to an incorrect position estimationof the POCC algorithm. To avoid this miscalculation theimaging coils were positioned at a distance of 2 cm fromthe bottles. The tracking sequence was executed for a totaltime of 10 min and no loss of tracking was observed. In thetrueFISP images a very low contrast was seen in the arti-ficial lesion. Previously acquired T2w images were used toconfirm that the PBPM was pointing at the lesion. Duringneedle positioning, an artifact with a width of about 3 mmwas seen in the real-time trueFISP images perpendicular tothe needle axis. Final T2w FSE confirmation imagesshowed that all three targets were correctly perforated bythe biopsy needle.

DISCUSSION AND CONCLUSION

In this work a real-time passive tracking sequence is pro-posed to increase the precision, which during manualalignment depends on the operator skills, and to reducethe duration of MR-guided prostate biopsies. The sequenceuses the POCC algorithm to continuously localize thePBPM in two different slices perpendicular to the biopsyneedle. Based on this information a third imaging slice ispositioned to acquire images aligned with the needle di-rection.

FIG. 7. Standard deviation of the measurement precision for differ-ent SNRs of the tracking FLASH image.

FIG. 8. Prostate biopsy phantomexperiment with the arrow show-ing a lesion in a T2w image (a).Using the automatic tracking se-quence, the PBPM is positionedover the lesion (b,c). Under true-FISP image guidance the needleis introduced into the target (d).Finally, a T2w image confirms theneedle position in the target (e).

1048 de Oliveira et al

The tracking pulse sequence extracts the position of thePBPM based on its expected shape in an image. In thisconcept a misalignment of the tracking slice and the PBPMaxis can generate errors since the acquired tracking imagessystematically deviate from the mask image of the POCC.Rather sharp fluctuations of the estimated rotation of up to3° were seen in the misalignment experiment, which canbe explained by gridding errors (Fig. 6): If the PBPM cross-section is exactly coincident with the image matrix, therotation estimation error is minimal, but under conditionswhere the PBPM is not perfectly aligned, an error propor-tional to the pixel size and to the distance between theslices can be observed. This tendency is seen in the resultswhere for higher (and subpixel) precision the amplitude ofoscillation decreases and the frequency of oscillation in-creases. Furthermore, the data show that the error in-creases with rotation angle, which might be a consequenceof the fact that the form of the PBPM changes from acircular to an elliptic ring so that the POCC algorithm isunable to precisely detect the position of the distortedshape.

Even though larger distortion errors are expected forlarger angulations, the initial misalignment experimentshows that the angle error never exceeds 4°, which trans-lates to a needle tip position error smaller than 0.21 cm.Therefore, for rotations smaller than 45° the initial angu-lation between the tracking slices and the PBPM does notsignificantly alter the reliability of the position detection.With two iterations the tracking slice will always be posi-tioned perpendicular to the needle axis in a region with anexpected estimation error of about 1°.

In very fast imaging high readout bandwidths are usedthat decrease SNR and could thus affect the performanceof the POCC algorithm. The SNR experiments indicate thatfor SNR values higher than 4 the SD of the estimatedposition of the PBPM is negligible. Thus, under typicalimaging conditions in a clinical environment a very pre-cise localization of the markers is guaranteed, since care istaken that SNRs much higher than 4 are achieved.

In clinical studies it was shown that the precisionneeded for MR-guided prostate biopsies is linked to thesmallest detectable target size. As discussed previously(7), only lesions with a diameter bigger than 10 mm couldbe targeted by the prostate biopsy device. Using activetracking in a prostate biopsy robot (8,9,12), a placementerror of around 2 mm (12) or 1.3 mm (8) was observed.Even though different methodologies were used to estab-lish the precision of the needle positioning, in this workthe passive tracking sequence provided an average error of1.5 mm, which is close to the pixel size and should pro-vide a significant improvement in the diagnostic perfor-mance of MR-guided prostate biopsies.

During studies with a prostate biopsy phantom the pas-sive tracking sequence was tested by two operators andproved to be very intuitive, allowing real-time navigationin the prostate. The fact that the prostate phantom wasdesigned for US-guided biopsies prevented an adequateevaluation of the contrast obtained by the trueFISP images.In fact, the embedded lesions could only faintly be ob-served in T2w images. Using the previously acquired T2wimages, the position of the lesions was estimated and thePBPM was moved until it reached the desired position.

Once the prostate biopsy device was locked, real-timetrueFISP imaging additionally helped in guiding the bi-opsy needle to the target, since real-time needle guidanceprovides depth control during needle advancement. In theclinical environment needle insertion is done only if theprostate lesion is seen in line with the PBPM. Therefore,this passive tracking technique poses no additional risk forthe patient, even in the case of a technical problem withthe MR-scanner.

FSE or Half Fourier Acquisition Single Shot Turbo SpinEcho (HASTE) sequences are used to obtain T2w imagesduring prostate biopsies (4–7). Unfortunately, these se-quences apply considerable amounts of RF energy duringreal-time acquisitions. Although they could in principlereplace the trueFISP acquisition as the imaging part of thepassive tracking sequence, the specific absorption rate(SAR) limits will lead to a significantly reduced acquisi-tion speed. Nevertheless, when a suspicious lesion isfound the slice position can simply be copied to an FSE orHASTE imaging protocol to acquire a high-resolution in-plane image of the needle with excellent T2 contrast.

Even though this new tracking approach proved to bereliable, on some occasions the PBPM position was lost.This could be due either to movements of the PBPM in thesame direction as the needle main axis or to quick move-ments of the device. In the first case, this limitation isintrinsic to the system, as the tracking images can onlylocalize movements of the PBPM in the directions perpen-dicular to the needle main axis. This movement can betolerated to a certain extent (up to 1.5 cm) due to the factthat the slices are positioned about 1.5 cm away from theborders of the PBPM. If large movements in this directionneed to be done during the intervention the sequence canbe paused and the tracking slices can be manually reposi-tioned. Under normal conditions it is not expected that thePBPM would move in this direction, as it would be fixed tothe prostate biopsy device. In the second case, duringclinical use, the limited space in the rectum should natu-rally avoid fast and wide movements, thus again prevent-ing loss of tracking. During use in the phantom a rotationspeed of up to 20°/s was well tolerated by the trackingsequence.

The POCC algorithm has shown incorrect position de-tection when trying to track the PBPM in images contain-ing regions with very strong signals (e.g., a bottle phan-tom). Low-frequency Fourier domain filtering could beused to attenuate signals arising from larger regions andimprove the robustness of the POCC in these cases. Thisfilter requires further optimization; however, in a clinicalsetup this situation is unlikely to happen, as the T1 valuesof human tissues are significantly higher than those of themarkers, so that a good tissue signal suppression can beachieved using short-TR pulse sequences with high flipangles.

Even though an endorectal coil was not integrated intothe PBPM, we expect that with an endorectal coil thehighest signal intensity would originate from the PBPMdue to its proximity to the coil. This high signal intensitycould be beneficial for localization. At the same time, themask would need to be modified to take the radial sensi-tivity profile of this coil into account.

Automatic Passive Tracking 1049

Redundant information such as extra tracking slicescould be used to improve the robustness and the precisionof the passive tracking sequence. This could be an optionto increase the precision during small movements of thebiopsy device to target very small lesions. Furthermore,loss of tracking events could be detected if physical priorinformation regarding the PBPM such as the device sizewere added as a constraint for the maximal distance be-tween the detected positions of the PBPM in the twotracking slices. On such occasions, a set of equally spacedslices could be acquired to relocalize the PBPM. Unfortu-nately, more tracking slices reduce the temporal resolutionand should thus only be used for specific applications.

Active tracking has often been used to track instrumentsduring MR-guided interventions (8,22) but heating effects(21) have up to now limited their use in clinical applications.Nevertheless, tracking sequences such as presented previ-ously (22) have been developed and successfully used tomonitor devices during interventions. The tracking part inthese sequences consists of a projection acquisition in the x,y, and z directions and subsequent peak detection to estab-lish the position of the instrument (23). Compared to theproposed solution with passive markers, the time spent ac-quiring new position information in the active tracking se-quence is significantly shorter, since for passive tracking thewhole k-space must be acquired instead of a single projectionline. Very high frame rates are important in intravascularprocedures but they may not be required during MR-guidedbiopsies. Considering that the movements of the PBPM areconfined by a relative small volume in the rectum, framerates as low as 0.5 image/s can be used to effectively track thePBPM and still provide enough temporal resolution forproper alignment of the device with suspect lesions. Bothtechniques yield similar precision that depend only on thepixel size. The main advantage of the passive tracking tech-nique over active tracking with marker coils is that it can beperformed using existing MR hardware. Furthermore, deviceheating due to long metallic structures in the active devices iscompletely avoided.

The presented pulse sequence provides a simple auto-matic tracking solution for MR-guided prostate biopsiesthat can easily be combined with other real-time MR pulsesequences. It exploits the fact that only minor changes ofboth orientation and position are present when the biopsydevice is reoriented in the rectum to target suspect regions.Using trueFISP images to visualize the expected path ofthe biopsy needle, the sequence currently offers a framerate of about 1 image/s. In the future this technique will betested in patients. The automatic passive tracking methodis expected to accelerate the time-consuming and impre-cise manual imaging plane orientation while providing asimple, cost-efficient, and safe alternative to other morehardware-dependent tracking systems.

ACKNOWLEDGMENTS

The authors thank Dr. Peter Speier (Siemens Medical So-lution) for support during the sequence implementation,Dr. Sven Muller who helped with the programming, andDr. Axel Winkel (Invivo Daum) for supplying the passivemarkers.

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