How to limit MarketData.GetBars?

Hi Guys, 

I'm sure it's possible, perhaps i'm just missing something. 

I want to get the high and low values of the 30 minute timeframe using MarketData.GetBars. so i have the following:

         var lothirty = MarketData.GetBars(TimeFrame.Minute30, Symbol.Name).LowPrices;
         var hithirty = MarketData.GetBars(TimeFrame.Minute30, Symbol.Name).HighPrices;

This works, as expected. Although, it gets ALL the thirty minute data available. How would i only request the last x instances?

If i only wanted to get the last 2 hours worth (4 x 30 mins), i was hoping something like this would work, but it doesn't:

         MarketData.GetBars(TimeFrame.Minute30, Symbol.Name, 4).LowPrices; 

Any help would be appreciated, 

Thanks,