the invention , as described herein , is directed to an implantable neurostimulator for delivering neurostimulation therapy to a patient in both a trial mode and a chronic mode . in the trial mode , the neurostimulator delivers therapy that permits a clinician and patient to evaluate the efficacy of the therapy . the trial period may run for a relatively short period of time , such as several hours , days , weeks , or months . if the trial mode indicates desirable efficacy , the neurostimulator enters a chronic mode in which the neurostimulator delivers neurostimulation on a permanent basis over an extended period of time , such as several months or years . although the invention may be described in the context of neurostimulation for pelvic floor disorders , for purposes of illustration , the invention may be readily applicable to other forms of neurostimulation or other neurostimulation applications in which a trial period is desirable , including but not limited to spinal cord stimulation for relief of chronic , intractable pain , gastric stimulation for treatment of gastric mobility disorders or obesity , and stimulation for the treatment of sexual dysfunction . sacral nerve stimulation will be described herein for purposes of illustration . however , the invention may be applied to applications involving stimulation of other nerves , such as pudendal nerves , perineal nerves , the spinal cord , the stomach , or other areas of the nervous system . fig1 is a diagram illustrating an implantable neurostimulation system 20 for sacral nerve stimulation via at least one lead 10 . neurostimulation system 20 includes an implantable neurostimulator 24 that supports both a trial stimulation mode and a chronic stimulation mode . neurostimulation system 20 delivers neurostimulation to the sacral nerves or other regions of the nervous system known to influence pelvic floor disorders , urinary control disorders , fecal control disorders , interstitial cystitis , sexual dysfunction , pelvic pain , or some combination thereof . neurostimulator 24 includes an implantable pulse generator , and delivers neurostimulation therapy to patient 12 in the form of electrical pulses generated by the implantable pulse generator . in the example of fig1 , neurostimulator 24 is implanted in the upper left buttock of patient 12 , but may be implanted at other locations . a proximal end of stimulation lead 10 is coupled to a connector block 26 associated with neurostimulator 24 . as shown in fig1 , lead 10 may be coupled to connector block 26 via a lead extension 28 and connector 30 . neurostimulator 24 may be implanted within a subcutaneous pocket 31 that serves as the implant site . subcutaneous pocket 31 is formed by a surgical procedure . lead 10 carries one or more stimulation electrodes to permit delivery of electrical stimulation to sacral nerves . for example , implantable neurostimulation system 20 may stimulate organs involved in urinary , fecal or sexual function via c - fibers or sacral nerves at the second , third , and fourth sacral nerve positions , commonly referred to as s2 , s3 , and s4 , respectively . also , in some embodiments , lead 10 may carry one or more sense electrodes to permit neurostimulation device 24 to sense electrical signals within sacrum 16 . accordingly , lead 10 includes an outer lead body that contains one or more conductors to electrically couple the electrodes to terminals within connector block 26 . in some embodiments , trial neurostimulator 24 may be coupled to two or more leads deployed at different positions relative to the spinal cord or sacral nerves , for example . implantable neurostimulator 24 eliminates the need for a percutaneous connection for trial stimulation , reducing the risk of infection and affording greater convenience and comfort to the patient . moreover , the absence of a percutaneous connection makes the neurostimulator easier to tolerate , and presents a reduced infection risk , permitting trial periods to run for extended period of times . the trial period may run for days , weeks or even months , in view of heightened patient tolerance . as examples , the trial period may be less than one month or , in some cases , less than six months . neurostimulator 24 supports both trial stimulation and chronic stimulation . consequently , neurostimulator 24 offers the convenience of fewer surgical procedures for patients by enabling both a trial period and chronic operation without the need to remove a trial neurostimulator and replace it with a chronic stimulator after effective trial therapy . in the trial mode , neurostimulator 24 delivers either a full set of stimulation parameters or a limited set of stimulation parameters specified by a clinician . the patient may be permitted to adjust stimulation parameters , such as amplitude , pulse width and pulse rate , i . e ., frequency , during the trial mode . alternatively , the clinician may specify a set of fixed stimulation parameters or a limited range of adjustment . in the chronic mode , neurostimulator 24 may deliver a full set of stimulation parameters , and may permit a full range of adjustment by patient 12 , subject to limits specified by the clinician . in addition , neurostimulator 24 may accept new parameters , adjusted parameters , or new programs containing parameter sets , via a physician programmer . in one embodiment of the invention , a device includes at least one sensor . the one or more sensor may provide a variety of information indicative of the level of efficacy achieved by the neurostimulation therapy delivered by neurostimulator . the information may be any information relating to the function of the bladder , or any other segment of the patient &# 39 ; s urinary tract , in storing releasing and passing urine . for example , the sensor may monitor parameters such as bladder pressure , bladder contractile force , urinary sphincter pressure , urine flow rate , urine flow pressure , voiding amount , and the like . other examples of sensed information include urine flow velocity , urine or bladder temperature , impedance , urinary ph , or chemical constituency of the urine . any of such information may reveal the effect of the neurostimulation therapy on the physiological function of the bladder , the urethra , or the urinary sphincter . for example , if the sensor indicates excessive pressure , excessive contractile force , or involuntary urine flow ( i . e ., leakage ) in response to a set of stimulation parameters , the information will be gathered by the one or more sensors . in still other embodiments , the one or more sensors may be implanted within a patient to sense a physiological state of the patient . for example , a sensor may be deployed to sense cardiac activity , respiratory activity , electromyographic activity , or the like , as an indication of patient activity level . such activity level information , in conjunction with other information , may be useful in determining the efficacy of the stimulation parameters . other types of sensors may detect a posture or activity level of the patient . for example , an accelerometer may detect an elevated activity level , e . g ., during exercise , while other sensors may detect whether the patient is sitting , standing or lying down . in addition , some of the information obtained by such sensors , such as respiration activity , may be analyzed to determine , e . g ., whether the patient is sleeping . in an embodiment including one or more sensors , the function of the one or more sensors can be different during the trial mode and the chronic mode . in one embodiment , the one or more sensors gathers information more frequently in the trial mode than it does in the chronic mode . in one embodiment , for example , the one or more sensors gathers information on an hourly or daily basis in the trial mode , and on a weekly basis in the chronic mode . in another embodiment , at one or more sensors gathers information in response to every voiding event , for example , that is indicated by the patient , and in the chronic mode , information is gathered only in response to every fifth , tenth , or twentieth ( for example ) voiding event . in another embodiment including one or more sensors , the information that is gathered during the trial mode includes more physiological parameters than that gathered during the chronic mode . in yet another embodiment including one or more sensors , both the information that is gathered , and the frequency upon which it is gathered is different in the trial mode and the chronic mode . an example of such an embodiment would be a device where more parameters are gathered more often during the trial mode than in the chronic mode . the one or more sensors , if included in an embodiment of the invention , can carry sufficient battery resources , a rechargeable battery , or an inductive power interface that can permit extended operation . the sensor ( s ) may be implanted by minimally invasive , endoscopic techniques for example . in some embodiments , the sensor ( s ) transmits sensed information continuously or periodically to the neurostimulator or the patient programmer . in this case , the sensor ( s ) can monitor physiological conditions continuously or periodically . alternatively , the neurostimulator or patient programmer may trigger activation of the sensor ( s ) to capture information at desired intervals . in some cases , triggered activation may occur when the patient enters information into the patient programmer to indicate a voiding event , for example . triggered activation of the sensor ( s ) may be useful in conserving battery life , if applicable , of the sensor ( s ) or neurostimulator . in one embodiment , multiple sensors may be provided and dedicated to different parameters or different locations within the urinary tract . in one embodiment of the invention , the sensor ( s ) can be programmed to gather information at a particular frequency ( either the same or different in both the trial and chronic mode ), and be configured to gather information when prompted by the patient programmer or the physician programmer . in another embodiment , the sensor ( s ) are programmed to gather information at a particular frequency during the trial mode and are programmed to be activated when prompted by the patient programmer or physician programmer during the chronic mode . in yet another embodiment , the sensor ( s ) are programmed to gather information at particular frequencies during the trial and chronic mode ( either the same frequency or different frequencies ) and can also be activated when prompted by the patient programmer or physician programmer during both the trial mode and the chronic mode . rather than immediately transmitting the information to the neurostimulator or the patient programmer , the sensor ( s ) may initially store the information internally for subsequent wireless transmission . hence , in some embodiments , the information may be stored within the sensor ( s ), and later transmitted to the neurostimulator or the patient programmer . in this case , the neurostimulator or the patient programmer may interrogate the sensor ( s ) to obtain the stored information . in one embodiment , the sensor ( s ) can include a sensor processor , a sensing element , memory , wireless telemetry interface , and a power source . the sensor ( s ) also may include an internal clock to track date and time of voiding events . the sensor ( s ) may have a capsule - like shape , and may be placed within the bladder or the urethra by endoscopic introduction via the urethra , or by hypodermic injection using a hypodermic needle . alternatively , the sensor ( s ) may be surgically implanted . in the case of minimally invasive endoscopic introduction , the sensor ( s ) may be constructed in a manner similar to the sensors described in u . s . patent application ser . no . 10 / 978 , 233 , to martin gerber , filed oct . 29 , 2004 , and entitled “ wireless urinary voiding diary system ,” which claims the benefit of u . s . provisional application no . 60 / 589 , 442 , filed jul . 20 , 2004 ; or u . s . patent application ser . no . 10 / 833 , 776 , to mark christopherson and warren starkebaum , filed apr . 28 , 2004 , entitled “ implantable urinary tract monitor ,” the entire content of each of which is incorporated herein by reference . the sensing element may be selected for any of a variety of urodynamic testing applications , and may include appropriate signal processing circuitry such as amplifier , filter , driver , and analog - to - digital conversion circuitry for presentation of sensed information to sensor processor . for urodynamic testing , sensing element may take the form of a pressure , flow , velocity , volume , temperature , impedance , or contractile force sensor . for pressure measurements , for example , sensing element may include one or more diaphragm sensors , strain gauge sensors , capacitive sensors , piezoelectric sensors , or other sensors used in conventional catheter - based urodynamic testing to sense pressure . as a further example , for bladder emptying , sensing element may include a conductive sensor to sense the presence of urine within the lower region of the bladder . for flow measurements , sensing element may comprise a pulsed doppler ultrasonic sensor , or a laser doppler flow sensor . doppler shifting of the frequency of the reflected energy indicates the velocity of the fluid flow passing over a surface of sensing element . consequently , in some embodiments , the sensor ( s ) may include circuitry , such as a quadrature phase detector , in order to enable the monitor to distinguish the direction of the flow of fluid in addition to its velocity . as a further example , sensing element may include any one or more thermal - convection velocity sensors . a thermal - convection velocity sensor may include a heating element upstream of a thermistor to heat urine within the urethra such that flow rate may be measured according to the temperature of the heated fluid when it arrives at the thermistor . in other embodiments , flow rate may be determined from the output of a concentration or temperature sensor using fick &# 39 ; s techniques . in some embodiments , the sensing element may include multiple sensors of a given type , as well as multiple types of sensors , e . g ., pressure , flow , bladder emptying , or the like . accordingly , the information obtained by the sensor ( s ) may then include different types of physiological parameters associated with a voiding event . alternatively , multiple the sensor ( s ) may be deployed within the bladder or the urethra . in this case , each of the sensor ( s ) may be configured with a different type or set of sensing elements to collect a variety of different urodynamic parameters during a voiding event . in some other embodiments , the sensing element may be chosen to sense a physiological state , such as an activity type , activity level , or posture of the patient . for example , the sensing element can include an accelerometer to detect an elevated activity level , or a decreased activity level . as further shown in fig1 , implantable neurostimulation system 20 also may include a clinician programmer 34 and a patient programmer 36 . clinician programmer 34 may be a handheld computing device that permits a clinician to program neurostimulation therapy for patient 12 , e . g ., using input keys and a display . for example , using clinician programmer 34 , the clinician may specify neurostimulation parameters for use in delivery of neurostimulation therapy . clinician programmer 34 supports radio frequency telemetry with neurostimulator 24 to download neurostimulation parameters and , optionally , upload operational or physiological data stored by neurostimulator 24 . in this manner , the clinician may periodically interrogate neurostimulator 24 to evaluate efficacy and , if necessary , modify the stimulation parameters . once the trial period has concluded and has been deemed effective , programmer 34 may have the ability to renew the operational “ subscription ” of neurostimulator 24 with neurostimulation parameters for chronic stimulation therapy . like clinician programmer 34 , patient programmer 36 may be a handheld computing device . patient programmer 36 may also include a display and input keys to allow patient 12 to interact with patient programmer 36 and implantable neurostimulator 24 . in this manner , patient programmer 36 provides patient 12 with an interface for control of neurostimulation therapy by neurostimulator 24 . for example , patient 12 may use patient programmer 36 to start , stop or adjust neurostimulation therapy during the trial or chronic mode . in particular , patient programmer 36 may permit patient 12 to adjust stimulation parameters such as duration , amplitude , pulse width and pulse rate within an adjustment range specified by the clinician via clinician programmer 34 . in one embodiment of the invention , different options may be provided to the patient during the trial period and the chronic period . neurostimulator 24 , clinician programmer 34 and patient programmer 36 may communicate via wireless communication , as shown in fig1 . clinician programmer 34 and patient programmer 36 may , for example , communicate via wireless communication with neurostimulator 12 using rf telemetry techniques known in the art . clinician programmer 34 and patient programmer 36 also may communicate with each other using any of a variety of local wireless communication techniques , such as rf communication according to the 802 . 11 or bluetooth specification sets , or other standard or proprietary telemetry protocols . the trial mode may be initiated by an enable signal transmitted by neurostimulator 24 from clinician programmer 34 or patient programmer 36 . likewise , the trial period may be terminated by a disable command transmitted by clinician programmer 34 or patient programmer 36 , e . g ., in response to a physician or patient command or upon expiration of a trial period timer . alternatively , the trial mode may be operative only so long as neurostimulator 24 continues to receive a periodic enable signal . in this case , if the enable signal is not received within a scheduled interval , neurostimulator 24 disables the trial mode . as a further alternative , once enabled , neurostimulator 24 may track the progress of the trial period using an internal clock or clock derived from an external source , and unilaterally disable the trial mode when the trial period has lapsed . upon disablement , neurostimulator 24 may remain idle until it is explanted , in the case of an unsuccessful trial , or until a clinician transmits an authorization command via the clinician programmer 34 or patient programmer 36 to enter the chronic mode , in the case of a successful trial . in the chronic mode , neurostimulator 24 is reactivated and enabled for continued delivery of neurostimulation therapy . in this case , neurostimulator 24 may receive simply the authorization command to invoke a set of parameters or a program stored in the neurostimulator to support the chronic mode . alternatively , neurostimulator 24 may receive a new parameter or program via the clinician programmer 34 or patient programmer 36 , or be entirely reprogrammed . in each case , there is no need for another surgical procedure . instead , the provisioning of neurostimulator 24 is accomplished by wireless telemetry . fig2 is a block diagram illustrating various components of an implantable neurostimulator 24 a . as shown in fig2 , device 12 delivers neurostimulation therapy via electrodes 37 a , 37 b , 37 c , 37 d of lead 10 ( collectively “ electrodes ”). electrodes 37 may be ring electrodes arranged on an axial lead or pad electrodes arranged in an array on a paddle lead . the configuration , type and number of electrodes 37 illustrated in fig2 are merely exemplary . electrodes 37 are electrically coupled to a therapy delivery circuit 36 via lead 10 . therapy delivery circuit 36 may , for example , include an implantable pulse generator coupled to a power supply 38 that generates stimulation energy from power delivered by a battery 40 . the implantable pulse generator within therapy delivery circuit 36 delivers electrical pulses to patient 12 via at least some of electrodes 37 under the control of a processor 42 . in one example , therapy delivery circuit 36 may deliver neurostimulation pulses with parameters selected to have values effective in controlling or managing symptoms of urinary incontinence , such as involuntary leakage . an exemplary range of neurostimulation stimulation pulse parameters likely to be effective in treating incontinence , e . g ., when applied to the sacral or pudendal nerves , are as follows : 1 . frequency : from approximately 0 . 5 hz to 500 hz , in one embodiment from approximately 10 hz to 250 hz , and in yet another embodiment from approximately 10 hz to 25 hz . 2 . amplitude : from approximately 0 . 1 volts to 50 volts , in one embodiment from approximately 0 . 5 volts to 20 volts , and in yet another embodiment from approximately 1 volt to 10 volts . 3 . pulse width : from about 10 microseconds to 5000 microseconds , in one embodiment from approximately 100 microseconds to 1000 microseconds , and in yet another embodiment from approximately 180 microseconds to 450 microseconds . processor 42 controls the implantable pulse generator within therapy delivery circuit 36 to deliver neurostimulation therapy according to selected stimulation parameters . specifically , processor 42 controls therapy delivery circuit 36 to deliver electrical pulses with selected amplitudes , pulse widths , and rates specified by the programs . in addition , processor 42 also controls therapy delivery circuit 36 to deliver the neurostimulation pulses via selected subsets of electrodes 37 with selected polarities . processor 42 may control therapy delivery circuit 36 to deliver each pulse according to a different program , thereby interleaving programs to simultaneously treat different symptoms or provide a combined therapeutic effect . for example , in addition to treatment of urinary incontinence , neurostimulator 24 a may be configured to deliver neurostimulation therapy to treat pain . processor 42 may include a microprocessor , a controller , a digital signal processor ( dsp ), an application - specific integrated circuit ( asic ), an field - programmable gate array ( fpga ), discrete logic circuitry , or the like . neurostimulator 24 a also includes a memory 46 . in some embodiments , memory 46 stores multiple sets of stimulation parameters that are available to be selected by patient 12 or a clinician for delivery of neurostimulation therapy . for example , memory 46 may store stimulation parameters transmitted by clinician programmer 34 . memory 46 also stores program instructions that , when executed by processor 42 , cause device 12 to deliver neurostimulation therapy . in the example of fig2 , memory 46 may store one or more trial programs 47 and one or more chronic programs 49 that control delivery of stimulation pulses for a trial mode and a chronic mode , respectively . in this case , an external device such as clinician programmer 34 or patient programmer 36 may send a signal to cause neurostimulator 24 to select either trial program 47 or chronic program 49 , which are already loaded into memory 46 . in other embodiments , trial and chronic programs may be selectively loaded into memory 46 , e . g ., by programming from clinician programmer 34 or patient programmer 36 , to cause neurostimulator 24 to enter either the trial mode or the chronic mode . memory 46 may include any volatile , non - volatile , magnetic , optical , or electrical media , such as a ram , rom , cd - rom , hard disk , removable magnetic disk , memory cards or sticks , nvram , eeprom , flash memory , and the like . accordingly , the invention also contemplates computer - readable media storing instructions to cause processor 42 to provide the functionality described herein . a telemetry circuit 44 supports wireless communication between device 12 , clinician programmer 34 , patient programmer 36 , or some combination thereof . in addition , in some embodiments , neurostimulator 24 a may optionally include a timer 48 to be used during the trial mode . timer 48 may serve to time the duration of the trial period , in some embodiments . for example , upon initiation of the trial period , timer 48 starts running to track the elapsed time in the trial period relative to a maximum trial period time . in some embodiments , when timer 48 expires , processor 42 responds by disabling therapy delivery circuit 36 to terminate the trial mode . in other words , processor 42 stops the trial period by stopping delivery of neurostimulation therapy to patient 12 by trial neurostimulator 24 a . in this manner , implantable neurostimulator 24 a can be configured to prevent patient 12 or a physician from prolonging the trial period beyond a prescribed period of time . rather , implantable neurostimulator 24 a has a finite period of operation , determined by a maximum trial period that may be specified by the manufacturer or in a programmable manner by a clinician . in chronic mode , timer 48 may be used in some embodiments to serve as a method to ensure clinician evaluation of stimulation throughout chronic therapy . in other words , the chronic mode may be periodically disabled , e . g ., with advance warning to the patient through patient programmer 36 , to compel a clinical visit by the patient for periodic evaluation . timer 48 may be implemented in hardware using a real - time clock , in software by processor 42 , or a combination thereof . accordingly , timer 48 is illustrated as a separate component in fig2 merely for exemplary purposes . battery 40 of implantable neurostimulator 24 a may be selected based on the stimulation therapy needed . in some cases , battery 40 may be a conventional lithium battery normally used in typical stimulation devices . the size and shape of battery 40 may be different to make implantable stimulator 24 smaller to fit in certain places on the patient or to allow a plurality of stimulation or sensing leads to be attached . in some embodiments , other battery technologies such as nickel - metal - hydride or nicad may be used in battery 40 . fig3 is a block diagram illustrating various components of another implantable neurostimulator 24 b . neurostimulator 24 b conforms substantially to neurostimulator 24 a , but incorporates a transcutaneous receiver coil 50 instead of a battery 40 . transcutaneous receiver coil 50 is implanted with neurostimulator 24 b and transduces electromagnetic energy received from an external coil into power for power supply 38 . in this embodiment , neurostimulator 24 b does not rely on battery resources as a power supply . instead , neurostimulator 24 b receives power via transcutaneous receiver coil 50 and controls the duration of the trial period and any subsequent chronic period in response to timer 48 . fig4 is a block diagram illustrating various components of a patient programmer 36 a for use with one embodiment of the implantable neurostimulation system 20 of fig1 . as shown in fig4 , patient programmer 36 a includes a processor 52 , a user interface 54 , and a telemetry circuit 56 for communication with telemetry circuit 44 of either of neurostimulators 24 a , 24 b , or clinician programmer 34 . patient programmer 36 a also includes memory 58 to store parameters , settings and instructions , and optionally a timer 60 for use during the trial mode , the chronic mode , or both . timer 60 may be implemented in hardware or software , and may operate as a programmable feature of processor 52 . patient programmer 36 a responds to user input entered via user interface 54 to adjust stimulation parameters , settings , and the like . the scope of adjustments permitted by patient programmer 36 a may vary according to whether neurostimulator 24 is in trial mode or chronic mode . in addition , patient programmer 36 a may interrogate neurostimulator 24 to obtain parameters , settings , and other operational data . for example , patient programmer 36 a may be used to initially program neurostimulator 24 for the trial period , and to upload parameters , settings , and other operational data from neurostimulator 24 upon expiration of the trial period . during the course of the trial period or chronic period , neurostimulator 24 may store a variety of information concerning adjustments made by the user , usage profiles and the like . notably , patient programmer 36 a is physically decoupled from neurostimulator 24 in the sense that neurostimulator 24 is implanted and the patient programmer is external to patient 12 . while patient programmer 36 a may be used during trial mode , chronic mode , or both , some embodiments of patient programmer 36 a may include separate patient programmers for use during trial mode and chronic mode . this distinction may make it possible to save cost on the trial programmer , which may be less durable and contain fewer features than the full chronic programmer . accordingly , patient programmer 36 a may upload applicable parameters , settings , and operational information from neurostimulator 24 at the end of the trial period , and then transfer that information directly to chronic programmer , providing significant convenience to the physician and patient . patient programmer 36 a may operate with a rechargeable or replaceable battery ( not shown ). in the example of fig4 , it is assumed that neurostimulator 24 includes its own battery . in other embodiments , as described herein , patient programmer 36 a may deliver power to neurostimulator 24 transcutaneously . in addition , rather than download operational parameters and settings to neurostimulator 24 for substantially independent operation by the neurostimulator , patient programmer 36 a may dynamically control the operation of the neurostimulator by continuous communication with the neurostimulator via telemetry circuit 44 . in addition , in some embodiments , patient programmer 36 a , rather than neurostimulator 24 , may control the end of the trial period . for example , processor 52 may be responsive to expiration of a finite period of time , as indicated by timer 60 . in this case , processor 52 transmits a signal to implanted neurostimulator 24 instructing the neurostimulator to cease operation . after the clinician analyzes the efficacy and safety of the stimulation therapy during the trial period , patient programmer 36 a may be used to renew a subscription for neurostimulator 24 and enable it to function in chronic mode for an unlimited time period , subject to longevity of battery resources in battery - powered systems . in some embodiments , the chronic mode may necessitate a fee or other action or commitment before the subscription to support chronic mode is activated . alternatively , processor 52 may simply terminate communication with neurostimulator 24 at the end of the trial period , in which case the neurostimulator terminates operation . accordingly , termination of the trial period may be initiated within neurostimulator 24 or within patient programmer 36 a , and may be accomplished in a variety of ways . fig5 is a block diagram illustrating various components of another patient programmer 36 b for use with the implantable neurostimulation system 20 of fig1 . patient programmer 36 b conforms substantially to patient programmer 36 a of fig4 . however , patient programmer 36 b further includes a transcutaneous transmitter coil 62 to deliver electromagnetic energy to transcutaneous receiver coil 50 of neurostimulator 24 b for transformation into operating power . accordingly , patient programmer 36 b is worn by the patient . patient programmer 36 b , or at least transcutaneous transmitter coil 62 , is positioned adjacent neurostimulator 24 to provide effective electromagnetic coupling between the transcutaneous transmitter coil and transcutaneous receiver coil 50 . in the example of fig5 , termination of the trial mode can be made by simply terminating the supply of power from transcutaneous transmitter coil 62 and transcutaneous receiver coil 50 . in chronic mode , electromagnetic coupling should not be terminated . however , if this occurs , the renewed subscription is not invalidated . once neurostimulator 24 receives power again , chronic mode may continue once again without interruption . fig6 is a diagram of an implantable trial neurostimulator 24 a with a small battery 40 as a power source . in the example of fig6 , battery 40 is depicted as a coin cell battery , although other battery configurations may be used . in general , battery 40 may have a power capacity and longevity of several years , similar to batteries used in other commercially available neurostimulators . fig6 also depicts patient programmer 36 a . as shown in fig6 , patient programmer 24 a may include buttons 66 , 68 to increase and decrease stimulation settings , respectively . in addition , patient programmer 36 a includes a display 64 , and navigational buttons 70 to permit navigation and selection of control options presented via the display . buttons 66 , 68 , 70 and display 64 form part of user interface 54 . in some cases , display 64 may present information advising patient 12 that expiration of the trial period is approaching . this information may help a patient receiving positive therapy by limiting the time between the end of the trial period and the start of the chronic period . fig7 is a diagram of an implantable neurostimulator 24 b with a transcutaneous receiver coil 50 for power delivery . in particular , patient programmer 36 b includes a transcutaneous receiver coil 50 , which either resides within or extends from a housing associated with neurostimulator 24 b . transcutaneous receiver coil 50 receives electromagnetic energy from transcutaneous transmitter coil 62 , which may be integrated with patient programmer 36 b or extend from a cable 72 , as shown in fig7 . with the exception of transcutaneous transmitter coil 62 , patient programmer 36 b may otherwise conform to patient programmer 36 a of fig6 . fig8 is a flow diagram illustrating implantation and use of an implantable neurostimulator system 20 in accordance with the invention . as shown in fig8 , a chronic stimulation lead 10 is first implanted ( 74 ), e . g ., as shown in fig1 . in some embodiments , lead 10 could be chronic or temporary . with neurostimulator 24 implanted in the chronic implant site , however , it may be desirable to use the chronic lead and thereby avoid the need to withdraw a temporary lead and replace it with the chronic lead after a successful trial period . upon surgical creation of a subcutaneous pocket ( 76 ), the neurostimulator is placed in the subcutaneous pocket for implantation ( 78 ). the chronic lead is subcutaneously tunneled through the body of patient 12 to the subcutaneous pocket ( 80 ). as shown in fig1 , for example , the lead may extend from sacrum 16 to a subcutaneous pocket 31 in the upper left buttock area of patient 12 . when lead 10 is tunneled to subcutaneous pocket 31 , the lead is connected to neurostimulator 24 either directly or via a lead extension and connector . before connecting lead 10 to neurostimulator 24 , and tunneling the lead , a test stimulator may be connected to the lead to deliver neurostimulation for assistance in determining lead placement , lead depth and electrode selection . once the subcutaneous pocket 31 is sealed , neurostimulator 24 is activated for use in trial mode for the duration of the trial period ( 82 ). patient 12 thereafter goes about his daily routine , and may control neurostimulator 24 via patient programmer 36 . when the trial period is complete ( 84 ), e . g ., as evidenced by expiration of a timer within trial neurostimulator 24 or patient programmer 36 , the neurostimulator ceases operation . patient programmer 36 , or physician programmer 34 , then may upload from neurostimulator 24 a set of neurostimulator parameters , settings , operational information or the like either pre - programmed or accumulated during the course of the trial period ( 86 ). the information can be uploaded to patient programmer 36 . if the results are unfavorable and stimulation is unsuccessful to treat the condition of the patient , neurostimulator 24 and lead 10 may be removed from the patient . if the results of the trial period are favorable , the physician may activate the chronic mode of neurostimulator 24 for operation in chronic mode ( 88 ). following the reactivation of neurostimulator 24 in chronic mode , patient programmer 36 or physician programmer 34 may be used to download at least some updated parameters , settings , and other operational information to modify stimulation therapy based upon information received from the trial period ( 90 ). in this manner , patient programmer 36 or physician programmer 34 can be used to quickly and conveniently program the chronic mode based on the results of the trial period . this feature can greatly simplify programming for the chronic period following the trial period . in other embodiments , instead of or , in addition to , authorizing transition from the trial mode to the chronic mode , authorization may serve to unlock additional features of the implanted stimulator . as examples , a clinician or administrator , or a manufacturer of the implanted stimulator , may provide an authorization to activate features such as a voiding diary , different algorithms , different stimulation patterns and the like . the authorization could be provided to any element within the neurostimulation system , such as a physician programmer , patient programmer or the implanted stimulator . the authorization may be an authorization key or code that is obtained from a clinician or a manufacturer of the neurostimulator . in some cases , the authorization may be downloaded or otherwise obtained from a manufacturer for an additional fee , somewhat like a software license . the authorization may be obtained on a recurring basis , for a recurring fee , to permit extended use of the neurostimulator or particular features of the neurostimulator . for example , an initial payment and authorization may support use of the neurostimulator or particular features of the neurostimulator for a period of time , e . g ., one year . after a year of use , the neurostimulator or features “ expire ” in the sense that they are deactivated . upon payment of a renewal fee , the neurostimulator or features are reactivated , providing another “ subscription ” period for the user . as a further variation , in the case of a rechargeable neurostimulator that is recharged at periodic intervals , a fee could be charged each time the battery in the neurostimulator is recharged . if the fee is not paid , the neurostimulator is inactivated . in each of the renewable scenarios , features can be extended or unlocked on a selective basis , providing a pay as you go approach for patients and the ability to select appropriate features on an a la carte basis . many embodiments of the invention have been described . various modifications may be made without departing from the scope of the claims . these and other embodiments are within the scope of the following claims .